0000875045-26-000045 | 10-Q | XBRL Viewer

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Collaborative and Other Relationships - Samsung Bioepis (Details) $ in Millions	1 Months Ended	3 Months Ended
	Dec. 31, 2019 product	Mar. 31, 2026 USD ($)	Mar. 31, 2025 USD ($)	Dec. 31, 2025 USD ($)
Collaborative arrangements and non-collaborative arrangement transactions
Collaboration profit sharing/(loss reimbursement)		$ 74.2	$ 58.1
Samsung Bioepis
Collaborative arrangements and non-collaborative arrangement transactions
Collaboration profit sharing/(loss reimbursement)		$ 57.2	$ 48.0
Samsung Bioepis
Collaborative arrangements and non-collaborative arrangement transactions
Contingent Commercialized Rights, Number Of Products \| product	2
Expected profit share percentage	45.00%	50.00%
Collaboration agreement term		5 years
Samsung Bioepis \| Related Party
Collaborative arrangements and non-collaborative arrangement transactions
Accounts receivable		$ 4.7		$ 4.4
Accounts payable		$ 154.4		$ 42.7

Derivative Instruments - Pre-tax Portion of the Fair Value of Foreign Currency Forward Contracts and Foreign Currency Options (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Unrealized gains	$ 9.7	$ 0.0
Unrealized (losses)	(24.4)	(73.3)
Net unrealized gains (losses)	$ (14.7)	$ (73.3)

- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References + Details

Dispositions - Narrative (Details) $ in Millions	1 Months Ended
Dispositions - Narrative (Details) $ in Millions	Mar. 31, 2025 USD ($)
Discontinued Operations and Disposal Groups [Abstract]
Proceeds from sale of intangible assets	$ 51.0

Litigation (Details) € in Thousands, $ in Millions	1 Months Ended		2 Months Ended
Litigation (Details) € in Thousands, $ in Millions	Nov. 30, 2025 USD ($)	Jun. 30, 2022 EUR (€)	Jan. 31, 2026 USD ($)	Mar. 31, 2026 action
Loss Contingencies [Line Items]
Estimate of possible loss \| $			$ 209.8
Securities Litigation
Loss Contingencies [Line Items]
Loss contingency, pending claims, number \| action				3,000,000
Derivative Actions
Loss Contingencies [Line Items]
Loss contingency, pending claims, number \| action				5,000,000
IMRALDI Patent Litigation
Loss Contingencies [Line Items]
Claims asserted \| €		€ 19,100
Genentech
Loss Contingencies [Line Items]
Claims asserted \| $	$ 124.3

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details

Collaborative and Other Relationships - Sage Therapeutics, Inc. (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Collaborative arrangements and non-collaborative arrangement transactions
Collaboration profit sharing/(loss reimbursement)	$ 74.2	$ 58.1
Share of net profit from sage therapeutics, Percent	50.00%
Sage Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Collaboration profit sharing/(loss reimbursement)	$ 17.0	$ 10.1

Acquisitions

3 Months Ended

Mar. 31, 2026

Business Combination [Abstract]

Acquisitions


Note 2:			Acquisitions

Proposed Acquisition of Apellis Pharmaceuticals, Inc.

In March 2026 we entered into an agreement to acquire all of the issued and outstanding shares of Apellis Pharmaceuticals, Inc., a commercial-stage biopharmaceutical company focused on the discovery, development and commercialization of novel therapeutic compounds to treat diseases with high unmet needs. As a result of this proposed acquisition we would acquire two FDA-approved products from Apellis: SYFOVRE (pegcetacoplan injection) for the treatment of geographic atrophy, or GA, an immune-mediated retinal disease; and EMPAVELI (pegcetacoplan) for the treatment of paroxysmal nocturnal hemoglobinuria, or PNH, a rare blood disorder, and C3 glomerulopathy, or C3G, and primary immune complex membranoproliferative glomerulonephritis, or primary IC-MPGN, in rare immune-

mediated kidney diseases. The addition of Apellis is expected to enhance our short- and long-term revenue growth profile by adding two commercialized differentiated immunology and rare disease medicines to our growth portfolio.

Under the terms of the proposed acquisition, we would pay Apellis shareholders $41.00 per share in cash, representing an expected total transaction value of approximately $5.6 billion, and one contractual, non-transferable contingent value right per share representing the right to receive contingent cash payments of up to an aggregate of $4.00 in cash, subject to the achievement of specified annual global net sales thresholds for SYFOVRE.

We plan to fund the proposed acquisition of Apellis through approximately $3.6 billion of available cash and marketable securities on hand, supplemented by approximately $2.0 billion in bank loans.

We expect this transaction to be accounted for as a business combination and to include the results of operations in our condensed consolidated financial statements from the acquisition date.

Alcyone Therapeutics, Inc.

In November 2025 we completed the acquisition of all of the issued and outstanding shares of Alcyone Therapeutics, Inc., a clinical-stage biotechnology company focused on pediatric care through precision CNS therapeutics and dosing platforms. The lead asset acquired is ThecaFlex DRx, an implantable subcutaneous port and catheter device being investigated for the intrathecal delivery of ASOs, including SPINRAZA, that is designed to provide an alternative to repeat lumbar punctures in chronic intrathecal administration of medicines.

Total consideration for this transaction was approximately $85.0 million, comprising of a $50.0 million payment made upon closing and a $35.0 million payment that was considered probable as of December 31, 2025, and made upon FDA approval of a supplemental application in January 2026. This consideration was recorded within acquired in-process research and development, upfront and milestone expense within our consolidated statements of income for the year ended December 31, 2025, included in our 2025 Form 10-K.

We may pay additional development and regulatory milestone payments to the former shareholders of Alcyone of up to a total of $75.0 million if approval is received for ThecaFlex DRx administration of SPINRAZA or other additional pipeline products.

We accounted for this transaction as an asset acquisition as the value being acquired primarily relates to a single asset. Under the terms of this acquisition, we will oversee the end-to-end development, manufacturing and commercialization of ThecaFlex DRx.

Alcyone's remaining therapeutic assets were divested from Alcyone into Neela Therapeutics, Inc., a newly formed independent company, prior to the closing of this acquisition.

- Definition

The entire disclosure for business combination.

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+ Details

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2026

Accounting Policies [Abstract]

Summary of significant accounting policies


Note 1:			Summary of Significant Accounting Policies

References in these notes to "Biogen," the "company," "we," "us" and "our" refer to Biogen Inc. and its consolidated subsidiaries.

Business Overview

Biogen is a global biopharmaceutical company focused on discovering, developing and delivering innovative therapies for people living with serious and complex diseases. We have a broad portfolio of medicines to treat MS, have introduced the first approved treatment for SMA, co-developed treatments to address a defining pathology of Alzheimer’s disease and launched the first approved treatment to target a genetic cause of ALS. We market the first and only drug approved in the U.S., the E.U. and certain international markets for the treatment of FA in adults and adolescents aged 16 years and older. We are focused on advancing our pipeline in neurology, specialized immunology and rare diseases. We support our drug discovery and development efforts through internal research and development programs, external collaborations and acquisitions.

Our marketed products include VUMERITY, TYSABRI, TECFIDERA, AVONEX and PLEGRIDY for the treatment of MS; SPINRAZA for the treatment of SMA; SKYCLARYS for the treatment of FA; and QALSODY for the treatment of ALS.

We also have collaborations with Eisai on the commercialization of LEQEMBI for the treatment of Alzheimer's disease and Supernus on the commercialization of ZURZUVAE for the treatment of PPD. We have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, CLL and other conditions; RITUXAN HYCELA for the treatment of non-Hodgkin's lymphoma and CLL; GAZYVA for the treatment of CLL, follicular lymphoma and lupus nephritis; OCREVUS for the treatment of PPMS and RMS; LUNSUMIO for the treatment of relapsed or refractory follicular lymphoma; COLUMVI, a bispecific antibody for the treatment of non-Hodgkin's lymphoma; and have the option to add other potential anti-CD20 therapies, pursuant to our collaboration arrangements with Genentech, a wholly owned member of the Roche Group.

We commercialize a portfolio of biosimilars of advanced biologics including: BENEPALI, an etanercept biosimilar referencing ENBREL; IMRALDI, an adalimumab biosimilar referencing HUMIRA; and FLIXABI, an infliximab biosimilar referencing REMICADE.

For additional information on our collaboration arrangements, please read Note 18, Collaborative and Other Relationships, to these unaudited condensed consolidated financial statements (condensed consolidated financial statements).

Basis of Presentation

In the opinion of management, our condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair statement of our financial statements for interim periods in accordance with U.S. GAAP. The information included in this quarterly report on Form 10-Q should be read in conjunction with our audited consolidated financial statements and the accompanying notes included in our 2025 Form 10-K. Our accounting policies are described in the Notes to Consolidated Financial Statements in our 2025 Form 10-K and updated, as necessary, in this report. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements, but does not include all disclosures required by U.S. GAAP. The results of operations for the three months ended March 31, 2026, are not necessarily indicative of the operating results for the full year or for any other subsequent interim period.

We operate as one operating segment, focused on discovering, developing and delivering innovative therapies for people living with serious and complex diseases.

Consolidation

Our condensed consolidated financial statements reflect our financial statements, those of our wholly owned subsidiaries and variable interest entities where we are the primary beneficiary. For consolidated entities where we own or are exposed to less than 100.0% of the economics, we record net income (loss) attributable to noncontrolling interests, net of tax in our condensed consolidated statements of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. Intercompany balances and transactions are eliminated in consolidation.

In determining whether we are the primary beneficiary of a variable interest entity, we apply a qualitative approach that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. We continuously assess whether we are the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in us consolidating or deconsolidating one or more of our collaborators or partners.

Use of Estimates

The preparation of our condensed consolidated financial statements requires us to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, equity, revenue and expense and related disclosure of contingent assets and liabilities. On an ongoing basis we evaluate our estimates, judgments and assumptions. We base our estimates on historical experience and on various other assumptions that we believe are reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenue and expense. Actual results may differ from these estimates.

Significant Accounting Policies

There have been no material changes to our significant accounting policies disclosed in Note 1, Summary of Significant Accounting Policies, to our audited consolidated financial statements included in our 2025 Form 10-K.

New Accounting Pronouncements

From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that we adopt as of the specified effective date. Unless otherwise discussed below, we do not believe that the adoption of recently issued standards have had, or may have, a material impact on our condensed consolidated financial statements or disclosures.

Standard

Description

Effective Date

Effects on the financial statements

ASU No. 2024-03, Income Statement (Subtopic 220-40): Reporting Comprehensive Income - Expense Disaggregation Disclosures

This standard requires disclosure in the notes to the financial statements, at each interim and annual reporting period, of specified information about certain costs and expense including purchases of inventory, employee compensation, depreciation and intangible asset amortization included in each relevant expense caption. This standard also requires a qualitative description of the amounts remaining in relevant expense captions that are not separately disaggregated, as well as disclosure of the total amount of selling expenses, and, in annual reporting periods, an entity’s definition of selling expenses.

Annual reporting for fiscal years beginning after December 15, 2026, and interim periods within fiscal years beginning after December 15, 2027. Early adoption is permitted.

We are currently evaluating the potential impact that this new standard will have on our consolidated financial statements and related disclosures, and expect to apply this standard prospectively upon adoption.

ASU No. 2025-06, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Targeted Improvements to the Accounting for Internal-Use Software

This standard modernizes the accounting for software costs, including updating guidance on the recognition and measurement of costs incurred in connection with development and implementation activities related to internal-use software.

Annual reporting for fiscal periods beginning after December 15, 2027, and interim periods within those annual reporting periods. Early adoption is permitted.

We are currently evaluating the potential impact that this new standard will have on our consolidated financial statements and related disclosures.

Income Taxes - Narrative (Details)	3 Months Ended
Income Taxes - Narrative (Details)	Mar. 31, 2026	Mar. 31, 2025
Income Tax Disclosure [Abstract]
Effective tax rate	15.40%	22.70%

Fair Value Measures - Level 3 Assets and Liabilities Held at Fair Value (Details) - Significant Unobservable Inputs (Level 3) $ in Millions	Mar. 31, 2026 USD ($)	Dec. 31, 2025 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration obligations	$ 266.9	$ 246.4
Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Discount rate	0.057
Minimum \| Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Discount rate		0.053
Maximum \| Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Discount rate		0.054
Weighted Average \| Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Discount rate	0.057	0.054

Financial Instruments - Proceeds from Marketable Debt Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Investments, Debt and Equity Securities [Abstract]
Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale	$ 625.2	$ 0.0

Other Consolidated Financial Statement Detail - Accrued Expense and Other (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Collaborative arrangements and non-collaborative arrangement transactions
Revenue-related reserves for discounts and allowances	$ 1,176.6	$ 1,210.8
Total accrued expense and other	2,546.8	2,802.6
Component of accrued expenses and other
Collaborative arrangements and non-collaborative arrangement transactions
Revenue-related reserves for discounts and allowances	961.1	1,000.4
Employee compensation and benefits	192.5	375.8
Collaboration expense	353.5	280.0
Royalties and licensing fees	308.5	302.4
Other	$ 731.2	$ 844.0

Property, Plant and Equipment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Property, Plant and Equipment [Abstract]
Accumulated depreciation	$ 2,998.3		$ 2,931.2
Depreciation	$ 67.9	$ 71.4

Collaborative and Other Relationships - Other Research and Discovery Arrangements and Funding Arrangements (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Collaborative arrangements and non-collaborative arrangement transactions
Research and development:	$ 539.0	$ 434.1
Reduction of research and development expense	25.0	50.0
Milestone Payment, Future Development
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones	250.0
Felzartamab And Izastobart/HIB210 \| Human Immunology Biosciences Pre-Existing In License Commitments \| Milestone Payment, One
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones	35.0
Felzartamab And Izastobart/HIB210 \| Human Immunology Biosciences Pre-Existing In License Commitments \| Milestone Payment, Two
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones	30.0
Felzartamab And Izastobart/HIB210 \| Human Immunology Biosciences Pre-Existing In License Commitments \| Milestone Payment, Future Development
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones	130.0
Felzartamab And Izastobart/HIB210 \| Human Immunology Biosciences Pre-Existing In License Commitments \| Milestone Payment, Regulatory
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones	230.0
Felzartamab And Izastobart/HIB210 \| Human Immunology Biosciences Pre-Existing In License Commitments \| Milestone Payment, Commercial
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones	640.0
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Collaborative arrangements and non-collaborative arrangement transactions
Expected receipts to co-fund development costs, current			$ 200.0
Expected receipts to co-fund development costs, noncurrent	50.0
Other research and discovery
Collaborative arrangements and non-collaborative arrangement transactions
Research and development:	$ 34.0	$ 1.0

Restructuring- Fit for Growth Initiative (Details) $ in Millions		3 Months Ended
	Sep. 26, 2023 USD ($)	Mar. 31, 2026 USD ($) Employee	Mar. 31, 2025 USD ($)	Jul. 31, 2023 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost Remaining				$ 320.0
Restructuring charges		$ 7.9	$ 35.3
2023 Cost Saving Initiatives
Restructuring Cost and Reserve [Line Items]
Expected operating expense reduction	$ 1,000.0
Expected number of positions eliminated \| Employee		1,400
Restructuring charges			$ 35.3

- Definition

Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

+ References + Details

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

Intangible Assets and Goodwill

3 Months Ended

Mar. 31, 2026

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible Assets and Goodwill


Note 7:			Intangible Assets and Goodwill

Intangible Assets

Intangible assets, net of accumulated amortization, impairment charges and adjustments are summarized as follows:


		As of March 31, 2026						As of December 31, 2025
(In millions)	Estimated Life	Cost		Accumulated Amortization		Net		Cost		Accumulated Amortization		Net



Completed technology	1-30 years	$	14,078.8	$	(6,824.3)	$	7,254.5	$	14,067.3	$	(6,687.8)	$	7,379.5
In-process research and development	Indefinite until commercialization	1,635.0		—		1,635.0		1,635.0		—		1,635.0
Priority review voucher	Indefinite	100.0		—		100.0		100.0		—		100.0
Trademarks and trade names	Indefinite	64.0		—		64.0		64.0		—		64.0
Total intangible assets		$	15,877.8	$	(6,824.3)	$	9,053.5	$	15,866.3	$	(6,687.8)	$	9,178.5

Amortization and Impairments

For the three months ended March 31, 2026, amortization and impairment of acquired intangible assets totaled $136.5 million, compared to $111.8 million in the prior year comparative period. The increase was primarily due to amortization for the acquired intangible assets associated with SKYCLARYS and TYSABRI.

For the three months ended March 31, 2026 and 2025, we had no impairment charges.

Completed Technology

Completed technology primarily relates to our other marketed products and programs acquired through asset acquisitions, licenses and business combinations. Completed technology intangible assets are amortized over their estimated useful lives, which range between approximately 1 to 30 years, with a remaining weighted average useful life of 11 years as of March 31, 2026.

IPR&D Related to Business Combinations

IPR&D represents the fair value assigned to research and development assets that we acquired as part of a business combination and had not yet reached technological feasibility at the date of acquisition.

The carrying value associated with our IPR&D assets as of March 31, 2026 and December 31, 2025, primarily relates to the IPR&D programs we acquired in connection with our acquisition of HI-Bio in July 2024, with an estimated fair value of approximately $1.6 billion.

Priority Review Voucher

In connection with our acquisition of Reata in September 2023 we acquired a rare pediatric disease PRV which may be used to obtain priority review by the FDA for a future regulatory submission or sold to a third party. We recorded the PRV based on its estimated fair value of $100.0 million as an intangible asset.

Estimated Future Amortization of Intangible Assets

The estimated future amortization of finite-lived intangible assets for the next five years is expected to be as follows:


(In millions)	As of March 31, 2026
2026 (remaining nine months)	$	400.0
2027	485.0
2028	525.0
2029	575.0
2030	650.0
2031	700.0

Goodwill

The following table provides a roll forward of the changes in our goodwill balance:


(In millions)	As of March 31, 2026
Goodwill, December 31, 2025	$	6,491.1

Other(1)	(2.4)
Goodwill, March 31, 2026	$	6,488.7

(1) Other includes adjustments related to foreign currency exchange rate fluctuations.

As of March 31, 2026, we had no impairment losses related to goodwill.

Collaborative and Other Relationships

3 Months Ended

Mar. 31, 2026

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Collaborative and Other Relationships


Note 18:			Collaborative and Other Relationships

Genentech, Inc. (Roche Group)

We have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, CLL and other conditions; RITUXAN HYCELA for the treatment of non-Hodgkin's lymphoma and CLL; GAZYVA for the treatment of CLL, follicular lymphoma and lupus nephritis; OCREVUS for the treatment of PPMS and RMS; LUNSUMIO for the treatment of relapsed or refractory follicular lymphoma; COLUMVI, a bispecific antibody for the treatment of non-Hodgkin's lymphoma; and have the option to add other potential anti-CD20 therapies, pursuant to our collaboration arrangements with Genentech, a wholly owned member of the Roche Group. For purposes of this footnote, we refer to RITUXAN and RITUXAN HYCELA collectively as RITUXAN.

RITUXAN

Under our collaboration with Genentech, we are entitled to a tiered share of co-promotion operating profits and losses of RITUXAN in the U.S., as summarized in the table below. Genentech and its affiliates are responsible for the worldwide manufacture of RITUXAN as well as all development and commercialization activities as follows:

•U.S.: We have co-exclusively licensed our rights to develop, commercialize and market RITUXAN in the U.S.

•Canada: We have co-exclusively licensed our rights to develop, commercialize and market RITUXAN in Canada.

GAZYVA

The Roche Group and its sub-licensees maintain sole responsibility for the development, manufacture and commercialization of GAZYVA and we are entitled to a tiered share of co-promotion operating profits and losses of GAZYVA in the U.S. The level of gross sales of GAZYVA in the U.S. has impacted our percentage of the co-promotion profits for RITUXAN and LUNSUMIO, as summarized in the table below.

OCREVUS

Pursuant to the terms of our collaboration arrangements with Genentech, we receive a tiered royalty on U.S. net sales from 13.5% and increasing up to 24.0% if annual net sales exceed $900.0 million. There will be a 50.0% reduction to these royalties upon the first entry of an FDA-approved biosimilar to OCREVUS.

In addition, we receive a gross 3.0% royalty on net sales of OCREVUS outside the U.S., with the royalty period lasting 11 years from the first commercial sale of OCREVUS on a country-by-country basis.

The commercialization of OCREVUS does not impact the percentage of the co-promotion profits we receive for RITUXAN, LUNSUMIO or GAZYVA. Genentech is solely responsible for development and commercialization of

OCREVUS and funding future costs. Genentech cannot develop OCREVUS in CLL, non-Hodgkin's lymphoma or rheumatoid arthritis.

OCREVUS royalty revenue is based on our estimates from third party and market research data of OCREVUS sales occurring during the corresponding period. Differences between actual and estimated royalty revenue will be adjusted for in the period in which they become known, which is generally expected to be the following quarter.

LUNSUMIO

Under our collaboration with Genentech, we are entitled to a tiered share of co-promotion operating profits and losses in the U.S., as summarized in the table below. In addition, we receive low-single digit royalties on sales of LUNSUMIO outside the U.S.

COLUMVI

Genentech has sole decision-making rights on the commercialization of COLUMVI within the U.S. and we receive tiered royalties in the mid-single digit range on net sales of COLUMVI in the U.S. The commercialization of COLUMVI does not impact the percentage of the co-promotion profits we receive for RITUXAN, LUNSUMIO or GAZYVA.

Profit-sharing Formulas

RITUXAN and LUNSUMIO Profit Share

Our current pre-tax co-promotion profit-sharing formula for RITUXAN and LUNSUMIO in the U.S. provides for a 30.0% share on the first $50.0 million of combined co-promotion operating profits earned each calendar year. Our share of the combined annual co-promotion profits for RITUXAN and LUNSUMIO in excess of $50.0 million varies upon the following events, as summarized in the table below:




After LUNSUMIO Approval until the First Threshold Date	37.5	%
After First Threshold Date until the Second Threshold Date	35.0	%
After Second Threshold Date	30.0	%

In March 2023 the First Threshold Date was achieved when U.S. gross sales of GAZYVA within a consecutive 12-month period reached $500.0 million. As a result, beginning in April 2023 the pre-tax profit share for RITUXAN and LUNSUMIO has been 35.0%. The Second Threshold Date would be achieved on the first date in any calendar year in which U.S. gross sales of LUNSUMIO have reached $350.0 million.

GAZYVA Profit Share

Our current pre-tax profit-sharing formula for GAZYVA provides for a 35.0% share of operating profits earned in the U.S. in each calendar year.

For additional information on our collaboration arrangements with Genentech, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2025 Form 10-K.

Ionis Pharmaceuticals, Inc.

2017 SMA Collaboration Agreement

In December 2017 we entered into a collaboration agreement with Ionis to identify new ASO drug candidates for the potential treatment of SMA. Under this agreement, we have options to license therapies arising out of this collaboration and will be responsible for the development and commercialization of such therapies.

In December 2021 we exercised our option with Ionis and obtained a worldwide, exclusive, royalty-bearing license to develop and commercialize salanersen (BIIB115), an investigational ASO in development for SMA.

We may pay Ionis up to $155.0 million in additional development and regulatory milestone payments related to salanersen, including a $45.0 million milestone payment due upon the initiation of a Phase 3 trial. Upon commercialization, we may also pay Ionis up to $400.0 million in additional performance-based milestone payments and tiered royalties on potential net sales of such therapies ranging from the mid-teens to high-twenties percentages.

For additional information on our collaboration arrangements with Ionis, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2025 Form 10-K.

Eisai Co., Ltd.

LEQEMBI (lecanemab) Collaboration

We have a collaboration agreement with Eisai to jointly develop and commercialize LEQEMBI (lecanemab), an anti-amyloid antibody for the treatment of Alzheimer's disease. The FDA granted traditional approval of LEQEMBI in July 2023. Prior to receiving traditional approval, LEQEMBI had been granted accelerated approval by the FDA in January 2023, at which time it became commercially available in the U.S. Outside of the U.S., LEQEMBI is now approved in the E.U. (April 2025), Australia (September 2025), China (January 2024), Japan (September 2023) and other international markets. LEQEMBI monthly IV maintenance dosing for the treatment of early Alzheimer's disease was approved in the U.S. and China in January 2025 and September 2025, respectively, and LEQEMBI subcutaneous autoinjector (IQLIK) for weekly maintenance dosing was approved in the U.S. in August 2025.

All costs, including research, development, sales and marketing expense, are shared equally between us and Eisai. We also share profits and losses equally. We currently have a supply agreement with Eisai to manufacture LEQEMBI drug substance and drug product through the end of 2031.

Subject to the limitations in the LEQEMBI Collaboration Agreement, Eisai has final decision-making authority on all matters relating to the collaboration and serves as the lead of LEQEMBI development and regulatory submissions globally. We co-commercialize and co-promote LEQEMBI with Eisai. The LEQEMBI Collaboration Agreement provides that each commercialization plan shall allocate the responsibilities for the activities under the plan in an equitable fashion taking into account Biogen's and Eisai's respective capabilities and provides a meaningful role for each party.

Upon commercialization of LEQEMBI in the U.S., we began recognizing our 50.0% share of LEQEMBI product revenue, net and cost of sales, including royalties, within Alzheimer's collaboration revenue in our condensed consolidated statements of income, as we are not the principal.

Our share of LEQEMBI sales and marketing expense and development expense are recorded within selling, general and administrative expense and research and development expense, respectively, within our condensed consolidated statements of income.

A summary of development and sales and marketing expense related to the LEQEMBI Collaboration is as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total development expense incurred by the collaboration related to the advancement of LEQEMBI	$	69.3	$	52.6
Biogen's share of the LEQEMBI Collaboration development expense reflected in research and development expense in our condensed consolidated statements of income	34.6		26.3
Total sales and marketing expense incurred by the LEQEMBI Collaboration	169.5		177.7
Biogen's share of the LEQEMBI Collaboration sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income	84.7		88.8

Amounts receivable from Eisai related to the agreements discussed above were approximately $7.0 million and $90.2 million as of March 31, 2026 and December 31, 2025, respectively. Amounts payable to Eisai related to the agreements discussed above were approximately $85.4 million and $95.5 million as of March 31, 2026 and December 31, 2025, respectively.

For additional information on our collaboration arrangements with Eisai, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2025 Form 10-K.

UCB

We have a collaboration agreement with UCB, effective November 2003, to jointly develop and commercialize dapirolizumab pegol, an anti-CD40L pegylated Fab, for the potential treatment of SLE and other future agreed indications. Either we or UCB may propose development of dapirolizumab pegol in additional indications. If the parties do not agree to add an indication as an agreed indication to the collaboration, we or UCB may, at the sole expense of the applicable party, pursue development in such excluded indication(s), subject to an opt-in right of the non-pursuing party after proof of clinical activity.

All costs incurred for agreed indications, including research, development, sales and marketing expense, are shared equally between us and UCB. If marketing approval is obtained, both companies will jointly commercialize dapirolizumab pegol and share profits and losses equally.

A summary of development expense related to the UCB collaboration agreement is as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total UCB collaboration development expense	$	29.4	$	21.8
Biogen's share of the UCB collaboration development expense reflected in research and development expense in our condensed consolidated statements of income	14.7		10.9

Supernus Pharmaceuticals, Inc. (previously Sage Therapeutics, Inc.)

In November 2020 we entered into a global collaboration and license agreement with Sage to jointly develop and commercialize ZURZUVAE (zuranolone) for the treatment of PPD. In July 2025 Sage was acquired by Supernus. ZURZUVAE was approved in the U.S. in August 2023 and in the E.U. in September 2025.

Under this collaboration, both companies will share equal responsibility and costs for development as well as profits and losses for commercialization in the U.S. Outside of the U.S., we are responsible for development and commercialization, excluding Japan, Taiwan and South Korea, with respect to zuranolone and may pay Supernus potential tiered royalties in the high-teens to low-twenties percentages.

We share 50.0% of the net collaboration results in the U.S. with Supernus, which are recognized in collaboration profit sharing/(loss reimbursement) in our condensed consolidated statements of income. For the three months ended March 31, 2026 and 2025, we recognized net profit-sharing expense of approximately $17.0 million and $10.1 million, respectively, to reflect Supernus' 50.0% share of the net collaboration results in the U.S.

A summary of development and sales and marketing expense related to the Supernus collaboration is as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total Supernus collaboration development expense	$	1.2	$	2.4
Biogen's share of the Supernus collaboration development expense reflected in research and development expense in our condensed consolidated statements of income	0.6		1.2
Total sales and marketing expense incurred by the Supernus collaboration	47.5		43.5
Biogen's share of the Supernus collaboration sales and marketing expense reflected in selling, general and administrative expense and collaboration profit sharing/(loss reimbursement) in our condensed consolidated statements of income	23.7		21.7

Denali Therapeutics Inc.

In August 2020 we entered into a collaboration and license agreement with Denali to co-develop and co-commercialize BIIB122, a small molecule inhibitor of LRRK2 for Parkinson's disease (LRRK2 Collaboration).

Under the LRRK2 Collaboration, both companies share responsibility and costs for global development based on specified percentages as well as profits and losses for commercialization in the U.S. and China. Outside the U.S. and China we are responsible for commercialization and may pay Denali potential tiered royalties.

A summary of development expense related to the Denali collaboration is as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total Denali collaboration development expense	$	10.3	$	12.8
Biogen's share of the Denali collaboration development expense reflected in research and development expense in our condensed consolidated statements of income	6.2		7.7

Stoke Therapeutics, Inc.

In February 2025 we entered into a collaboration and license agreement with Stoke to co-develop and commercialize zorevunersen, an investigational ASO that targets the SCN1A gene for the potential treatment of Dravet syndrome, a rare form of genetic epilepsy associated with refractory seizures and neurodevelopmental impairments. Zorevunersen dosed its first patient in August 2025, advancing zorevunersen to a global Phase 3 trial.

Under the terms of this agreement, Stoke will continue to lead global development and retain exclusive development and commercialization rights for zorevunersen in the U.S., Canada and Mexico and we will have exclusive rights to commercialize zorevunersen in the rest of the world. Both companies will share responsibility for external clinical development costs, where Stoke is responsible for 70.0% of these development costs and we are responsible for the remaining 30.0% of these development costs.

In connection with the closing of this transaction we made an upfront payment of $165.0 million to Stoke, which was recognized in acquired in-process research and development, upfront and milestone expense within our condensed consolidated statements of income for the three months ended March 31, 2025. We may also pay Stoke potential development and commercial milestone payments of up to $50.0 million and $335.0 million, respectively, if all the specified milestones set forth in this collaboration are achieved. In addition, we may pay Stoke tiered royalties on potential net sales of any products developed under this collaboration in the low-double digit to high-teen percentages.

We also have an exclusive option to license certain future follow-on ASO products targeting the SCN1A gene in all territories worldwide other than the U.S., Canada and Mexico, in exchange for separate milestone, cost sharing and royalty considerations.

A summary of development expense related to the Stoke collaboration agreement is as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total Stoke collaboration development expense	$	12.2	$	3.9
Biogen's share of the Stoke collaboration development expense reflected in research and development expense in our condensed consolidated statements of income	3.7		1.2

Samsung Bioepis Co., Ltd.

2019 Development and Commercialization Agreement

In December 2019 we completed a transaction with Samsung Bioepis and secured the exclusive rights to commercialize two potential ophthalmology biosimilar products, BYOOVIZ, a ranibizumab biosimilar referencing LUCENTIS, and OPUVIZ, an aflibercept biosimilar referencing EYLEA, in major markets worldwide, including the U.S., Canada, Europe, Japan and Australia. The agreement established that Samsung Bioepis will be responsible for development and will supply both products to us at a pre-specified gross margin of approximately 45.0%.

In October 2024 we notified Samsung Bioepis of our decision to terminate our 2019 Development and Commercialization Agreement (the DCA Agreement) solely within the U.S. and Canada. The transfer of commercialization rights for BYOOVIZ and OPUVIZ in the U.S. and Canada back to Samsung Bioepis was completed as of December 31, 2025.

In October 2025 we completed the sale of our remaining commercial rights to BYOOVIZ and OPUVIZ in Europe. Samsung Bioepis will have full responsibility for commercialization of BYOOVIZ upon the transfer of commercial rights from Biogen back to Samsung Bioepis, which became effective as of January 2026.

We reflected revenue on sales of BYOOVIZ to third parties in product revenue, net in our condensed consolidated statements of income and recorded the related cost of revenue and sales and marketing expense in our condensed consolidated statements of income to their respective line items when these costs are incurred.

2013 Commercial Agreement

In December 2013 we entered into an agreement with Samsung Bioepis to commercialize, over a 10-year term, three anti-TNF biosimilar product candidates which includes IMRALDI, an adalimumab biosimilar referencing HUMIRA, FLIXABI, an infliximab biosimilar referencing REMICADE, and BENEPALI, an etanercept biosimilar referencing ENBREL, in Europe. In July 2024 we exercised an option to extend this agreement by an additional five years.

We reflect revenue on sales of BENEPALI, IMRALDI and FLIXABI to third parties in product revenue, net in our condensed consolidated statements of income and record the related cost of revenue and sales and marketing expense in our condensed consolidated statements of income to their respective line items when these costs are incurred. Royalty payments to AbbVie on sales of IMRALDI are recognized in cost of sales within our condensed consolidated statements of income.

We share 50.0% of the profit or loss related to our 2013 commercial agreement with Samsung Bioepis, which is recognized in collaboration profit sharing/(loss reimbursement) in our condensed consolidated statements of income. For the three months ended March 31, 2026 and 2025, we recognized net profit-sharing expense of approximately $57.2 million and $48.0 million, respectively, to reflect Samsung Bioepis' 50.0% sharing of the net collaboration profits.

Other Services

Simultaneous with the formation of Samsung Bioepis, we also entered into a license agreement with Samsung Bioepis. Under this license agreement, we granted Samsung Bioepis an exclusive license to use, develop, manufacture and commercialize biosimilar products created by Samsung Bioepis using Biogen product-specific technology. In exchange, we receive single-digit royalties on biosimilar products developed and commercialized by Samsung Bioepis. Royalty revenue under the license agreement is recognized as a component of contract manufacturing, royalty and other revenue in our condensed consolidated statements of income.

Amounts receivable from Samsung Bioepis related to the agreements discussed above were approximately $4.7 million and $4.4 million as of March 31, 2026 and December 31, 2025, respectively. Amounts payable to Samsung Bioepis related to the agreements discussed above were approximately $154.4 million and $42.7 million as of March 31, 2026 and December 31, 2025, respectively.

For additional information on our collaboration arrangements with Samsung Bioepis and our other significant collaboration arrangements, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2025 Form 10-K.

Other Research and Discovery Arrangements and Funding Arrangements

These arrangements may include the potential for future milestone payments based on the achievement of certain clinical and commercial development payable over a period of several years.

Other

For the three months ended March 31, 2026 and 2025, we recorded approximately $34.0 million and less than $1.0 million, respectively, as acquired in-process research and development, upfront and milestone expense in our condensed consolidated statements of income related to other research and discovery related arrangements.

Royalty Pharma Funding Arrangement

In February 2025 we entered into a funding agreement with Royalty Pharma under which we received $200.0 million in 2025 and $50.0 million in 2026 to co-fund our development costs for the litifilimab program. As there is a substantive transfer of risk to the financial partner for the amount invested, the development funding will be recognized by us as an obligation to perform contractual services. This funding is being recognized as a reduction to research and development expense within our condensed consolidated statements of income, proportionate to the related expense. For the three months ended March 31, 2026 and 2025, we received $25.0 million and $50.0 million, respectively, from Royalty Pharma, which we recorded as reductions to research and development expense within our condensed consolidated statements of income. The final payment related to the funding agreement of $25.0 million was received in April 2026.

If the litifilimab clinical trials are successful for the indications based on the applicable clinical trials, upon regulatory approval in the U.S. or certain major markets in the world, Royalty Pharma will be eligible to receive approval-based fixed milestone payments of up to $250.0 million. The milestone payments due upon approval will be recorded as a component of other (income) expense, net within our condensed consolidated statements of income, when incurred.

If litifilimab receives regulatory approval, Royalty Pharma will be eligible to receive royalties of a mid-single digit percentage of the applicable net sales. Royalties on net sales will be recorded as cost of sales within our condensed consolidated statements of income.

MorphoSys AG

As part of our acquisition of HI-Bio in July 2024, we acquired HI-Bio's pre-existing in-license commitments under third-party agreements with MorphoSys, which included tiered royalties on potential future sales ranging from high-single digit to mid-teen percentages, as well as potential development, regulatory and commercial milestone payments of up to $130.0 million, $230.0 million and $640.0 million, respectively. These amounts included milestone payments due upon the first patient dosed in a Phase 3 clinical trial of felzartamab in a first and second indication of $35.0 million and $30.0 million, respectively.

In April 2026 we entered into a definitive agreement with TJ Bio, where we assumed regulatory and sales milestone obligations under a pre-existing agreement between TJ Bio and MorphoSys and may pay MorphoSys tiered royalties on potential net sales of felzartamab in the greater China region. For additional information on TJ Bio, please read Note 22, Subsequent Events, to these condensed consolidated financial statements.

During the first quarter of 2025 we accrued a milestone payment due to MorphoSys of $35.0 million upon the first patient dosed in a Phase 3 clinical trial of felzartamab for the treatment of AMR, which was recorded within acquired in-process research and development, upfront and milestone expense in our condensed consolidated statements of income for the three months ended March 31, 2025, and paid in April 2025.

During the second quarter of 2025 we accrued a milestone payment due to MorphoSys of $30.0 million upon the first patient dosed in a Phase 3 clinical trial of felzartamab for the treatment of IgAN, which was recorded within acquired in-process research and development, upfront and milestone expense in our condensed consolidated statements of income, and paid in July 2025.

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Mar. 31, 2026
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Collaborative and Other Relationships (Tables)

3 Months Ended

Mar. 31, 2026

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Co Promotion Profit Sharing Formula

Our share of the combined annual co-promotion profits for RITUXAN and LUNSUMIO in excess of $50.0 million varies upon the following events, as summarized in the table below:




After LUNSUMIO Approval until the First Threshold Date	37.5	%
After First Threshold Date until the Second Threshold Date	35.0	%
After Second Threshold Date	30.0	%

Summary of Activity Related to BAN2401 and Elenbecestat Collaboration

A summary of development and sales and marketing expense related to the LEQEMBI Collaboration is as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total development expense incurred by the collaboration related to the advancement of LEQEMBI	$	69.3	$	52.6
Biogen's share of the LEQEMBI Collaboration development expense reflected in research and development expense in our condensed consolidated statements of income	34.6		26.3
Total sales and marketing expense incurred by the LEQEMBI Collaboration	169.5		177.7
Biogen's share of the LEQEMBI Collaboration sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income	84.7		88.8

Summary of activity related to the UCB collaboration

A summary of development expense related to the UCB collaboration agreement is as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total UCB collaboration development expense	$	29.4	$	21.8
Biogen's share of the UCB collaboration development expense reflected in research and development expense in our condensed consolidated statements of income	14.7		10.9

Summary of Activity Related to Sage Therapeutics

A summary of development and sales and marketing expense related to the Supernus collaboration is as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total Supernus collaboration development expense	$	1.2	$	2.4
Biogen's share of the Supernus collaboration development expense reflected in research and development expense in our condensed consolidated statements of income	0.6		1.2
Total sales and marketing expense incurred by the Supernus collaboration	47.5		43.5
Biogen's share of the Supernus collaboration sales and marketing expense reflected in selling, general and administrative expense and collaboration profit sharing/(loss reimbursement) in our condensed consolidated statements of income	23.7		21.7

Summary of Activity Related to Denali Therapeutics Collaboration

A summary of development expense related to the Denali collaboration is as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total Denali collaboration development expense	$	10.3	$	12.8
Biogen's share of the Denali collaboration development expense reflected in research and development expense in our condensed consolidated statements of income	6.2		7.7

Summary of Activity Related to Stoke Therapeutics Collaboration

A summary of development expense related to the Stoke collaboration agreement is as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total Stoke collaboration development expense	$	12.2	$	3.9
Biogen's share of the Stoke collaboration development expense reflected in research and development expense in our condensed consolidated statements of income	3.7		1.2

Intangible Assets and Goodwill (Tables)

3 Months Ended

Mar. 31, 2026

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible assets

Intangible assets, net of accumulated amortization, impairment charges and adjustments are summarized as follows:


		As of March 31, 2026						As of December 31, 2025
(In millions)	Estimated Life	Cost		Accumulated Amortization		Net		Cost		Accumulated Amortization		Net



Completed technology	1-30 years	$	14,078.8	$	(6,824.3)	$	7,254.5	$	14,067.3	$	(6,687.8)	$	7,379.5
In-process research and development	Indefinite until commercialization	1,635.0		—		1,635.0		1,635.0		—		1,635.0
Priority review voucher	Indefinite	100.0		—		100.0		100.0		—		100.0
Trademarks and trade names	Indefinite	64.0		—		64.0		64.0		—		64.0
Total intangible assets		$	15,877.8	$	(6,824.3)	$	9,053.5	$	15,866.3	$	(6,687.8)	$	9,178.5

Estimated future amortization for acquired intangible assets

The estimated future amortization of finite-lived intangible assets for the next five years is expected to be as follows:


(In millions)	As of March 31, 2026
2026 (remaining nine months)	$	400.0
2027	485.0
2028	525.0
2029	575.0
2030	650.0
2031	700.0

Summary of roll forward of the changes in goodwill

The following table provides a roll forward of the changes in our goodwill balance:


(In millions)	As of March 31, 2026
Goodwill, December 31, 2025	$	6,491.1

Other(1)	(2.4)
Goodwill, March 31, 2026	$	6,488.7

(1) Other includes adjustments related to foreign currency exchange rate fluctuations.

- Definition

Tabular disclosure of goodwill by reportable segment and in total which includes a rollforward schedule.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 24
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482548/350-20-55-24

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (h)
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-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
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-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
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-Name Accounting Standards Codification
-Section 50
-Paragraph 1
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-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
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-Section 50
-Paragraph 1
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
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-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 11: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1A

Reference 12: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-2

+ Details

Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2026

Earnings Per Share [Abstract]

Basic and diluted earnings per share

Basic and diluted shares outstanding used in our earnings per share calculation are calculated as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Numerator:
Net income attributable to Biogen Inc.	$	319.5	$	240.5
Denominator:
Weighted average number of common shares outstanding	147.2		146.1
Effect of dilutive securities:

Time-vested restricted stock units	1.0		0.5

Performance stock units settled in stock	0.2		—
Dilutive potential common shares	1.2		0.5
Shares used in calculating diluted earnings per share	148.4		146.6

Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2026

Investments, Debt and Equity Securities [Abstract]

Summary of financial assets with maturities of less than 90 days included within cash and cash equivalents

The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included in cash and cash equivalents in our condensed consolidated balance sheets:


(In millions)	As of March 31, 2026		As of December 31, 2025
Money market funds	$	1,949.4	$	2,027.7
Overnight reverse repurchase agreements	34.3		70.0

Short-term debt securities	391.8		15.4
Commercial paper	137.3		120.1
Total	$	2,512.8	$	2,233.2

Marketable debt and equity securities

The following tables summarize our marketable debt and equity securities, classified as available-for-sale:


	As of March 31, 2026
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable debt securities
Corporate debt securities:
Current	$	418.2	$	—	$	(0.7)	$	417.5
Non-current	317.8		—		(1.1)		316.7
Government securities:
Current	482.6		—		(0.1)		482.5
Non-current	64.8		—		—		64.8
Mortgage and other asset backed securities:

Non-current	84.2		—		(0.1)		84.1
Total marketable debt securities	$	1,367.6	$	—	$	(2.0)	$	1,365.6

Marketable equity securities

Marketable equity securities, non-current	$	227.8	$	—	$	(90.4)	$	137.4
Total marketable equity securities	$	227.8	$	—	$	(90.4)	$	137.4


	As of December 31, 2025
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable debt securities
Corporate debt securities:
Current	$	246.8	$	—	$	—	$	246.8
Non-current	290.6		0.2		—		290.8
Government securities:
Current	560.4		—		—		560.4
Non-current	88.4		—		—		88.4
Mortgage and other asset backed securities:

Non-current	52.7		—		—		52.7
Total marketable debt securities	$	1,238.9	$	0.2	$	—	$	1,239.1

Marketable equity securities

Marketable equity securities, non-current	$	227.7	$	—	$	(109.6)	$	118.1
Total marketable equity securities	$	227.7	$	—	$	(109.6)	$	118.1

The estimated fair value and amortized cost of our marketable debt securities classified as available-for-sale by contractual maturity are summarized as follows:


	As of March 31, 2026				As of December 31, 2025
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	900.0	$	900.8	$	807.2	$	807.2
Due after one year through five years	454.7		455.9		419.5		419.3
Due after five years	10.9		10.9		12.4		12.4
Total marketable debt securities	$	1,365.6	$	1,367.6	$	1,239.1	$	1,238.9

Realized Gain (Loss) on Investments

The proceeds from maturities and sales of marketable debt securities and any resulting realized gains and losses are summarized as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Proceeds from maturities and sales	$	625.2	$	—

- Definition

Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-9

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 11: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 12: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

+ Details

Segment Information - Narrative (Details)	3 Months Ended
Segment Information - Narrative (Details)	Mar. 31, 2026 segment
Segment Reporting [Abstract]
Number of reportable segment	1

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2026

Fair Value Disclosures [Abstract]

Summary of assets and liabilities recorded at fair value

The tables below present information about our assets and liabilities that are regularly measured and carried at fair value and indicate the level within the fair value hierarchy of the valuation techniques we utilized to determine such fair value:


	Fair Value Measurements on a Recurring Basis
	As of March 31, 2026
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	2,512.8	$	—	$	2,512.8	$	—
Marketable debt securities:
Corporate debt securities	734.2		—		734.2		—
Government securities	547.3				547.3		—
Mortgage and other asset backed securities	84.1				84.1		—
Marketable equity securities	137.4		137.4		—		—
Other current assets:

Derivative contracts	19.1		—		19.1		—
Other non-current assets:
Convertible note(1)	35.0		—		—		35.0
Plan assets for deferred compensation	56.8		—		56.8		—
Derivative contracts	5.9		—		5.9		—
Total	$	4,132.6	$	137.4	$	3,960.2	$	35.0
Liabilities:
Other current liabilities:
Derivative contracts	$	38.2	$	—	$	38.2	$	—

Other non-current liabilities:

Contingent consideration obligations	266.9		—		—		266.9
Total	$	305.1	$	—	$	38.2	$	266.9

(1) Convertible notes includes a $30.0 million convertible note we invested in as part of our strategic research arrangement with City Therapeutics during 2025, as well as a $5.0 million convertible note we invested into Neela Therapeutics, Inc. during 2025. We elected the fair value option for both convertible notes. For additional information on the arrangement with City Therapeutics, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2025 Form 10-K.


	Fair Value Measurements on a Recurring Basis
	As of December 31, 2025
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	2,233.2	$	—	$	2,233.2	$	—
Marketable debt securities:
Corporate debt securities	537.6		—		537.6		—
Government securities	648.8		—		648.8		—
Mortgage and other asset backed securities	52.7		—		52.7		—
Marketable equity securities	118.1		118.1		—		—
Other current assets:
Derivative contracts	10.0		—		10.0		—
Other non-current assets:
Convertible notes(1)	35.0		—		—		35.0
Plan assets for deferred compensation	52.2		—		52.2		—
Derivative contracts	0.4		—		0.4		—
Total	$	3,688.0	$	118.1	$	3,534.9	$	35.0
Liabilities:
Other current liabilities:
Derivative contracts	$	56.7	$	—	$	56.7	$	—

Other non-current liabilities:
Derivative contracts	2.2		—		2.2		—
Contingent consideration obligations	246.4		—		—		246.4
Total	$	305.3	$	—	$	58.9	$	246.4

Fair Value Measurement Inputs and Valuation Techniques

The following tables present quantitative information, as of the dates indicated, about the valuation techniques and significant unobservable inputs used in the valuation of our Level 3 financial assets and liabilities measured at fair value on a recurring basis:

Quantitative Information about Level 3 Fair Value Measurements

As of March 31, 2026

(In millions)

Fair Value

Valuation Technique

Significant
Unobservable Input(s)

Range

Weighted Average

Liabilities:

Contingent consideration obligations

266.9

Discounted cash flow

Discount rate

5.7%

Expected timing of achievement of development milestones

2028

—

Quantitative Information about Level 3 Fair Value Measurements

As of December 31, 2025

(In millions)

Fair Value

Valuation Technique

Significant
Unobservable Input(s)

Range

Weighted Average

Liabilities:

Contingent consideration obligations

246.4

Discounted cash flow

Discount rate

5.3% - 5.4%

5.4%

Expected timing of achievement of development milestones

2028 - 2030

—

Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation

The following table provides a roll forward of the fair value of our contingent consideration obligations, which were classified as Level 3 measurements:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Fair value, beginning of period	$	246.4	$	512.8

Changes in fair value	20.5		9.6

Fair value, end of period	$	266.9	$	522.4

Summary of fair and carrying value of debt instruments

The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows:


	As of March 31, 2026				As of December 31, 2025
(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value






Non-current portion:


2.250% Senior Notes due May 1, 2030	$	1,369.5	$	1,495.9	$	1,378.7	$	1,495.7
5.050% Senior Notes due January 15, 2031	407.5		398.1		413.0		398.0
5.750% Senior Notes due May 15, 2035	672.5		645.6		684.5		645.5
5.200% Senior Notes due September 15, 2045	1,004.8		1,101.6		1,029.5		1,101.5
3.150% Senior Notes due May 1, 2050	943.9		1,475.8		973.0		1,475.6
3.250% Senior Notes due February 15, 2051	443.6		481.3		461.8		480.3
6.450% Senior Notes due May 15, 2055	725.9		690.2		737.6		690.2
Non-current portion of notes payable	5,567.7		6,288.5		5,678.1		6,286.8
Total notes payable	$	5,567.7	$	6,288.5	$	5,678.1	$	6,286.8

Equity - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications	$ 21.1	$ (28.0)
Amounts reclassified from AOCI	20.7	(10.3)
Net current period other comprehensive income (loss)	41.8	(38.3)
Gains (losses) on securities available for sale
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	0.2
Other comprehensive income (loss) before reclassifications	(1.8)
Amounts reclassified from AOCI	0.0
Net current period other comprehensive income (loss)	(1.8)
Ending Balance	(1.6)
Gains (losses) on cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(58.9)	51.6
Other comprehensive income (loss) before reclassifications	31.9	(47.5)
Amounts reclassified from AOCI	20.7	(10.3)
Net current period other comprehensive income (loss)	52.6	(57.8)
Ending Balance	(6.3)	(6.2)
Unfunded status of postretirement benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(9.4)	(16.6)
Other comprehensive income (loss) before reclassifications	(0.5)	0.4
Amounts reclassified from AOCI	0.0	0.0
Net current period other comprehensive income (loss)	(0.5)	0.4
Ending Balance	(9.9)	(16.2)
Currency translation adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(113.9)	(171.2)
Other comprehensive income (loss) before reclassifications	(8.5)	19.1
Amounts reclassified from AOCI	0.0	0.0
Net current period other comprehensive income (loss)	(8.5)	19.1
Ending Balance	(122.4)	(152.1)
Accumulated other comprehensive income (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(182.0)	(136.2)
Ending Balance	$ (140.2)	$ (174.5)

Dispositions

3 Months Ended

Mar. 31, 2026

Divestitures [Abstract]

Dispositions


Note 3:			Dispositions

Sale of TOFIDENCE

In March 2025 we completed the sale of our regulatory and commercial rights in the U.S. for TOFIDENCE, a tocilizumab biosimilar referencing ACTEMRA, to Organon. Under the terms of this transaction, we received a payment of approximately $51.0 million in July 2025 and recognized a de minimis loss within our condensed consolidated statements of income for the three months ended March 31, 2025.

Restructuring (Tables)

3 Months Ended

Mar. 31, 2026

Restructuring and Related Activities [Abstract]

Schedule of Restructuring Reserve by Type of Cost

Charges and spending related to workforce reductions are summarized as follows:


	Workforce Reductions
(In millions)	2026		2025

Restructuring reserve as of January 1	$	15.8		$	31.9
Expense	7.9			35.3
Payment	(10.3)			(25.8)
Foreign currency and other adjustments	0.2			(1.0)
Restructuring reserve as of March 31	$	13.6		$	40.4

Share-Based Payments (Tables)

3 Months Ended

Mar. 31, 2026

Share-Based Payment Arrangement [Abstract]

Share-based compensation expense included in condensed consolidated statements of income

The following table summarizes share-based compensation expense included in our condensed consolidated statements of income:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Research and development	$	34.8	$	32.1
Selling, general and administrative	53.0		52.0

Subtotal	87.8		84.1
Capitalized share-based compensation costs	(3.8)		(3.1)
Share-based compensation expense included in total cost and expense	84.0		81.0
Income tax effect	(16.5)		(16.0)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	67.5	$	65.0

Summary of share-based compensation expense associated with each of our share-based compensating programs

The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Time-vested restricted stock units	$	70.9	$	66.1


Performance stock units settled in stock	13.3		13.6
Employee stock purchase plan	3.6		3.5

Stock options	—		0.9



Subtotal	87.8		84.1
Capitalized share-based compensation costs	(3.8)		(3.1)
Share-based compensation expense included in total cost and expense	$	84.0	$	81.0

Acquisitions - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended			3 Months Ended
	Dec. 31, 2026	Jul. 02, 2024	Mar. 31, 2026	Jan. 31, 2026	Nov. 30, 2025	Mar. 31, 2026	Mar. 31, 2025
Business Combination [Line Items]
Payments to acquire in process research and development						$ 35.0	$ 0.0
Alcyone Therapeutics, Inc.
Business Combination [Line Items]
Payments to acquire productive assets		$ 85.0
Payments to acquire in process research and development				$ 35.0	$ 50.0
Asset acquisition, contingent consideration, range of outcomes, maximum, amount					$ 75.0
Apellis Pharmaceuticals, Inc.
Business Combination [Line Items]
Price per share (in dollars per share)			$ 41.00			$ 41.00
Business combination, consideration transferred			$ 5,600.0
Payment to shareholder (in dollars per share)			$ 4.00			$ 4.00
Apellis Pharmaceuticals, Inc. \| Forecast
Business Combination [Line Items]
Cash paid for acquisition	$ 3,600.0
Bank loans used for acquisiton	$ 2,000.0

Subsequent Events

3 Months Ended

Mar. 31, 2026

Subsequent Events [Abstract]

Subsequent Events


Note 22:			Subsequent Events

TJ Biopharma Co., Ltd.

In April 2026 we entered into a definitive agreement with TJ Biopharma Co., Ltd. to acquire TJ Bio's exclusive rights to felzartamab in the greater China region. With this agreement, we will own exclusive worldwide rights to felzartamab.

Under the terms of this agreement we made an upfront payment of $100.0 million to TJ Bio, which will be recognized in acquired in-process research and development, upfront and milestone expense within our condensed consolidated statements of income during the second quarter of 2026.

TJ Bio will also be eligible to receive potential commercial and sales milestone payments of up to $20.0 million and $730.0 million, respectively, if all specified milestones set forth in this collaboration are achieved. In addition, we may pay TJ Bio tiered royalties on potential net sales of felzartamab in the greater China region in the mid-single digit to low-double digit percentages.

Additionally, we assumed regulatory and sales milestone obligations under a pre-existing agreement between TJ Bio and MorphoSys and may pay MorphoSys tiered royalties on potential net sales of felzartamab in the greater China region.

Collaborative and Other Relationships - OCREVUS (Details) - OCREVUS $ in Millions	1 Months Ended
	Mar. 31, 2017 USD ($)
Collaborative arrangements and non-collaborative arrangement transactions
Royalty operating profit threshold for highest royalty rate percentage	$ 900.0
Reduction in royalty rate	50.00%
Collaboration agreement term	11 years
Foreign Tax Authority
Collaborative arrangements and non-collaborative arrangement transactions
Royalty percentage to be received	3.00%
Minimum
Collaborative arrangements and non-collaborative arrangement transactions
Royalty percentage to be received	13.50%
Maximum
Collaborative arrangements and non-collaborative arrangement transactions
Royalty percentage to be received	24.00%

Fair Value Measurements - Narrative (Details)	3 Months Ended	12 Months Ended
Fair Value Measurements - Narrative (Details)	Mar. 31, 2026 USD ($)	Dec. 31, 2025 USD ($)	Jun. 30, 2025 USD ($)
Debt Instrument [Line Items]
Asset impairment charges	$ 0	$ 0
Contingent consideration obligations	$ 266,900,000	$ 246,400,000
Minimum \| Human Immunology Biosciences \| Measurement Input, Probability Rate
Debt Instrument [Line Items]
Discount rate	0.750
Maximum \| Human Immunology Biosciences \| Phase 3 Clinical Trial, Felzartamab AMR
Debt Instrument [Line Items]
Contingent consideration obligations			$ 150,000,000.0
Maximum \| Human Immunology Biosciences \| Measurement Input, Probability Rate
Debt Instrument [Line Items]
Discount rate	0.950

Financial Instruments - Summary of financial assets with maturities of less than 90 days included within cash and cash equivalents (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Net Investment Income [Line Items]
Cash equivalents, at carrying value	$ 2,512.8	$ 2,233.2
Money market funds
Net Investment Income [Line Items]
Cash equivalents, at carrying value	1,949.4	2,027.7
Overnight reverse repurchase agreements
Net Investment Income [Line Items]
Cash equivalents, at carrying value	34.3	70.0
Short-term debt securities
Net Investment Income [Line Items]
Cash equivalents, at carrying value	391.8	15.4
Commercial Paper
Net Investment Income [Line Items]
Cash equivalents, at carrying value	$ 137.3	$ 120.1

Other Consolidated Financial Statement Detail

3 Months Ended

Mar. 31, 2026

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Other Consolidated Financial Statement Detail


Note 17:			Other Consolidated Financial Statement Detail

Other (Income) Expense, Net

Components of other (income) expense, net, are summarized as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Interest income	$	(37.9)	$	(23.9)
Interest expense	67.6		60.0
(Gains) losses on investments, net	(22.3)		35.6
Litigation related expense	4.7		3.0
Foreign exchange (gains) losses, net	6.5		(3.8)
Other, net	1.1		(2.5)
Total other (income) expense, net	$	19.7	$	68.4

The (gains) losses on investments, net, as reflected in the table above, relate to debt securities, equity securities of certain biotechnology companies, venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities.

The following table summarizes our (gains) losses on investments, net that relate to our equity securities held during the following periods:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Net (gains) losses recognized on equity securities	$	(22.3)	$	35.6
Less: Net (gains) losses realized on equity securities	(3.2)		(5.4)
Net unrealized (gains) losses recognized on equity securities	$	(19.1)	$	41.0

The net unrealized gains recognized during the three months ended March 31, 2026, primarily reflect an increase in the aggregate fair value of our investment in Denali common stock of approximately $19.2 million.

The net unrealized losses recognized during the three months ended March 31, 2025, primarily reflect a decrease in the aggregate fair value of our investment in Denali common stock of approximately $48.5 million, partially offset by an increase in the fair value of Sage common stock of approximately $15.7 million, which was later disposed of during the third quarter of 2025.

Other Current Assets

Other current assets includes prepaid taxes of $661.6 million and $693.6 million as of March 31, 2026 and December 31, 2025, respectively.

Accrued Expense and Other

Accrued expense and other consists of the following:


(In millions)	As of March 31, 2026		As of December 31, 2025
Revenue-related reserves for discounts and allowances	$	961.1	$	1,000.4
Employee compensation and benefits	192.5		375.8
Collaboration expense	353.5		280.0
Royalties and licensing fees	308.5		302.4


Other	731.2		844.0
Total accrued expense and other	$	2,546.8	$	2,802.6

Other Long-term Liabilities

Other long-term liabilities were $787.1 million and $748.5 million as of March 31, 2026 and December 31, 2025, respectively, and included accrued income taxes totaling $174.2 million and $166.4 million, respectively

Collaborative and Other Relationships - Ionis Pharmaceuticals, Inc (Details) - Ionis $ in Millions	Mar. 31, 2026 USD ($)
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones	$ 155.0
Estimated additional payments upon initiation of new phases	45.0
Expected additional milestone payments when meeting certain sales threshold	$ 400.0

Equity (Tables)

3 Months Ended

Mar. 31, 2026

Equity [Abstract]

Schedule of Accumulated Other Comprehensive Income (Loss)

The following tables summarize the changes in AOCI, net of tax by component:


	For the Three Months Ended March 31, 2026
(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax		Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax		Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax		Currency Translation Adjustments, Net of Tax		Total
Balance, December 31, 2025	$	0.2	$	(58.9)	$	(9.4)	$	(113.9)	$	(182.0)
Other comprehensive income (loss) before reclassifications	(1.8)		31.9		(0.5)		(8.5)		21.1
Amounts reclassified from AOCI	—		20.7		—		—		20.7
Net current period other comprehensive income (loss)	(1.8)		52.6		(0.5)		(8.5)		41.8
Balance, March 31, 2026	$	(1.6)	$	(6.3)	$	(9.9)	$	(122.4)	$	(140.2)


	For the Three Months Ended March 31, 2025
(In millions)	Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax		Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax		Currency Translation Adjustments, Net of Tax		Total
Balance, December 31, 2024	$	51.6	$	(16.6)	$	(171.2)	$	(136.2)
Other comprehensive income (loss) before reclassifications	(47.5)		0.4		19.1		(28.0)
Amounts reclassified from AOCI	(10.3)		—		—		(10.3)
Net current period other comprehensive income (loss)	(57.8)		0.4		19.1		(38.3)
Balance, March 31, 2025	$	(6.2)	$	(16.2)	$	(152.1)	$	(174.5)

Reclassification out of Accumulated Other Comprehensive Income

The following table summarizes the amounts reclassified from AOCI:


(In millions)	Amounts Reclassified from AOCI				Income Statement Location
	For the Three Months Ended March 31,
	2026		2025



Gains (losses) on cash flow hedges	$	(24.3)	$	10.9		Revenue
	0.7		0.7			Operating expense
	—		(0.1)			Other (income) expense, net
	2.9		(1.2)			Income tax (benefit) expense





Total reclassifications, net of tax	$	(20.7)	$	10.3

Fair Value Measurements - Contingent Consideration Obligations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Business Combination, Contingent Consideration, Liability [Roll Forward]
Fair value, beginning of period	$ 246.4	$ 512.8
Changes in fair value	20.5	9.6
Fair value, end of period	$ 266.9	$ 522.4

Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,477.8	$ 2,431.0
Product revenue, net
Disaggregation of Revenue [Line Items]
Total revenue	1,752.3	1,726.5
Product revenue, net \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	839.2	753.8
Product revenue, net \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	913.1	972.7
MS Product Revenues
Disaggregation of Revenue [Line Items]
Total revenue	957.5	953.0
MS Product Revenues \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	559.4	490.4
MS Product Revenues \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	398.1	462.6
Fumarate
Disaggregation of Revenue [Line Items]
Total revenue	288.5	344.9
Fumarate \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	184.8	156.9
Fumarate \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	103.7	188.0
TECFIDERA
Disaggregation of Revenue [Line Items]
Total revenue	109.5	206.1
TECFIDERA \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	31.4	39.8
TECFIDERA \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	78.1	166.3
VUMERITY
Disaggregation of Revenue [Line Items]
Total revenue	179.0	138.8
VUMERITY \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	153.4	117.1
VUMERITY \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	25.6	21.7
Interferon
Disaggregation of Revenue [Line Items]
Total revenue	227.5	226.3
Interferon \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	132.8	132.7
Interferon \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	94.7	93.6
AVONEX
Disaggregation of Revenue [Line Items]
Total revenue	163.2	166.8
AVONEX \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	108.5	108.6
AVONEX \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	54.7	58.2
PLEGRIDY
Disaggregation of Revenue [Line Items]
Total revenue	64.3	59.5
PLEGRIDY \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	24.3	24.1
PLEGRIDY \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	40.0	35.4
TYSABRI
Disaggregation of Revenue [Line Items]
Total revenue	441.5	381.5
TYSABRI \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	241.8	200.8
TYSABRI \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	199.7	180.7
FAMPYRA
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.3
FAMPYRA \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
FAMPYRA \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.3
Rare Disease Product Revenue
Disaggregation of Revenue [Line Items]
Total revenue	557.2	563.3
Rare Disease Product Revenue \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	224.5	231.0
Rare Disease Product Revenue \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	332.7	332.3
SPINRAZA
Disaggregation of Revenue [Line Items]
Total revenue	374.0	423.9
SPINRAZA \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	142.2	154.4
SPINRAZA \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	231.8	269.5
SKYCLARYS
Disaggregation of Revenue [Line Items]
Total revenue	150.7	123.9
SKYCLARYS \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	71.8	69.1
SKYCLARYS \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	78.9	54.8
QALSODY
Disaggregation of Revenue [Line Items]
Total revenue	32.5	15.5
QALSODY \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	10.5	7.5
QALSODY \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	22.0	8.0
Biosimilars
Disaggregation of Revenue [Line Items]
Total revenue	182.2	180.8
Biosimilars \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	4.3
Biosimilars \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	182.2	176.5
BENEPALI
Disaggregation of Revenue [Line Items]
Total revenue	122.1	111.3
BENEPALI \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
BENEPALI \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	122.1	111.3
IMRALDI
Disaggregation of Revenue [Line Items]
Total revenue	49.6	47.4
IMRALDI \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
IMRALDI \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	49.6	47.4
FLIXABI
Disaggregation of Revenue [Line Items]
Total revenue	10.5	13.1
FLIXABI \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
FLIXABI \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	10.5	13.1
BYOOVIZ
Disaggregation of Revenue [Line Items]
Total revenue	0.0	8.9
BYOOVIZ \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	4.2
BYOOVIZ \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.0	4.7
TOFIDENCE
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.1
TOFIDENCE \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.1
TOFIDENCE \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Total revenue	55.4	29.4
Other \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	55.3	28.1
Other \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.1	1.3
ZURZUVAE
Disaggregation of Revenue [Line Items]
Total revenue	55.4	27.7
ZURZUVAE \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	55.3	27.7
ZURZUVAE \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	0.1	0.0
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Total revenue	0.0	1.7
FUMADERM And ADUHELM \| U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.4
FUMADERM And ADUHELM \| Rest of World
Disaggregation of Revenue [Line Items]
Total revenue	$ 0.0	$ 1.3

Derivative Instruments

3 Months Ended

Mar. 31, 2026

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivative Instruments


Note 10:			Derivative Instruments

Foreign Currency Forward Contracts - Hedging Instruments

Due to the global nature of our operations, portions of our revenue and operating expense are recorded in currencies other than the U.S. dollar. The value of revenue and operating expense measured in U.S. dollars is therefore subject to changes in foreign currency exchange rates. We enter into foreign currency forward contracts and foreign currency options with financial institutions with the primary objective to mitigate the impact of foreign currency exchange rate fluctuations on our international revenue and operating expense.

Foreign currency forward contracts and foreign currency options in effect as of March 31, 2026 and December 31, 2025, had durations of 1 to 18 months and 1 to 21 months, respectively. These contracts have been designated as cash flow hedges and unrealized gains and losses on the portion of these foreign currency forward contracts and foreign currency options that are included in the effectiveness test are reported in AOCI. Realized gains and losses of such contracts and options are recognized in revenue when the sale of product in the currency being hedged is recognized and in operating expense when the expense in the currency being hedged is recorded. We recognize all cash flow hedge reclassifications from AOCI and fair value changes of excluded portions in the same line item in our condensed consolidated statements of income that have been impacted by the hedged item.

The notional amount of foreign currency forward contracts and foreign currency options that were entered into to hedge forecasted revenue and operating expense is summarized as follows:


	Notional Amount
(In millions)	As of March 31, 2026		As of December 31, 2025
Euro	$	1,332.6	$	1,531.0
Swiss franc	165.6		—
British pound	76.2		—

Polish zloty	33.0		—
Canadian dollar	24.7		—
Total foreign currency forward contracts and options	$	1,632.1	$	1,531.0

The pre-tax portion of the fair value of these foreign currency forward contracts and foreign currency options that were included in AOCI in total equity is summarized as follows:



(In millions)	As of March 31, 2026		As of December 31, 2025
Unrealized gains	$	9.7	$	—
Unrealized (losses)	(24.4)		(73.3)
Net unrealized gains (losses)	$	(14.7)	$	(73.3)

We expect net unrealized losses of approximately $14.7 million to be settled over the next 18 months, of which approximately $16.4 million of these net unrealized losses are expected to be settled over the next 12 months, with any amounts in AOCI to be reported as an adjustment to revenue or operating expense. We consider the impact of our and our counterparties’ credit risk on the fair value of the contracts as well as the ability of each party to execute its contractual obligations. As of March 31, 2026 and December 31, 2025, credit risk did not materially change the fair value of our foreign currency forward contracts and forward currency options.

The following table summarizes the effect of foreign currency forward contracts and forward currency options designated as hedging instruments in our condensed consolidated statements of income:


For the Three Months Ended March 31,
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)					Net Gains/(Losses) Excluded from Effectiveness Testing and Recognized in Operating Income (in millions)
Location	2026		2025		Location	2026		2025
Revenue	$	(24.3)	$	10.9	Revenue	$	4.1	$	0.4
Operating expense	0.7		0.7		Operating expense	(1.1)		(1.0)

Foreign Currency Forward Contracts - Other Derivative Instruments

We also enter into other foreign currency forward contracts, usually with durations of one month or less, to mitigate the foreign currency risk related to certain balance sheet positions. We have not elected hedge accounting for these transactions.

The aggregate notional amount of these outstanding foreign currency forward contracts was $1,241.1 million and $1,193.7 million as of March 31, 2026 and December 31, 2025, respectively. Net losses of $18.8 million related to these contracts was recorded as a component of other (income) expense, net for the three months ended March 31, 2026, compared to net gains of $7.7 million in the prior year comparative period.

Summary of Derivative Instruments

While certain of our derivative instruments are subject to netting arrangements with our counterparties, we do not offset derivative assets and liabilities in our condensed consolidated balance sheets. The amounts in the table below would not be substantially different if the derivative assets and liabilities were offset.

The following table summarizes the fair value and presentation in our condensed consolidated balance sheets of our outstanding derivative instruments, including those designated as hedging instruments:


(In millions)	Balance Sheet Location	As of March 31, 2026		As of December 31, 2025
Cash Flow Hedging Instruments:
Asset derivative instruments	Other current assets	$	12.2	$	0.1
	Investments and other assets	5.9		0.4
Liability derivative instruments	Accrued expense and other	12.2		54.0
	Other long-term liabilities	—		2.2





Other Derivative Instruments:
Asset derivative instruments	Other current assets	6.9		9.9
Liability derivative instruments	Accrued expense and other	26.0		2.7

- Definition

The entire disclosure for derivative instruments and hedging activities including, but not limited to, risk management strategies, non-hedging derivative instruments, assets, liabilities, revenue and expenses, and methodologies and assumptions used in determining the amounts.

+ References

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+ Details

Segment Information (Tables)

3 Months Ended

Mar. 31, 2026

Segment Reporting [Abstract]

Reconciliation of revenue from segments to consolidated

The following table includes additional information about reported segment revenue, significant segment expense and segment measure of profitability:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total revenue	$	2,477.8	$	2,431.0
Less cost and expense:
Cost of sales, excluding amortization and impairment of acquired intangible assets:
Product cost of sales	478.7		462.2
Royalty cost of sales	182.3		167.1
Research and development:
Research and discovery	40.5		44.3
Early stage programs	69.0		72.4
Late stage programs	102.5		49.4
Marketed products	154.2		89.5
Other research and development costs(1)	172.8		178.5
Acquired in-process research and development, upfront and milestone expense	34.0		200.7
Selling, general and administrative	607.3		572.5
Other segment expense(2)	317.0		353.9
Net Income attributable to Biogen Inc.	$	319.5	$	240.5

(1) Other research and development costs primarily consist of indirect costs incurred in support of overall research and development activities and non-specific programs, including activities that benefit multiple programs, such as management costs, as well as depreciation, information technology and facility-based expenses and are not allocated to a specific program or stage.

(2) Other segment expense includes: amortization and impairment of acquired intangible assets; collaboration profit sharing/(loss reimbursement); (gain) loss on fair value remeasurement of contingent consideration; restructuring charges; other (income) expense, net; and income tax (benefit) expense

Intangible Assets and Goodwill - Estimated Future Amortization of Intangible Assets (Details) $ in Millions	Mar. 31, 2026 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2026 (remaining nine months)	$ 400.0
2027	485.0
2028	525.0
2029	575.0
2030	650.0
2031	$ 700.0

Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,477.8	$ 2,431.0
Total revenues from anti-CD20 therapeutic programs	2,477.8	2,431.0
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	94.7	83.7
Other revenues from anti-CD20 therapeutic programs	7.2	5.7
Royalty Attributed To OCREVUS
Disaggregation of Revenue [Line Items]
Total revenue	317.2	288.8
Total revenues from anti-CD20 therapeutic programs	317.2	288.8
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenue	419.1	378.2
Total revenues from anti-CD20 therapeutic programs	$ 419.1	$ 378.2

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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+ Details

Collaborative and Other Relationships Summary of Activity Related to Stoke Therapeutics (Details) - Research and Development Expense [Member] - Stoke Therapeutics Inc - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration	$ 12.2	$ 3.9
Expense reflected within statements of income	$ 3.7	$ 1.2

Intangible Assets and Goodwill - Summary of Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Finite-Lived Intangible Assets, Net [Abstract]
Finite-Lived Intangible Assets, Gross	$ 14,078.8	$ 14,067.3
Accumulated Amortization	(6,824.3)	(6,687.8)
Net	7,254.5	7,379.5
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Cost	15,877.8	15,866.3
Accumulated Amortization	6,824.3	6,687.8
Net	$ 9,053.5	$ 9,178.5
Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated life (in years)	1 year	1 year
Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated life (in years)	30 years	30 years
In-process research and development
Indefinite-Lived Intangible Assets [Line Items]
Cost and net	$ 1,635.0	$ 1,635.0
Priority review voucher
Indefinite-Lived Intangible Assets [Line Items]
Cost and net	100.0	100.0
Trademarks and trade names
Indefinite-Lived Intangible Assets [Line Items]
Cost and net	$ 64.0	$ 64.0

Revenues Reserves for Discounts and Allowances (Details 1) $ in Millions	3 Months Ended
	Mar. 31, 2026 USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 1,210.8
Current Provisions Relating To Sales In Current Year	905.0
Adjustments Relating To Prior Years	(17.2)
Payments/Returns Relating To Sales in Current Year	(388.8)
Payments/Returns Relating To Sales in Prior Year	(533.2)
Ending Balance	1,176.6
Discounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	115.7
Current Provisions Relating To Sales In Current Year	214.0
Adjustments Relating To Prior Years	0.8
Payments/Returns Relating To Sales in Current Year	(133.4)
Payments/Returns Relating To Sales in Prior Year	(92.3)
Ending Balance	104.8
Contractual adjustments
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	1,046.1
Current Provisions Relating To Sales In Current Year	683.7
Adjustments Relating To Prior Years	(28.4)
Payments/Returns Relating To Sales in Current Year	(255.3)
Payments/Returns Relating To Sales in Prior Year	(426.0)
Ending Balance	1,020.1
Returns
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	49.0
Current Provisions Relating To Sales In Current Year	7.3
Adjustments Relating To Prior Years	10.4
Payments/Returns Relating To Sales in Current Year	(0.1)
Payments/Returns Relating To Sales in Prior Year	(14.9)
Ending Balance	$ 51.7

Financial Instruments

3 Months Ended

Mar. 31, 2026

Investments, Debt and Equity Securities [Abstract]

Financial Instruments


Note 9:			Financial Instruments

The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included in cash and cash equivalents in our condensed consolidated balance sheets:


(In millions)	As of March 31, 2026		As of December 31, 2025
Money market funds	$	1,949.4	$	2,027.7
Overnight reverse repurchase agreements	34.3		70.0

Short-term debt securities	391.8		15.4
Commercial paper	137.3		120.1
Total	$	2,512.8	$	2,233.2

The carrying values of our money market funds, overnight reverse repurchase agreements, short-term debt securities and commercial paper, including accrued interest, approximate fair value due to their short-term maturities.

Our marketable equity securities gains (losses) are recorded in other (income) expense, net in our condensed consolidated statements of income. The following tables summarize our marketable debt and equity securities, classified as available-for-sale:


	As of March 31, 2026
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable debt securities
Corporate debt securities:
Current	$	418.2	$	—	$	(0.7)	$	417.5
Non-current	317.8		—		(1.1)		316.7
Government securities:
Current	482.6		—		(0.1)		482.5
Non-current	64.8		—		—		64.8
Mortgage and other asset backed securities:

Non-current	84.2		—		(0.1)		84.1
Total marketable debt securities	$	1,367.6	$	—	$	(2.0)	$	1,365.6

Marketable equity securities

Marketable equity securities, non-current	$	227.8	$	—	$	(90.4)	$	137.4
Total marketable equity securities	$	227.8	$	—	$	(90.4)	$	137.4


	As of December 31, 2025
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable debt securities
Corporate debt securities:
Current	$	246.8	$	—	$	—	$	246.8
Non-current	290.6		0.2		—		290.8
Government securities:
Current	560.4		—		—		560.4
Non-current	88.4		—		—		88.4
Mortgage and other asset backed securities:

Non-current	52.7		—		—		52.7
Total marketable debt securities	$	1,238.9	$	0.2	$	—	$	1,239.1

Marketable equity securities

Marketable equity securities, non-current	$	227.7	$	—	$	(109.6)	$	118.1
Total marketable equity securities	$	227.7	$	—	$	(109.6)	$	118.1

Summary of Contractual Maturities: Available-for-Sale Debt Securities

The estimated fair value and amortized cost of our marketable debt securities classified as available-for-sale by contractual maturity are summarized as follows:


	As of March 31, 2026				As of December 31, 2025
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	900.0	$	900.8	$	807.2	$	807.2
Due after one year through five years	454.7		455.9		419.5		419.3
Due after five years	10.9		10.9		12.4		12.4
Total marketable debt securities	$	1,365.6	$	1,367.6	$	1,239.1	$	1,238.9

The average maturity of our marketable debt securities classified as available-for-sale as of March 31, 2026, was approximately 10 months.

Proceeds from Marketable Debt Securities

The proceeds from maturities and sales of marketable debt securities and any resulting realized gains and losses are summarized as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Proceeds from maturities and sales	$	625.2	$	—

Strategic Investments

Our strategic investment portfolio includes investments in equity securities of certain biotechnology companies, which are reflected within our disclosures included in Note 8, Fair Value Measurements, to these condensed consolidated financial statements, as well as venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities.

As of March 31, 2026 and December 31, 2025, our strategic investment portfolio was comprised of investments totaling $205.3 million and $186.6 million, respectively, which are included in investments and other assets within our condensed consolidated balance sheets.

The increase in our strategic investment portfolio as of March 31, 2026, compared to December 31, 2025, was primarily due to the increase in the fair value of our investment in Denali common stock.

For additional information on our investments in Denali common stock, please read Note 8, Fair Value Measurements, and Note 17, Other Consolidated Financial Statement Detail, to these condensed consolidated financial statements.

Collaborative and Other Relationships - Co-promotion Profit Sharing Formula (Details) $ in Millions	Mar. 31, 2026 USD ($)
RITUXAN
Collaborative arrangements and non-collaborative arrangement transactions
After LUNSUMIO Approval until the First Threshold Date	37.50%
After First Threshold Date until the Second Threshold Date	35.00%
After Second Threshold Date	30.00%
LUNSUMIO
Collaborative arrangements and non-collaborative arrangement transactions
After First Threshold Date until the Second Threshold Date	35.00%
Co promotion operating profit threshold to determine share of co promotion operating profit prior to amendment	$ 50.0

Cover Page - shares	3 Months Ended
Cover Page - shares	Mar. 31, 2026	Apr. 27, 2026
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2026
Document Transition Report	false
Entity File Number	0-19311
Entity Registrant Name	BIOGEN INC.
Entity Central Index Key	0000875045
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2026
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	33-0112644
Entity Address, Address Line One	225 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	679-2000
Title of 12(b) Security	Common Stock, $0.0005 par value
Trading Symbol	BIIB
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		147,637,117

Collaborative and Other Relationships - Summary of Activity Related To Denali Therapeutics (Details) - Research and Development Expense [Member] - Denali Therapeutics - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration	$ 10.3	$ 12.8
Expense reflected within statements of income	$ 6.2	$ 7.7

Restructuring - Restructuring Reserve Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Restructuring Cost and Reserve [Line Items]
Expense	$ 7.9	$ 35.3
Workforce Reductions
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, beginning	15.8	31.9
Expense	7.9	35.3
Payment	(10.3)	(25.8)
Foreign currency and other adjustments	0.2	(1.0)
Restructuring reserve, ending	$ 13.6	$ 40.4

- Definition

Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

+ References + Details

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

Indebtedness

3 Months Ended

Mar. 31, 2026

Debt Disclosure [Abstract]

Indebtedness


Note 12:			Indebtedness

2025 Senior Notes

On May 12, 2025, we issued senior unsecured notes for an aggregate principal amount of $1.75 billion, consisting of the following:

•$400.0 million of 5.050% Senior Notes due January 15, 2031, valued at 99.981% of par;

•$650.0 million of 5.750% Senior Notes due May 15, 2035, valued at 99.924% of par; and

•$700.0 million of 6.450% Senior Notes due May 15, 2055, valued at 99.657% of par.

Our 2025 Senior Notes are senior unsecured obligations and may be redeemed at our option at any time at 100% of the principal amount plus accrued interest and, until a specified period before maturity, a specified make-whole amount. Our 2025 Senior Notes contain a change-of-control provision that, under certain circumstances, may require us to purchase our 2025 Senior Notes at a price equal to 101% of the principal amount plus accrued and unpaid interest to the date of repurchase.

We incurred approximately $13.9 million of costs associated with this offering which have been recorded as a reduction to the carrying amount of the debt on our condensed consolidated balance sheets. These costs will be amortized as additional interest expense using the effective interest rate method over the period from issuance through maturity. The discounts will be amortized as additional interest expense over the period from issuance through maturity using the effective interest rate method.

Interest on our 2031 Senior Notes is payable January 15 and July 15 of each year, commencing January 15, 2026. Interest on our 2035 Senior Notes and 2055 Senior Notes is payable May 15 and November 15 of each year, commencing on November 15, 2025.

4.050% Senior Notes due September 15, 2025

On September 15, 2015, we issued $1.75 billion aggregate principal amount of 4.050% Senior Notes due September 15, 2025, at 99.764% of par. In June 2025 we used the net proceeds from the sale of our 2025 Senior Notes to redeem our 4.050% Senior Notes due September 15, 2025, prior to maturity. No gain or loss was recognized upon redemption.

- Definition

The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
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-Name Accounting Standards Codification
-Section 50
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-URI https://asc.fasb.org/1943274/2147480434/815-10-50-8A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(c))
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-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Share-Based Payments - Share-Based Compensation Expense Included In Statements Of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Share-based Compensation Expense included in consolidated statements of income
Subtotal	$ 87.8	$ 84.1
Capitalized share-based compensation costs	(3.8)	(3.1)
Share-based compensation expense included in total cost and expense	84.0	81.0
Income tax effect	(16.5)	(16.0)
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	34.8	32.1
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	53.0	52.0
Share-based compensation expense included in net income attributable to Biogen Inc.
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ 67.5	$ 65.0

Revenues - Narrative (Details) $ in Millions	3 Months Ended
Revenues - Narrative (Details) $ in Millions	Mar. 31, 2026 USD ($) wholesaler	Mar. 31, 2025 USD ($) wholesaler
Disaggregation of Revenue [Line Items]
Number of wholesalers \| wholesaler	2	2
Total revenue	$ 2,477.8	$ 2,431.0
Alzheimer's collaboration revenue
Disaggregation of Revenue [Line Items]
Share of non-controling interest recognized	50.00%
Total revenue	$ 59.5	33.0
Alzheimer's Collaboration
Disaggregation of Revenue [Line Items]
Total revenue	$ 59.5	$ 33.0
Distributor One
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	27.30%	25.90%
Distributor Two
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	14.70%	14.10%

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Income Taxes

3 Months Ended

Mar. 31, 2026

Income Tax Disclosure [Abstract]

Income Taxes


Note 16:			Income Taxes

Tax Rate

For the three months ended March 31, 2026 and 2025, our effective tax rate was 15.4% and 22.7%, respectively. The decrease in our effective tax rate was partially driven by favorable impacts of a current year settlement of a foreign tax audit and the vesting of certain share based awards, partially offset by the higher rate of tax on NCTI due to the OBBBA enactment.

Accounting for Uncertainty in Income Taxes

We and our subsidiaries are routinely examined by various taxing authorities. We file income tax returns in various U.S. states and in U.S. federal and other foreign jurisdictions. With few exceptions, we are no longer subject to U.S. federal tax examination for years before 2022 or state, local or non-U.S. income tax examinations for years before 2013.

It is reasonably possible that we will adjust the value of our uncertain tax positions related to certain transfer pricing, collaboration matters, withholding taxes and other issues as we receive additional information from various taxing authorities, including reaching settlements with such authorities.

- Definition

The entire disclosure for income tax.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/exampleRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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Reference 15: http://www.xbrl.org/2003/role/disclosureRef
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Reference 16: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Other Consolidated Financial Statement Detail (Tables)

3 Months Ended

Mar. 31, 2026

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Other income (expense), net

Components of other (income) expense, net, are summarized as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Interest income	$	(37.9)	$	(23.9)
Interest expense	67.6		60.0
(Gains) losses on investments, net	(22.3)		35.6
Litigation related expense	4.7		3.0
Foreign exchange (gains) losses, net	6.5		(3.8)
Other, net	1.1		(2.5)
Total other (income) expense, net	$	19.7	$	68.4

Gain (loss) on investments in equity securities

The following table summarizes our (gains) losses on investments, net that relate to our equity securities held during the following periods:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Net (gains) losses recognized on equity securities	$	(22.3)	$	35.6
Less: Net (gains) losses realized on equity securities	(3.2)		(5.4)
Net unrealized (gains) losses recognized on equity securities	$	(19.1)	$	41.0

Accrued expenses and other

Accrued expense and other consists of the following:


(In millions)	As of March 31, 2026		As of December 31, 2025
Revenue-related reserves for discounts and allowances	$	961.1	$	1,000.4
Employee compensation and benefits	192.5		375.8
Collaboration expense	353.5		280.0
Royalties and licensing fees	308.5		302.4


Other	731.2		844.0
Total accrued expense and other	$	2,546.8	$	2,802.6

Summary of Significant Accounting Policies (Details)	3 Months Ended
Summary of Significant Accounting Policies (Details)	Mar. 31, 2026 segment
Accounting Policies [Abstract]
Number of reportable segments	1
Interest in subsidiary (less than given percentage)	100.00%

Collaborative and Other Relationships - Profit Sharing (Details) $ in Millions	3 Months Ended
	Mar. 31, 2026 USD ($)
RITUXAN
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of co promotion operating profits first fifty million	30.00%
After Second GAZYVA Threshold Date	35.00%
Until Second GAZYVA Threshold Date	37.50%
LUNSUMIO
Collaborative arrangements and non-collaborative arrangement transactions
Limit of gross sale of GAZYVA to be achieved in preceding 12 months under option one	$ 500.0
Sales trigger gross sales threshold	$ 350.0
After Second GAZYVA Threshold Date	35.00%
GAZYVA
Collaborative arrangements and non-collaborative arrangement transactions
After Second GAZYVA Threshold Date	35.00%

Equity

3 Months Ended

Mar. 31, 2026

Equity [Abstract]

Equity


Note 13:			Equity

Accumulated Other Comprehensive Income (Loss)

The following tables summarize the changes in AOCI, net of tax by component:


	For the Three Months Ended March 31, 2026
(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax		Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax		Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax		Currency Translation Adjustments, Net of Tax		Total
Balance, December 31, 2025	$	0.2	$	(58.9)	$	(9.4)	$	(113.9)	$	(182.0)
Other comprehensive income (loss) before reclassifications	(1.8)		31.9		(0.5)		(8.5)		21.1
Amounts reclassified from AOCI	—		20.7		—		—		20.7
Net current period other comprehensive income (loss)	(1.8)		52.6		(0.5)		(8.5)		41.8
Balance, March 31, 2026	$	(1.6)	$	(6.3)	$	(9.9)	$	(122.4)	$	(140.2)


	For the Three Months Ended March 31, 2025
(In millions)	Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax		Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax		Currency Translation Adjustments, Net of Tax		Total
Balance, December 31, 2024	$	51.6	$	(16.6)	$	(171.2)	$	(136.2)
Other comprehensive income (loss) before reclassifications	(47.5)		0.4		19.1		(28.0)
Amounts reclassified from AOCI	(10.3)		—		—		(10.3)
Net current period other comprehensive income (loss)	(57.8)		0.4		19.1		(38.3)
Balance, March 31, 2025	$	(6.2)	$	(16.2)	$	(152.1)	$	(174.5)

The following table summarizes the amounts reclassified from AOCI:


(In millions)	Amounts Reclassified from AOCI				Income Statement Location
	For the Three Months Ended March 31,
	2026		2025



Gains (losses) on cash flow hedges	$	(24.3)	$	10.9		Revenue
	0.7		0.7			Operating expense
	—		(0.1)			Other (income) expense, net
	2.9		(1.2)			Income tax (benefit) expense





Total reclassifications, net of tax	$	(20.7)	$	10.3

- Definition

The entire disclosure for equity.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-8A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-14

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477968/946-235-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477968/946-235-50-2

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478448/946-505-50-6

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-6

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480008/505-10-S99-1

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org/505/tableOfContent

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-14

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-14

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-16

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-18

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-18

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-18

+ Details

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2026

Accounting Policies [Abstract]

Business Overview

Basis of presentation

We operate as one operating segment, focused on discovering, developing and delivering innovative therapies for people living with serious and complex diseases.

Consolidation

Use of estimates

Significant Accounting Policies

New accounting pronouncements

New Accounting Pronouncements

Standard

Description

Effective Date

Effects on the financial statements

ASU No. 2024-03, Income Statement (Subtopic 220-40): Reporting Comprehensive Income - Expense Disaggregation Disclosures

Annual reporting for fiscal years beginning after December 15, 2026, and interim periods within fiscal years beginning after December 15, 2027. Early adoption is permitted.

ASU No. 2025-06, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Targeted Improvements to the Accounting for Internal-Use Software

Annual reporting for fiscal periods beginning after December 15, 2027, and interim periods within those annual reporting periods. Early adoption is permitted.

We are currently evaluating the potential impact that this new standard will have on our consolidated financial statements and related disclosures.

- Definition

Disclosure of accounting policy for the use of estimates in the preparation of financial statements in conformity with generally accepted accounting principles.

+ References

+ Details

Collaborative and Other Relationships - Summary of Activity Related to LEQEMBI Collaboration (Details) - LEQEMBI - Alzheimer's collaboration revenue - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI	$ 69.3	$ 52.6
Expense reflected within statements of income	34.6	26.3
Selling, General and Administrative Expenses [Member]
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income	84.7	88.8
Expense incurred by the collaboration	$ 169.5	$ 177.7

Revenues

3 Months Ended

Mar. 31, 2026

Revenue from Contract with Customer [Abstract]

Revenues


Note 5:			Revenue

Product Revenue

Revenue by product is summarized as follows:


	For the Three Months Ended March 31,
	2026						2025
(In millions)	United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis:
TECFIDERA	$	31.4	$	78.1	$	109.5	$	39.8	$	166.3	$	206.1
VUMERITY	153.4		25.6		179.0		117.1		21.7		138.8
Total Fumarate	184.8		103.7		288.5		156.9		188.0		344.9
AVONEX	108.5		54.7		163.2		108.6		58.2		166.8
PLEGRIDY	24.3		40.0		64.3		24.1		35.4		59.5
Total Interferon	132.8		94.7		227.5		132.7		93.6		226.3
TYSABRI	241.8		199.7		441.5		200.8		180.7		381.5
FAMPYRA(1)	—		—		—		—		0.3		0.3
Subtotal: Multiple Sclerosis	559.4		398.1		957.5		490.4		462.6		953.0

Rare Disease:
SPINRAZA	142.2		231.8		374.0		154.4		269.5		423.9
SKYCLARYS	71.8		78.9		150.7		69.1		54.8		123.9
QALSODY	10.5		22.0		32.5		7.5		8.0		15.5
Subtotal: Rare Disease	224.5		332.7		557.2		231.0		332.3		563.3

Biosimilars:
BENEPALI	—		122.1		122.1		—		111.3		111.3
IMRALDI	—		49.6		49.6		—		47.4		47.4
FLIXABI	—		10.5		10.5		—		13.1		13.1
BYOOVIZ(2)	—		—		—		4.2		4.7		8.9
TOFIDENCE(2)	—		—		—		0.1		—		0.1
Subtotal: Biosimilars	—		182.2		182.2		4.3		176.5		180.8

Other:
ZURZUVAE	55.3		0.1		55.4		27.7		—		27.7


Other(3)	—		—		—		0.4		1.3		1.7
Subtotal: Other	55.3		0.1		55.4		28.1		1.3		29.4
Total product revenue, net	$	839.2	$	913.1	$	1,752.3	$	753.8	$	972.7	$	1,726.5

(1) Effective January 1, 2025, our collaboration and license agreement for FAMPYRA global commercialization rights was terminated.

(2) In 2025 we completed the sale of our rights to TOFIDENCE and BYOOVIZ.

(3) Other includes FUMADERM and ADUHELM.

We recognized revenue from two wholesalers accounting for 27.3% and 14.7% of gross product revenue for the three months ended March 31, 2026, compared to 25.9% and 14.1% of gross product revenue for the three months ended March 31, 2025.

An analysis of the change in reserves for discounts and allowances is summarized as follows:


(In millions)	Discounts		Contractual Adjustments		Returns		Total
Balance, December 31, 2025	$	115.7	$	1,046.1	$	49.0	$	1,210.8
Current provisions relating to sales in current year	214.0		683.7		7.3		905.0
Adjustments relating to prior years	0.8		(28.4)		10.4		(17.2)
Payments/credits relating to sales in current year	(133.4)		(255.3)		(0.1)		(388.8)
Payments/credits relating to sales in prior years	(92.3)		(426.0)		(14.9)		(533.2)
Balance, March 31, 2026	$	104.8	$	1,020.1	$	51.7	$	1,176.6

The total reserves above, which are included in our condensed consolidated balance sheets, are summarized as follows:


(In millions)	As of March 31, 2026		As of December 31, 2025
Component of accrued expense and other	$	961.1	$	1,000.4
Reduction of accounts receivable	215.5		210.4
Total revenue-related reserves	$	1,176.6	$	1,210.8

Revenue from Anti-CD20 Therapeutic Programs

Revenue from anti-CD20 therapeutic programs is summarized in the table below. For the purposes of this footnote, we refer to RITUXAN and RITUXAN HYCELA collectively as RITUXAN.


	For the Three Months Ended March 31,
(In millions)	2026		2025
Royalty revenue on sales of OCREVUS	$	317.2	$	288.8
Biogen’s share of pre-tax profits in the U.S. for RITUXAN, GAZYVA and LUNSUMIO	94.7		83.7
Other revenue from anti-CD20 therapeutic programs	7.2		5.7
Total revenue from anti-CD20 therapeutic programs	$	419.1	$	378.2

For additional information on our collaboration arrangements with Genentech, please read Note 18, Collaborative and Other Relationships, to these condensed consolidated financial statements.

Alzheimer's Collaboration Revenue

Alzheimer's collaboration revenue consists of our 50.0% share of LEQEMBI product revenue, net and cost of sales, including royalties, as we are not the principal. We began recognizing Alzheimer's collaboration revenue upon the accelerated approval of LEQEMBI in the U.S. during the first quarter of 2023.

For the three months ended March 31, 2026 and 2025, we recognized Alzheimer's collaboration revenue of approximately $59.5 million and $33.0 million, respectively.

For additional information on our collaboration arrangements with Eisai, please read Note 18, Collaborative and Other Relationships, to these condensed consolidated financial statements.

Contract Manufacturing, Royalty and Other Revenue

Contract manufacturing, royalty and other revenue is summarized as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Contract manufacturing revenue	$	237.2	$	282.3
Royalty and other revenue	9.7		11.0
Total contract manufacturing, royalty and other revenue	$	246.9	$	293.3

Contract Manufacturing Revenue

Contract manufacturing revenue primarily reflects amounts earned under contract manufacturing agreements with our strategic customers and batches of LEQEMBI related to our collaboration with Eisai.

Royalty and Other Revenue

Royalty and other revenue primarily reflects royalty revenue on biosimilar products from our license arrangements with Samsung Bioepis and royalties we receive from net sales on products related to patents that we have out-licensed.

For additional information on our license arrangements with Samsung Bioepis and our collaboration arrangements with Eisai, please read Note 18, Collaborative and Other Relationships, to these condensed consolidated financial statements.

- Definition

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-9

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-10

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 15
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-15

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-12

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-12

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-12

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-12

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-12

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-13

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org/606/tableOfContent

+ Details

Fair Value Measurements - Debt Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025	May 12, 2025
Debt Instrument [Line Items]
Fair Value	$ 5,567.7	$ 5,678.1
Carrying Value	$ 6,288.5	6,286.8
2.250% Senior Notes due May 1, 2030
Debt Instrument [Line Items]
Fair Value		1,378.7
Debt instrument, interest rate, stated percentage	2.25%
2.250% Senior Notes due May 1, 2030 \| Senior Notes
Debt Instrument [Line Items]
Fair Value	$ 1,369.5
Carrying Value	$ 1,495.9	1,495.7
5.050% Senior Notes due January 15, 2031
Debt Instrument [Line Items]
Fair Value		413.0
Debt instrument, interest rate, stated percentage	5.05%
5.050% Senior Notes due January 15, 2031 \| Senior Notes
Debt Instrument [Line Items]
Fair Value	$ 407.5
Carrying Value	$ 398.1	398.0
Debt instrument, interest rate, stated percentage			5.05%
5.750% Senior Notes due May 15, 2035
Debt Instrument [Line Items]
Fair Value		684.5
Debt instrument, interest rate, stated percentage	5.75%
5.750% Senior Notes due May 15, 2035 \| Senior Notes
Debt Instrument [Line Items]
Fair Value	$ 672.5
Carrying Value	$ 645.6	645.5
Debt instrument, interest rate, stated percentage			5.75%
5.200% Senior Notes due September 15, 2045
Debt Instrument [Line Items]
Fair Value		1,029.5
Debt instrument, interest rate, stated percentage	5.20%
5.200% Senior Notes due September 15, 2045 \| Senior Notes
Debt Instrument [Line Items]
Fair Value	$ 1,004.8
Carrying Value	$ 1,101.6	1,101.5
3.150% Senior Notes due May 1, 2050
Debt Instrument [Line Items]
Fair Value		973.0
Debt instrument, interest rate, stated percentage	3.15%
3.150% Senior Notes due May 1, 2050 \| Senior Notes
Debt Instrument [Line Items]
Fair Value	$ 943.9
Carrying Value	$ 1,475.8	1,475.6
3.250% Senior Notes due February 15, 2051
Debt Instrument [Line Items]
Fair Value		461.8
Debt instrument, interest rate, stated percentage	3.25%
3.250% Senior Notes due February 15, 2051 \| Senior Notes
Debt Instrument [Line Items]
Fair Value	$ 443.6
Carrying Value	$ 481.3	480.3
6.450% Senior Notes due May 15, 2055
Debt Instrument [Line Items]
Fair Value		737.6
Debt instrument, interest rate, stated percentage	6.45%
6.450% Senior Notes due May 15, 2055 \| Senior Notes
Debt Instrument [Line Items]
Fair Value	$ 725.9
Carrying Value	690.2	690.2
Debt instrument, interest rate, stated percentage			6.45%
Non-current portion of notes payable \| Senior Notes
Debt Instrument [Line Items]
Fair Value	5,567.7	5,678.1
Carrying Value	$ 6,288.5	$ 6,286.8

Share-Based Payments - Share-Based Compensation Expense by Share-Based Compensation Program (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Summary of share based compensation expense associated with different programs [Abstract]
Subtotal	$ 87.8	$ 84.1
Capitalized share-based compensation costs	(3.8)	(3.1)
Share-based compensation expense included in total cost and expense	84.0	81.0
Time-vested restricted stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	70.9	66.1
Performance stock units settled in stock
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	13.3	13.6
Employee stock purchase plan
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	3.6	3.5
Stock options
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ 0.0	$ 0.9

Other Consolidated Financial Statement Detail - (Gains) Losses on Investments, Net (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Net (gains) losses recognized on equity securities	$ (22.3)	$ 35.6
Less: Net (gains) losses realized on equity securities	(3.2)	(5.4)
Net unrealized (gains) losses recognized on equity securities	$ (19.1)	$ 41.0

Revenues- Reserves for Discounts and Allowances (Details 2) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Disaggregation of Revenue [Line Items]
Total Reserves	$ 1,176.6	$ 1,210.8
Component of accrued expenses and other
Disaggregation of Revenue [Line Items]
Total Reserves	961.1	1,000.4
Reduction of accounts receivable
Disaggregation of Revenue [Line Items]
Total Reserves	$ 215.5	$ 210.4

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Financial Instruments - Summary of Marketable Securities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Available-for-Sale, Amortized Cost [Abstract]
Amortized Cost	$ 1,367.6	$ 1,238.9
Gross unrealized gains	0.0	0.2
Gross Unrealized Losses	(2.0)	0.0
Fair Value	1,365.6	1,239.1
Debt Securities, Trading, and Equity Securities, FV-NI, Cost [Abstract]
Amortized Cost	227.8	227.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(90.4)	(109.6)
Fair Value	137.4	118.1
Corporate Debt Securities Current
Debt Securities, Available-for-Sale, Amortized Cost [Abstract]
Amortized Cost	418.2	246.8
Gross unrealized gains	0.0	0.0
Gross Unrealized Losses	(0.7)	0.0
Fair Value	417.5	246.8
Corporate debt securities, noncurrent
Debt Securities, Available-for-Sale, Amortized Cost [Abstract]
Amortized Cost	317.8	290.6
Gross unrealized gains	0.0	0.2
Gross Unrealized Losses	(1.1)	0.0
Fair Value	316.7	290.8
Government Securities Current
Debt Securities, Available-for-Sale, Amortized Cost [Abstract]
Amortized Cost	482.6	560.4
Gross unrealized gains	0.0	0.0
Gross Unrealized Losses	(0.1)	0.0
Fair Value	482.5	560.4
Government securities, noncurrent
Debt Securities, Available-for-Sale, Amortized Cost [Abstract]
Amortized Cost	64.8	88.4
Gross unrealized gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	64.8	88.4
Mortgage and other asset backed securities, noncurrent
Debt Securities, Available-for-Sale, Amortized Cost [Abstract]
Amortized Cost	84.2	52.7
Gross unrealized gains	0.0	0.0
Gross Unrealized Losses	(0.1)	0.0
Fair Value	84.1	52.7
Equity Securities, Non-Current
Debt Securities, Trading, and Equity Securities, FV-NI, Cost [Abstract]
Amortized Cost	227.8	227.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(90.4)	(109.6)
Fair Value	$ 137.4	$ 118.1

Revenues - Contract Manufacturing, Royalty and Other Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,477.8	$ 2,431.0
Contract manufacturing revenue
Disaggregation of Revenue [Line Items]
Total revenue	237.2	282.3
Royalty and other revenue
Disaggregation of Revenue [Line Items]
Total revenue	9.7	11.0
Contract manufacturing, royalty and other revenue
Disaggregation of Revenue [Line Items]
Total revenue	$ 246.9	$ 293.3

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Inventory - Components of Inventory (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Inventory Disclosure [Abstract]
Raw materials	$ 278.7	$ 293.4
Work in process	1,351.6	1,595.2
Finished goods	458.2	424.9
Inventory	2,088.5	2,313.5
Inventory, current	1,949.0	2,168.1
Inventory, noncurrent	$ 139.5	$ 145.4

Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Total revenue	$ 2,477.8	$ 2,431.0
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	661.0	629.3
Research and development	539.0	434.1
Acquired in-process research and development, upfront and milestone expense	34.0	200.7
Selling, general and administrative	607.3	572.5
Amortization and impairment of acquired intangible assets	136.5	111.8
Collaboration profit sharing/(loss reimbursement)	74.2	58.1
Business Combination, Contingent Consideration, Change in Contingent Consideration, Liability, Increase (Decrease)	20.5	9.6
Restructuring charges	7.9	35.3
Other (income) expense, net	19.7	68.4
Total cost and expense	2,100.1	2,119.8
Income before income tax (benefit) expense	377.7	311.2
Income tax (benefit) expense	58.2	70.7
Net income attributable to Biogen Inc.	$ 319.5	$ 240.5
Net income per share:
Basic earnings per share attributable to Biogen Inc. (in USD per share)	$ 2.17	$ 1.65
Diluted earnings per share attributable to Biogen Inc. (in USD per share)	$ 2.15	$ 1.64
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc. (in USD per share)	147.2	146.1
Diluted earnings per share attributable to Biogen Inc. (in USD per share)	148.4	146.6
Product revenue, net
Total revenue	$ 1,752.3	$ 1,726.5
Revenue from anti-CD20 therapeutic programs
Total revenue	419.1	378.2
Alzheimer's collaboration revenue
Total revenue	59.5	33.0
Contract manufacturing, royalty and other revenue
Total revenue	$ 246.9	$ 293.3

Intangible Assets and Goodwill - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025	Sep. 30, 2023
Goodwill [Line Items]
Amortization and impairment of acquired intangible assets	$ 136,500,000	$ 111,800,000
Impairment of intangible assets	0	$ 0
Accumulated impairment losses related to goodwill	$ 0
Acquired and in-licensed rights and patents
Goodwill [Line Items]
Weighted-Average remaining useful life	11 years		11 years
In-process research and development \| Human Immunology Biosciences
Goodwill [Line Items]
Business combination, recognized asset acquired, identifiable intangible asset, finite-lived	$ 1,600,000,000		$ 1,600,000,000
Priority review voucher \| Reata Pharmaceuticals, Inc
Goodwill [Line Items]
Business combination, recognized asset acquired, identifiable intangible asset, finite-lived				$ 100,000,000.0
Minimum
Goodwill [Line Items]
Estimated life (in years)	1 year		1 year
Maximum
Goodwill [Line Items]
Estimated life (in years)	30 years		30 years

Financial Instruments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Dec. 31, 2025
Net Investment Income [Line Items]
Average maturity of our marketable debt securities classified as available-for-sale	10 months
Other noncurrent assets \| Strategic Investments
Net Investment Income [Line Items]
Strategic investment portfolio	$ 205.3	$ 186.6

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Current assets:
Cash and cash equivalents	$ 3,382.7	$ 3,008.5
Current portion of marketable securities	900.0	807.2
Inventory	1,949.0	2,168.1
Other current assets	1,168.3	1,123.3
Total current assets	9,190.4	8,974.1
Marketable securities	465.6	431.9
Property, plant and equipment, net	3,017.9	3,055.4
Operating lease assets	251.3	265.4
Intangible assets, net	9,053.5	9,178.5
Goodwill	6,488.7	6,491.1
Deferred tax asset	238.2	292.5
Investments and other assets	777.5	750.6
Total assets	29,483.1	29,439.5
Current liabilities:
Taxes payable	93.6	114.8
Accounts payable	358.5	432.0
Accrued expense and other	2,546.8	2,802.6
Total current liabilities	2,998.9	3,349.4
Notes payable	6,288.5	6,286.8
Deferred tax liability	483.5	507.6
Long-term operating lease liabilities	273.4	290.4
Other long-term liabilities	787.1	748.5
Total liabilities	10,831.4	11,182.7
Commitments, contingencies and guarantees
Biogen Inc. shareholders’ equity:
Preferred stock, par value $0.001 per share	0.0	0.0
Common stock, par value $0.0005 per share	0.1	0.1
Additional paid-in capital	896.7	863.1
Accumulated other comprehensive income (loss)	(140.2)	(182.0)
Retained earnings	20,872.2	20,552.7
Treasury stock, at cost	(2,977.1)	(2,977.1)
Total equity	18,651.7	18,256.8
Total liabilities and equity	29,483.1	29,439.5
Nonrelated Party
Current assets:
Accounts receivable, net of allowance for doubtful accounts of $3.0 and $3.0, respectively	1,369.2	1,342.4
Related Party
Current assets:
Accounts receivable, net of allowance for doubtful accounts of $3.0 and $3.0, respectively	$ 421.2	$ 524.6

Fair Value Measurements

3 Months Ended

Mar. 31, 2026

Fair Value Disclosures [Abstract]

Fair Value Measurements


Note 8:			Fair Value Measurements


	Fair Value Measurements on a Recurring Basis
	As of March 31, 2026
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	2,512.8	$	—	$	2,512.8	$	—
Marketable debt securities:
Corporate debt securities	734.2		—		734.2		—
Government securities	547.3				547.3		—
Mortgage and other asset backed securities	84.1				84.1		—
Marketable equity securities	137.4		137.4		—		—
Other current assets:

Derivative contracts	19.1		—		19.1		—
Other non-current assets:
Convertible note(1)	35.0		—		—		35.0
Plan assets for deferred compensation	56.8		—		56.8		—
Derivative contracts	5.9		—		5.9		—
Total	$	4,132.6	$	137.4	$	3,960.2	$	35.0
Liabilities:
Other current liabilities:
Derivative contracts	$	38.2	$	—	$	38.2	$	—

Other non-current liabilities:

Contingent consideration obligations	266.9		—		—		266.9
Total	$	305.1	$	—	$	38.2	$	266.9


	Fair Value Measurements on a Recurring Basis
	As of December 31, 2025
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	2,233.2	$	—	$	2,233.2	$	—
Marketable debt securities:
Corporate debt securities	537.6		—		537.6		—
Government securities	648.8		—		648.8		—
Mortgage and other asset backed securities	52.7		—		52.7		—
Marketable equity securities	118.1		118.1		—		—
Other current assets:
Derivative contracts	10.0		—		10.0		—
Other non-current assets:
Convertible notes(1)	35.0		—		—		35.0
Plan assets for deferred compensation	52.2		—		52.2		—
Derivative contracts	0.4		—		0.4		—
Total	$	3,688.0	$	118.1	$	3,534.9	$	35.0
Liabilities:
Other current liabilities:
Derivative contracts	$	56.7	$	—	$	56.7	$	—

Other non-current liabilities:
Derivative contracts	2.2		—		2.2		—
Contingent consideration obligations	246.4		—		—		246.4
Total	$	305.3	$	—	$	58.9	$	246.4

Our marketable equity securities represent investments in publicly traded equity securities. Our ability to liquidate our investment in Denali may be limited by the size of our interest, the volume of market-related activity, our concentrated level of ownership and potential restrictions resulting from our status as a collaborator. Therefore, we may realize significantly less than the current value of such investments. For additional information on our investment in Denali common stock, please read Note 9, Financial Instruments, and Note 17, Other Consolidated Financial Statement Detail, to these condensed consolidated financial statements.

There have been no material impairments of our assets measured and carried at fair value as of March 31, 2026 and December 31, 2025. In addition, there have been no changes to our valuation techniques as of March 31, 2026 and December 31, 2025.

For a description of our validation procedures related to prices provided by third-party pricing services and our option pricing valuation model, please read Note 1, Summary of Significant Accounting Policies - Fair Value Measurements, to our consolidated financial statements included in our 2025 Form 10-K.

Level 3 Assets and Liabilities Held at Fair Value

Quantitative Information about Level 3 Fair Value Measurements

As of March 31, 2026

(In millions)

Fair Value

Valuation Technique

Significant
Unobservable Input(s)

Range

Weighted Average

Liabilities:

Contingent consideration obligations

266.9

Discounted cash flow

Discount rate

5.7%

Expected timing of achievement of development milestones

2028

—

Quantitative Information about Level 3 Fair Value Measurements

As of December 31, 2025

(In millions)

Fair Value

Valuation Technique

Significant
Unobservable Input(s)

Range

Weighted Average

Liabilities:

Contingent consideration obligations

246.4

Discounted cash flow

Discount rate

5.3% - 5.4%

5.4%

Expected timing of achievement of development milestones

2028 - 2030

—

The weighted average discount rates were calculated based on the relative fair values of each distinct contingent consideration obligation related to our acquisition of HI-Bio in July 2024. In addition, we apply various probabilities of technological and regulatory success to the valuation models to estimate the fair values of these contingent consideration obligations, which ranged from approximately 75.0% to 95.0% as of March 31, 2026.

There were no transfers of assets or liabilities into or out of Level 3 as of March 31, 2026 and December 31, 2025.

Contingent Consideration Obligations

In connection with our acquisition of HI-Bio in July 2024 we agreed to make additional payments based upon the achievement of certain milestone events. The following table provides a roll forward of the fair value of our contingent consideration obligations, which were classified as Level 3 measurements:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Fair value, beginning of period	$	246.4	$	512.8

Changes in fair value	20.5		9.6

Fair value, end of period	$	266.9	$	522.4

Changes in the fair value of our contingent consideration obligations, other than changes due to payments, are recognized as a (gain) loss on fair value remeasurement of contingent consideration in our condensed consolidated statements of income. The fair values of the contingent consideration liabilities were based on a probability-adjusted discounted cash flow calculation using Level 3 fair value measurements and inputs. For additional information on the valuation techniques and inputs utilized in the valuation of our financial assets and liabilities, please read Note 1, Summary of Significant Accounting Policies, to our consolidated financial statements included in our 2025 Form 10-K.

As of March 31, 2026 and December 31, 2025, approximately $266.9 million and $246.4 million, respectively, of the fair value of our contingent consideration obligations were classified as long-term and reflected as a component of other long-term liabilities in our condensed consolidated balance sheets.

For the three months ended March 31, 2026, changes in the fair value of our contingent consideration obligations were primarily due to changes in the probabilities of success and expected timing of the achievement of certain remaining developmental milestones.

During the second quarter of 2025 the first milestone related to the fourth patient dosed in a phase 3 clinical trial of felzartamab for AMR was achieved, resulting in a $150.0 million milestone payment made to the former shareholders of HI-Bio, which was paid during the third quarter of 2025. In October 2025 the second milestone related to the fourth patient dosed in a phase 3 clinical trial of felzartamab for IgAN was achieved, resulting in a $150.0 million milestone payment made to the former shareholders of HI-Bio during the fourth quarter of 2025.

Financial Instruments Not Carried at Fair Value

Other Financial Instruments

Due to the short-term nature of certain financial instruments, the carrying value reflected in our condensed consolidated balance sheets for current accounts receivable, due from anti-CD20 therapeutic programs, other current assets, accounts payable and accrued expense and other, approximates fair value.

Debt Instruments

The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows:


	As of March 31, 2026				As of December 31, 2025
(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value






Non-current portion:


2.250% Senior Notes due May 1, 2030	$	1,369.5	$	1,495.9	$	1,378.7	$	1,495.7
5.050% Senior Notes due January 15, 2031	407.5		398.1		413.0		398.0
5.750% Senior Notes due May 15, 2035	672.5		645.6		684.5		645.5
5.200% Senior Notes due September 15, 2045	1,004.8		1,101.6		1,029.5		1,101.5
3.150% Senior Notes due May 1, 2050	943.9		1,475.8		973.0		1,475.6
3.250% Senior Notes due February 15, 2051	443.6		481.3		461.8		480.3
6.450% Senior Notes due May 15, 2055	725.9		690.2		737.6		690.2
Non-current portion of notes payable	5,567.7		6,288.5		5,678.1		6,286.8
Total notes payable	$	5,567.7	$	6,288.5	$	5,678.1	$	6,286.8

The fair values of each of our series of Senior Notes were determined through market, observable and corroborated sources. The changes in the fair values of our Senior Notes as of March 31, 2026, compared to December 31, 2025, are primarily related to increases in U.S. treasury yields and credit spreads used to value our Senior Notes since December 31, 2025. For additional information related to our Senior Notes, please read Note 13, Indebtedness, to our consolidated financial statements included in our 2025 Form 10-K.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
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-Section 50
-Paragraph 2
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Reference 2: http://www.xbrl.org/2003/role/exampleRef
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Reference 3: http://www.xbrl.org/2003/role/exampleRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Investments in Variable Interest Entities [Abstract]
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	$ 48.4	$ 49.8

Derivative Instruments - Effect of Foreign Currency Forward Contracts and Forward Currency Options Designated as Hedging Instruments (Details) - Foreign Exchange Contract - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Revenue
Derivatives [Line Items]
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)	$ (24.3)	$ 10.9
Revenue \| Cash flows, revenue
Derivatives [Line Items]
Net Gains/(Losses) Excluded from Effectiveness Testing and Recognized in Operating Income (in millions)	4.1	0.4
Operating expense
Derivatives [Line Items]
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)	0.7	0.7
Operating expense \| Cash flows, operating expenses
Derivatives [Line Items]
Net Gains/(Losses) Excluded from Effectiveness Testing and Recognized in Operating Income (in millions)	$ (1.1)	$ (1.0)

Equity - Reclassified From AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Total revenue	$ 2,477.8	$ 2,431.0
Other (income) expense, net	(19.7)	(68.4)
Income Tax Expense (Benefit)	58.2	70.7
Net income attributable to Biogen Inc.	319.5	240.5
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income attributable to Biogen Inc.	(20.7)	10.3
Gains (losses) on cash flow hedges \| Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Total revenue	(24.3)	10.9
Operating Expenses	0.7	0.7
Other (income) expense, net	0.0	(0.1)
Income Tax Expense (Benefit)	$ 2.9	$ (1.2)

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Inventory

3 Months Ended

Mar. 31, 2026

Inventory Disclosure [Abstract]

Inventory


Note 6:			Inventory

The components of inventory are summarized as follows:


(In millions)	As of March 31, 2026		As of December 31, 2025
Raw materials	$	278.7	$	293.4
Work in process	1,351.6		1,595.2
Finished goods	458.2		424.9
Total inventory	$	2,088.5	$	2,313.5

Balance Sheet Classification:
Inventory	$	1,949.0	$	2,168.1
Investments and other assets	139.5		145.4
Total inventory	$	2,088.5	$	2,313.5

Long-term inventory is included in investments and other assets within our condensed consolidated balance sheets.

As a result of our acquisition of Reata in September 2023 we recorded a fair value step-up adjustment related to the acquired inventory of SKYCLARYS of approximately $1.3 billion. This fair value step-up adjustment is being amortized to cost of sales as the inventory is sold or research and development expense as the inventory is used for clinical purposes within our condensed consolidated statements of income. We expect this amount to be fully amortized by the end of 2028. For the three months ended March 31, 2026 and 2025, amortization from the fair value step-up adjustment was approximately $107.6 million and $51.4 million, respectively. For the three months ended March 31, 2026, amortization from the fair value step-up adjustment includes approximately $56.8 million of inventory used for clinical purposes, which is reflected in research and development expense within our condensed consolidated statements of income. For additional information on our acquisition of Reata, please read Note 2, Acquisitions, to our consolidated financial statements included in our 2025 Form 10-K.

Collaborative and Other Relationships - Eisai (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Amounts receivable	$ 7.0	$ 90.2
Amounts payable	$ 85.4	$ 95.5
Alzheimer's collaboration revenue
Collaborative arrangements and non-collaborative arrangement transactions
Share of non-controling interest recognized	50.00%

Share-based Payments

3 Months Ended

Mar. 31, 2026

Share-Based Payment Arrangement [Abstract]

Share-based Payments


Note 15:			Share-Based Payments

Share-based Compensation Expense

The following table summarizes share-based compensation expense included in our condensed consolidated statements of income:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Research and development	$	34.8	$	32.1
Selling, general and administrative	53.0		52.0

Subtotal	87.8		84.1
Capitalized share-based compensation costs	(3.8)		(3.1)
Share-based compensation expense included in total cost and expense	84.0		81.0
Income tax effect	(16.5)		(16.0)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	67.5	$	65.0

The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Time-vested restricted stock units	$	70.9	$	66.1


Performance stock units settled in stock	13.3		13.6
Employee stock purchase plan	3.6		3.5

Stock options	—		0.9



Subtotal	87.8		84.1
Capitalized share-based compensation costs	(3.8)		(3.1)
Share-based compensation expense included in total cost and expense	$	84.0	$	81.0

Fair Value Measurements - Summary of Assets and Liabilities Recorded at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Assets:
Cash equivalents	$ 2,512.8	$ 2,233.2
Marketable debt securities:	1,365.6	1,239.1
Marketable equity securities	137.4	118.1
Other current assets:
Derivative contracts	19.1	10.0
Other non-current assets:
Convertible notes	35.0	35.0
Plan assets for deferred compensation	56.8	52.2
Derivative contracts	5.9	0.4
Total	4,132.6	3,688.0
Other current liabilities:
Derivative contracts	38.2	56.7
Other non-current liabilities:
Derivative contracts		2.2
Contingent consideration obligations	266.9	246.4
Total	305.1	305.3
City Therapeutics, Inc. \| Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Other non-current liabilities:
Collaborative arrangements, investments in counterparty	30.0
Neela Therapeutics, Inc \| Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Other non-current liabilities:
Collaborative arrangements, investments in counterparty	5.0
Corporate debt securities
Assets:
Marketable debt securities:	734.2	537.6
Government securities
Assets:
Marketable debt securities:	547.3	648.8
Mortgage and other asset backed securities
Assets:
Marketable debt securities:	84.1	52.7
Quoted Prices in Active Markets (Level 1)
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	137.4	118.1
Other current assets:
Derivative contracts	0.0	0.0
Other non-current assets:
Convertible notes	0.0	0.0
Plan assets for deferred compensation	0.0	0.0
Derivative contracts	0.0	0.0
Total	137.4	118.1
Other current liabilities:
Derivative contracts	0.0	0.0
Other non-current liabilities:
Derivative contracts		0.0
Contingent consideration obligations	0.0	0.0
Total	0.0	0.0
Quoted Prices in Active Markets (Level 1) \| Corporate debt securities
Assets:
Marketable debt securities:	0.0	0.0
Quoted Prices in Active Markets (Level 1) \| Government securities
Assets:
Marketable debt securities:		0.0
Quoted Prices in Active Markets (Level 1) \| Mortgage and other asset backed securities
Assets:
Marketable debt securities:		0.0
Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	2,512.8	2,233.2
Marketable equity securities	0.0	0.0
Other current assets:
Derivative contracts	19.1	10.0
Other non-current assets:
Convertible notes	0.0	0.0
Plan assets for deferred compensation	56.8	52.2
Derivative contracts	5.9	0.4
Total	3,960.2	3,534.9
Other current liabilities:
Derivative contracts	38.2	56.7
Other non-current liabilities:
Derivative contracts		2.2
Contingent consideration obligations	0.0	0.0
Total	38.2	58.9
Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Assets:
Marketable debt securities:	734.2	537.6
Significant Other Observable Inputs (Level 2) \| Government securities
Assets:
Marketable debt securities:	547.3	648.8
Significant Other Observable Inputs (Level 2) \| Mortgage and other asset backed securities
Assets:
Marketable debt securities:	84.1	52.7
Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	0.0	0.0
Other current assets:
Derivative contracts	0.0	0.0
Other non-current assets:
Convertible notes	35.0	35.0
Plan assets for deferred compensation	0.0	0.0
Derivative contracts	0.0	0.0
Total	35.0	35.0
Other current liabilities:
Derivative contracts	0.0	0.0
Other non-current liabilities:
Derivative contracts		0.0
Contingent consideration obligations	266.9	246.4
Total	266.9	246.4
Significant Unobservable Inputs (Level 3) \| Corporate debt securities
Assets:
Marketable debt securities:	0.0	0.0
Significant Unobservable Inputs (Level 3) \| Government securities
Assets:
Marketable debt securities:	0.0	0.0
Significant Unobservable Inputs (Level 3) \| Mortgage and other asset backed securities
Assets:
Marketable debt securities:	$ 0.0	$ 0.0

Derivative Instruments (Tables)

3 Months Ended

Mar. 31, 2026

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Foreign currency forward contracts that were entered into to hedge forecasted revenue

The notional amount of foreign currency forward contracts and foreign currency options that were entered into to hedge forecasted revenue and operating expense is summarized as follows:


	Notional Amount
(In millions)	As of March 31, 2026		As of December 31, 2025
Euro	$	1,332.6	$	1,531.0
Swiss franc	165.6		—
British pound	76.2		—

Polish zloty	33.0		—
Canadian dollar	24.7		—
Total foreign currency forward contracts and options	$	1,632.1	$	1,531.0

Derivative Instruments, Unrealized Gain (Loss)

The pre-tax portion of the fair value of these foreign currency forward contracts and foreign currency options that were included in AOCI in total equity is summarized as follows:



(In millions)	As of March 31, 2026		As of December 31, 2025
Unrealized gains	$	9.7	$	—
Unrealized (losses)	(24.4)		(73.3)
Net unrealized gains (losses)	$	(14.7)	$	(73.3)

Summary of the effect of cash flow derivatives designated as hedging instruments on the condensed consolidated statements of income

The following table summarizes the effect of foreign currency forward contracts and forward currency options designated as hedging instruments in our condensed consolidated statements of income:


For the Three Months Ended March 31,
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)					Net Gains/(Losses) Excluded from Effectiveness Testing and Recognized in Operating Income (in millions)
Location	2026		2025		Location	2026		2025
Revenue	$	(24.3)	$	10.9	Revenue	$	4.1	$	0.4
Operating expense	0.7		0.7		Operating expense	(1.1)		(1.0)

Summary of fair value and presentation of derivatives


(In millions)	Balance Sheet Location	As of March 31, 2026		As of December 31, 2025
Cash Flow Hedging Instruments:
Asset derivative instruments	Other current assets	$	12.2	$	0.1
	Investments and other assets	5.9		0.4
Liability derivative instruments	Accrued expense and other	12.2		54.0
	Other long-term liabilities	—		2.2





Other Derivative Instruments:
Asset derivative instruments	Other current assets	6.9		9.9
Liability derivative instruments	Accrued expense and other	26.0		2.7

Inventory (Tables)

3 Months Ended

Mar. 31, 2026

Inventory Disclosure [Abstract]

Components of inventory

The components of inventory are summarized as follows:


(In millions)	As of March 31, 2026		As of December 31, 2025
Raw materials	$	278.7	$	293.4
Work in process	1,351.6		1,595.2
Finished goods	458.2		424.9
Total inventory	$	2,088.5	$	2,313.5

Balance Sheet Classification:
Inventory	$	1,949.0	$	2,168.1
Investments and other assets	139.5		145.4
Total inventory	$	2,088.5	$	2,313.5

Derivative Instruments - Summary of Fair Value and Presentation of Derivatives (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Other current assets \| Designated as Hedging Instrument
Derivative [Line Items]
Derivative asset, fair value	$ 12.2	$ 0.1
Other current assets \| Not designated as hedging instrument
Derivative [Line Items]
Derivative asset, fair value	6.9	9.9
Accrued expenses and other \| Designated as Hedging Instrument
Derivative [Line Items]
Derivative liability, fair value	12.2	54.0
Accrued expenses and other \| Not designated as hedging instrument
Derivative [Line Items]
Derivative liability, fair value	26.0	2.7
Other long-term liabilities \| Foreign Exchange Contract \| Cash Flow Hedging [Member]
Derivative [Line Items]
Derivative liability, fair value	0.0	2.2
Other noncurrent assets \| Designated as Hedging Instrument
Derivative [Line Items]
Derivative asset, fair value	$ 5.9	$ 0.4

Intangible Assets and Goodwill - Changes in Goodwill (Details) $ in Millions	3 Months Ended
	Mar. 31, 2026 USD ($)
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period	$ 6,491.1
Other	(2.4)
Goodwill, end of period	$ 6,488.7

- Definition

Amount, after accumulated impairment loss, of asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

+ References

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Reference 3: http://www.xbrl.org/2003/role/exampleRef
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Numerator:
Net income attributable to Biogen Inc.	$ 319.5	$ 240.5
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	147.2	146.1
Effect of dilutive securities:
Dilutive potential common shares	1.2	0.5
Shares used in calculating diluted earnings per share	148.4	146.6
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	1.0	0.5
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0.2	0.0

Restructuring

3 Months Ended

Mar. 31, 2026

Restructuring and Related Activities [Abstract]

Restructuring, Business Transformation and Other Cost Saving Initiatives


Note 4:			Restructuring

2023 Fit for Growth Restructuring Program

In 2023 we initiated cost saving measures as part of our Fit for Growth program to reduce operating costs, while improving operating efficiency and effectiveness. The Fit for Growth program generated approximately $1.0 billion in gross operating expense savings by the end of 2025, some of which has been reinvested in various initiatives. The Fit for Growth program included net headcount reductions of approximately 1,400 employees and we incurred total restructuring charges of approximately $320.0 million by the end of 2025.

For the three months ended March 31, 2025, we recorded approximately $35.3 million in restructuring charges related to severance costs from our Fit for Growth program within restructuring charges in our condensed consolidated statements of income.

Restructuring Reserve

Charges and spending related to workforce reductions are summarized as follows:


	Workforce Reductions
(In millions)	2026		2025

Restructuring reserve as of January 1	$	15.8		$	31.9
Expense	7.9			35.3
Payment	(10.3)			(25.8)
Foreign currency and other adjustments	0.2			(1.0)
Restructuring reserve as of March 31	$	13.6		$	40.4

Condensed Consolidated Statement of Equity Statement - USD ($) shares in Millions, $ in Millions	Total	Preferred stock	Common stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Retained earnings	Treasury stock
Beginning balance, shares at Dec. 31, 2024		0.0	169.5
Beginning balance at Dec. 31, 2024	$ 16,716.0	$ 0.0	$ 0.1	$ 569.4	$ (136.2)	$ 19,259.8	$ (2,977.1)
Beginning balance, treasury stock, shares at Dec. 31, 2024							(23.8)
Net income	240.5					240.5
Other comprehensive income (loss), net of tax	(38.3)				(38.3)
Issuance of common stock under stock option and stock purchase plans (in shares)			0.2
Issuance of common stock under stock option and stock purchase plans	15.2			15.2
Issuance of common stock under stock award plan (in shares)			0.6
Issuance of common stock under stock award plan	(39.5)			(39.5)
Compensation related to share-based payments	84.1			84.1
Other	(0.7)			(0.7)
Ending balance at Mar. 31, 2025	16,978.7	$ 0.0	$ 0.1	629.9	(174.5)	19,500.3	$ (2,977.1)
Ending balance, shares at Mar. 31, 2025		0.0	170.3
Ending balance, treasury stock, shares at Mar. 31, 2025							(23.8)
Beginning balance, shares at Dec. 31, 2025		0.0	170.5
Beginning balance at Dec. 31, 2025	18,256.8	$ 0.0	$ 0.1	863.1	(182.0)	20,552.7	$ (2,977.1)
Beginning balance, treasury stock, shares at Dec. 31, 2025							(23.8)
Net income	319.5					319.5
Other comprehensive income (loss), net of tax	41.8				41.8
Issuance of common stock under stock option and stock purchase plans (in shares)			0.1
Issuance of common stock under stock option and stock purchase plans	15.2			15.2
Issuance of common stock under stock award plan (in shares)			0.8
Issuance of common stock under stock award plan	(70.3)			(70.3)
Compensation related to share-based payments	88.1			88.1
Other	(0.6)			(0.6)
Ending balance at Mar. 31, 2026	$ 18,651.7	$ 0.0	$ 0.1	$ 896.7	$ (140.2)	$ 20,872.2	$ (2,977.1)
Ending balance, shares at Mar. 31, 2026		0.0	171.4
Ending balance, treasury stock, shares at Mar. 31, 2026							(23.8)

Collaborative and Other Relationships - Pretax Profit Sharing Formula (Details) $ in Millions	3 Months Ended
	Mar. 31, 2026 USD ($)
GAZYVA
Collaborative arrangements and non-collaborative arrangement transactions
After Second GAZYVA Threshold Date	35.00%
LUNSUMIO
Collaborative arrangements and non-collaborative arrangement transactions
After Second GAZYVA Threshold Date	35.00%
Threshold of Gross Sales Of To Be Achieved In Preceding Twelve Consecutive Months Under Option One	$ 500.0

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Cash flow from operating activities:
Net income	$ 319.5	$ 240.5
Adjustments to reconcile net income to net cash flow from operating activities:
Depreciation and amortization	204.4	183.2
Excess and obsolescence charges related to inventory	4.8	7.3
Amortization of acquired inventory step-up	107.6	51.4
Share-based compensation	84.0	81.0
Contingent consideration	20.5	9.6
Deferred income taxes	23.6	(27.8)
(Gain) loss on strategic investments	(19.0)	40.9
Other	28.0	6.7
Changes in operating assets and liabilities, net of effects of business acquired:
Inventory	116.2	62.0
Accrued expense and other current liabilities	(243.2)	(300.5)
Income tax assets and liabilities	18.4	69.1
Other changes in operating assets and liabilities, net	(89.0)	(53.5)
Net cash flow provided by (used in) operating activities	645.5	259.3
Cash flow from investing activities:
Purchases of property, plant and equipment	(51.2)	(37.1)
Proceeds from sales and maturities of marketable securities	625.2	0.0
Purchases of marketable securities	(748.8)	0.0
Acquired in-process research and development	(35.0)	0.0
Acquisitions of intangible assets	0.0	(10.0)
Proceeds from sales of strategic investments	0.5	0.0
Other	(0.2)	(0.2)
Net cash flow provided by (used in) investing activities	(209.5)	(47.3)
Cash flow from financing activities:
Payments related to issuance of stock for share-based compensation arrangements, net	(55.1)	(24.3)
Other	11.3	1.3
Net cash flow provided by (used in) financing activities	(43.8)	(23.0)
Net increase (decrease) in cash and cash equivalents	392.2	189.0
Effect of exchange rate changes on cash and cash equivalents	(18.0)	34.3
Cash and cash equivalents, beginning of the period	3,008.5	2,375.0
Cash and cash equivalents, end of the period	3,382.7	2,598.3
Nonrelated Party
Changes in operating assets and liabilities, net of effects of business acquired:
Accounts receivable	(33.7)	(181.0)
Related Party
Changes in operating assets and liabilities, net of effects of business acquired:
Accounts receivable	$ 103.4	$ 70.4

Derivative Instruments - Notional Amount of Foreign Currency Forward Contracts and Foreign Currency Options (Details) - Foreign Exchange Contract - Designated as Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Derivatives [Line Items]
Total foreign currency forward contracts and options	$ 1,632.1	$ 1,531.0
Euro
Derivatives [Line Items]
Total foreign currency forward contracts and options	1,332.6	1,531.0
Swiss franc
Derivatives [Line Items]
Total foreign currency forward contracts and options	165.6	0.0
British pound
Derivatives [Line Items]
Total foreign currency forward contracts and options	76.2	0.0
Polish zloty
Derivatives [Line Items]
Total foreign currency forward contracts and options	33.0	0.0
Canadian dollar
Derivatives [Line Items]
Total foreign currency forward contracts and options	$ 24.7	$ 0.0

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 319.5	$ 240.5
Other comprehensive income (loss):
Unrealized gains (losses) on securities available for sale, net of tax	(1.8)	0.0
Unrealized gains (losses) on cash flow hedges, net of tax	52.6	(57.8)
Unrealized gains (losses) on pension benefit obligation, net of tax	(0.5)	0.4
Currency translation adjustments, net of tax	(8.5)	19.1
Total other comprehensive income (loss), net of tax	41.8	(38.3)
Comprehensive income (loss) attributable to Biogen Inc.	$ 361.3	$ 202.2

- Definition

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

+ References + Details

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

+ Details

Segment Information

3 Months Ended

Mar. 31, 2026

Segment Reporting [Abstract]

Segment Information


Note 21:			Segment Information

We operate and are managed as one operating segment, and derive revenue from activities related to the discovery, development and delivery of innovative therapies for people living with serious and complex diseases.

Our research and development organization is responsible for the research and discovery of new product candidates and supports development and registration efforts for potential future products. Our pharmaceutical, operations and technology organization manages the development of the manufacturing processes, clinical trial supply, commercial product supply, distribution, buildings and facilities. Our commercial organization is responsible for U.S. and international development of our commercial products. We are also supported by corporate staff functions.

Our CEO, as the CODM, manages and allocates resources to the operations of our company on a total company basis by assessing the overall level of resources available and deciding how to best deploy these resources across functions, therapeutic areas and research and development projects that are in line with our long-term company-wide strategic goals. In making these decisions, our CEO is provided with and uses consolidated financial information for purposes of evaluating performance, forecasting future period financial results, allocating resources and setting incentive targets. The CODM performs this assessment based on the segment’s net income. Through this analysis, which includes a comparison to budgeted results, the CODM assesses performance and how to allocate resources across the functions discussed above. The measure of segment assets used in determining how to manage and allocate resources is reported within our condensed consolidated balance sheets as total assets.

The table presented below, which was prepared in accordance with the accounting policies discussed in Note 1, Summary of Significant Accounting Policies, to our consolidated financial statements included in our 2025 Form 10-K, contains additional information on our segment’s revenue and profits, including significant segment expense and other segment items.

Additional Segment Information

The following table includes additional information about reported segment revenue, significant segment expense and segment measure of profitability:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Total revenue	$	2,477.8	$	2,431.0
Less cost and expense:
Cost of sales, excluding amortization and impairment of acquired intangible assets:
Product cost of sales	478.7		462.2
Royalty cost of sales	182.3		167.1
Research and development:
Research and discovery	40.5		44.3
Early stage programs	69.0		72.4
Late stage programs	102.5		49.4
Marketed products	154.2		89.5
Other research and development costs(1)	172.8		178.5
Acquired in-process research and development, upfront and milestone expense	34.0		200.7
Selling, general and administrative	607.3		572.5
Other segment expense(2)	317.0		353.9
Net Income attributable to Biogen Inc.	$	319.5	$	240.5

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

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+ Details

Investments in Variable Interest Entities

3 Months Ended

Mar. 31, 2026

Investments in Variable Interest Entities [Abstract]

Investments in Variable Interest Entities


Note 19:			Investments in Variable Interest Entities

Unconsolidated Variable Interest Entities

We have relationships with various variable interest entities that we do not consolidate as we lack the power to direct the activities that significantly impact the economic success of these entities. These relationships include investments in certain biotechnology companies and research collaboration agreements.

As of March 31, 2026 and December 31, 2025, the carrying value of our investments in certain biotechnology companies representing potential unconsolidated variable interest entities totaled $48.4 million and $49.8 million, respectively. Our maximum exposure to loss related to these variable interest entities is limited to the carrying value of our investments.

We have also entered into research collaboration agreements with certain variable interest entities where we are required to fund certain development activities. These development activities are included in research and development expense in our condensed consolidated statements of income as they are incurred. We have provided no financing to these variable interest entities other than previous contractually required amounts.

For additional information on our investments in variable interest entities, please read Note 20, Investments in Variable Interest Entities, to our consolidated financial statements included in our 2025 Form 10-K.

Property, Plant and Equipment

3 Months Ended

Mar. 31, 2026

Property, Plant and Equipment [Abstract]

Property, Plant and Equipment


Note 11:			Property, Plant and Equipment

Property, plant and equipment are recorded at historical cost, net of accumulated depreciation. Accumulated depreciation on property, plant and equipment was $2,998.3 million and $2,931.2 million as of March 31, 2026 and December 31, 2025, respectively. For the three months ended March 31, 2026 and 2025, depreciation expense totaled approximately $67.9 million and $71.4 million,

Litigation

3 Months Ended

Mar. 31, 2026

Loss Contingency, Information about Litigation Matters [Abstract]

Litigation


Note 20:			Litigation

We are currently involved in various claims, investigations and legal proceedings, including the matters described below. For information as to our accounting policies relating to claims and legal proceedings, including use of estimates and contingencies, please read Note 1, Summary of Significant Accounting Policies, to our consolidated financial statements included in our 2025 Form 10-K.

With respect to some loss contingencies, an estimate of the possible loss or range of loss cannot be made until management has further information, including, for example, (i) which claims, if any, will survive dispositive motion practice; (ii) information to be obtained through discovery; (iii) information as to the parties' damages claims and supporting evidence; (iv) the parties’ legal theories; and (v) the parties' settlement positions. If an estimate of the possible loss or range of loss can be made at this time, it is included in the potential loss contingency description below.

The claims and legal proceedings in which we are involved also include challenges to the scope, validity or enforceability of the patents relating to our products, pipeline or processes and challenges to the scope, validity or

enforceability of the patents held by others. These include claims by third parties that we infringe their patents. An adverse outcome in any of these proceedings could result in one or more of the following and have a material impact on our business or consolidated results of operations and financial position: (i) loss of patent protection; (ii) inability to continue to engage in certain activities; and (iii) payment of significant damages, royalties, penalties and/or license fees to third parties.

Loss Contingencies

Securities Litigation

We and certain current and former officers are defendants in three securities actions pending in the District Court, one filed by Nadia Shash and Amjad Khan in November 2020, which relates to statements about ADUHELM, one filed by the Oklahoma Firefighters Pension and Retirement System in February 2022, which relates to statements about ADUHELM, and one filed by Thomas Allen Gray and Frances Clarity Stokes in May 2024, which relates to statements about LEQEMBI, TECFIDERA and VUMERITY. All allege violations of federal securities laws under 15 U.S.C. §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seek declarations of the actions as class actions and monetary relief. In March 2026 the District Court dismissed the action brought by Thomas Allen Gray and Frances Clarity Stokes and they have appealed. In April 2026 the parties in the action brought by the Oklahoma Firefighters Pension and Retirement System reached an agreement in principle to settle the action.

Derivative Actions

We and members of the Board of Directors are named as defendants in five derivative actions pending in the District Court, one filed by The Booth Family Trust (Booth) in February 2022, one filed by Elaine Wang (Wang) in July 2022, one filed by Jonathan Blaufarb (Blaufarb I) in July 2024, one filed by Lawrence Hollin (Hollin) in October 2024 and one filed by Jonathan Blaufarb (Blaufarb II) in October 2024. The Booth, Wang and Blaufarb II actions relate to ADUHELM and other matters, and the Blaufarb I and Hollin actions relate to statements about LEQEMBI, our compliance controls, 2023 earnings guidance and other matters. The actions allege breach of fiduciary duty, waste of corporate assets and other common law claims, and violations of the Securities Exchange Act of 1934, 15 U.S.C. §78a et seq. The actions seek declaratory and injunctive relief, monetary relief payable to Biogen, and attorneys’ fees and costs payable to the plaintiffs. All derivative actions are stayed.

IMRALDI Patent Litigation

IMRALDI is an adalimumab biosimilar manufactured by Samsung Bioepis that Biogen commercializes in Europe.

Fresenius Kabi has alleged infringement of national counterparts of its EP '3 145 488 Patent (the EP '488 Patent), including in Spain, Italy, France and Germany. In June 2022, Fresenius Kabi filed a claim for damages and injunctive relief against Biogen France SAS in the Tribunal de Grande Instance de Paris alleging infringement of the French counterpart of the EP '488 Patent by a formulation of IMRALDI no longer commercialized in France. Fresenius Kabi alleges damages of approximately €19.1 million plus interest and costs. Biogen disputes infringement and the validity of the patent. Trial is set for June 2026.

In May 2025 the Higher Regional Court of Düsseldorf, Germany held that a formulation of IMRALDI we no longer commercialize in Germany infringed the German counterpart of the EP '488 Patent, enjoined infringement and declared Fresenius Kabi's right to seek damages. Biogen has requested review of the decision by Germany's Federal Court of Justice and has challenged the validity of the patent in a separate proceeding.

Humana Patient Assistance Litigation

In February 2025 Humana filed suit against Biogen Inc., Biogen U.S. Corp. and Advanced Care Scripts, Inc. in Jefferson Circuit Court in Kentucky alleging damages related to providing MS patients with free medications and to charitable contributions to non-profit organizations that provide financial assistance to MS patients. Humana alleges breach of contract, fraud and other claims under various state laws and seeks damages, attorneys' fees and costs.

Genentech Litigation

In February 2023 Genentech Inc. filed suit in the U.S. District Court for the Northern District of California claiming that it was owed royalties on sales of TYSABRI that occurred after the expiration of a patent licensed by Genentech to Biogen. In November 2025 the court entered judgment against us for approximately $124.3 million. We appealed and the appeal is pending.

Antitrust Litigation

In October 2025 Local No. 1 Health Fund, the Mayor and City Council of Baltimore, Teamsters Local 237 Welfare Fund, Teamsters Local 237 Retirees' Benefit Fund, UFCW Local 1500 Welfare Fund, and Jacksonville Police Officers and Fire Fighters Health Insurance Trust filed an amended complaint against us in now consolidated proceedings in the U.S. District Court for the Northern District of Illinois (the Illinois federal court). The first complaint was filed in August 2024. The plaintiffs allege violations of federal antitrust laws including 15 U.S.C. §§ 1, 2 and 13(c), the Racketeer Influenced and Corrupt Organizations Act, 18 U.S.C. §1962(c) and of various state laws, based on allegations about our contracts with pharmacy benefit managers related to TECFIDERA and VUMERITY and other allegations. Plaintiffs seek declarations of the actions as class actions, monetary, declaratory and equitable relief, and attorneys' fees and costs.

In addition, in September 2025 Walgreen Co. and The Kroger Co. sued us in the Illinois federal court, alleging violations of 15 U.S.C. §§ 1 and 2 based on allegations about our contracts with pharmacy benefit managers related to TECFIDERA and VUMERITY and other allegations. They seek monetary, declaratory and equitable relief, and attorneys' fees and costs.

Neurimmune Litigation

In May 2025 we sued Neurimmune Holding AG and Neurimmune Subone AG (collectively, "Neurimmune") in the District Court seeking declaratory judgment and permanent injunctive relief regarding our rights under a terminated collaboration agreement related to aducanumab. In September 2025 Neurimmune counterclaimed for declaratory judgment, breach of contract and unfair competition under Massachusetts G.L. 93A and seeking monetary, declaratory and equitable relief and attorneys' fees and costs.

TECFIDERA E.U. Litigation

We have sued certain generic companies for damages for sales of generic versions of TECFIDERA in violation of our regulatory market protection, including Neuraxpharm Pharmaceuticals S.L., Neuraxpharm Netherlands B.V., Zaklady Farmaceutyczne Polpharma S.A., Sandoz B.V., Mylan Ireland Ltd. and Mylan B.V. in the District Court of Amsterdam Netherlands in November 2023; Sandoz A/S in the Danish Maritime and Commercial High Court in June 2024; and STADA Arzneimittel AG, STADA Nordic ApS, Sandoz A/S, Sandoz (Denmark) and Sandoz A/S (Finland), Glenmark Arzneimittel AG and Glenmark Pharmaceuticals Nordic AB in the Finnish Market Court in July 2024. All of these parties have counterclaimed for damages based on our actions to enforce TECFIDERA's regulatory marketing protection.

In September 2025 the European General Court annulled the May 2023 European Commission decision granting TECFIDERA an additional year of regulatory marketing protection extending until February 2025. We and the European Commission appealed and the appeal is pending.

In November 2025 the Technical Boards of Appeal of the European Patent Office revoked our EP 2 653 873 patent related to TECFIDERA, after which we stopped enforcing this patent and its national counterparts. Certain generic companies have filed claims in the Danish Maritime and Commercial High Court for alleged damages due to injunctions we obtained prior to patent revocation, including Sandoz A/S in October 2025, Viatris ApS in January 2024, Glenmark Pharmaceuticals Nordic AB in July 2025 and Neuraxpharm Sweden AB and Neuraxpharm Pharmaceuticals S.L. in February 2026.

Germany Tax Matter

In December 2025 and January 2026 a German tax authority issued assessments against us of approximately €209.8 million including interest, which continues to accrue. We are challenging the assessments.

Other Matters

Government Investigations

In May 2024 the Italian Competition Authority informed us that it is investigating Biogen and other companies in relation to our biosimilar product BYOOVIZ.

In September 2025 we received a Civil Investigative Demand from the Louisiana Department of Justice for information regarding our policies relating to the purchase of drugs by healthcare organizations that are covered entities under Section 340B of the Public Health Service Act.

In January 2026 we received a request for information regarding TECFIDERA from the European Commission Directorate-General for Competition.

TYSABRI Biosimilar Patent Matter

In September 2022 we filed an action in the U.S. District Court for the District of Delaware against Sandoz Inc., other Sandoz entities and Polpharma Biologics S.A. under the Biologics Price Competition and Innovation Act, 42 U.S.C. §262, seeking a declaratory judgment of patent infringement. Trial against Sandoz Inc. is scheduled for April 2027.

Eisai Matter

In June 2025 we filed a request for arbitration in the International Court of Arbitration of the International Chamber of Commerce seeking adoption of a budget and commercialization plan for the European Territory that allocates commercialization activities to Biogen and Eisai in an equitable fashion taking into account our respective capabilities and provides a meaningful role for each party.

Product Liability and Other Legal Proceedings

We are also involved in product liability claims and other legal proceedings incidental to our normal business activities. While the outcome of any of these proceedings cannot be accurately predicted, we do not believe the ultimate resolution of any of these existing matters would have a material adverse effect on our business or financial condition.

Collaborative and Other Relationships - Summary of Activity Related to the UCB Collaboration (Details) - Research and Development Expense [Member] - UCB Pharma S.A. - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration	$ 29.4	$ 21.8
Expense reflected within statements of income	$ 14.7	$ 10.9

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Statement of Financial Position [Abstract]
Accounts Receivable, Allowance for Credit Loss, Current	$ 3.0	$ 3.0
Preferred stock, par value (in dollars per share)	$ 0.001
Common stock, par value (in dollars per share)	$ 0.0005

Indebtedness - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Indebtedness - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2026	May 12, 2025	Sep. 15, 2015
5.050% Senior Notes due January 15, 2031
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	5.05%
5.750% Senior Notes due May 15, 2035
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	5.75%
6.450% Senior Notes due May 15, 2055
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	6.45%
Senior Notes
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 1,750.0
Senior Notes \| 5.050% Senior Notes due January 15, 2031
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 400.0
Debt instrument, interest rate, stated percentage		5.05%
Percentage par value of senior notes		99.981%
Senior Notes \| 5.750% Senior Notes due May 15, 2035
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 650.0
Debt instrument, interest rate, stated percentage		5.75%
Percentage par value of senior notes		99.924%
Senior Notes \| 6.450% Senior Notes due May 15, 2055
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 700.0
Debt instrument, interest rate, stated percentage		6.45%
Percentage par value of senior notes		99.657%
Senior Notes \| 2025 Senior Notes
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage	100.00%
Debt instrument, make whole provision redemption price, percentage	101.00%
Payments of financing costs	$ 13.9
Senior Notes \| 4.050% Senior Notes due 2025
Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 1,750.0
Debt instrument, interest rate, stated percentage	4.05%
Debt instrument, redemption price, percentage	99.764%

Other Consolidated Financial Statement Detail - Other (Income) Expense, Net (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Interest income	$ (37.9)	$ (23.9)
Interest expense	67.6	60.0
(Gains) losses on investments, net	(22.3)	35.6
Litigation related expense	4.7	3.0
Foreign exchange (gains) losses, net	6.5	(3.8)
Other, net	1.1	(2.5)
Other (income) expense, net	$ 19.7	$ 68.4

- Definition

Amount of interest expense classified as operating and nonoperating. Includes, but is not limited to, cost of borrowing accounted for as interest expense.

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+ Details

Revenues (Tables)

3 Months Ended

Mar. 31, 2026

Revenue from Contract with Customer [Abstract]

Revenues by product

Revenue by product is summarized as follows:


	For the Three Months Ended March 31,
	2026						2025
(In millions)	United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis:
TECFIDERA	$	31.4	$	78.1	$	109.5	$	39.8	$	166.3	$	206.1
VUMERITY	153.4		25.6		179.0		117.1		21.7		138.8
Total Fumarate	184.8		103.7		288.5		156.9		188.0		344.9
AVONEX	108.5		54.7		163.2		108.6		58.2		166.8
PLEGRIDY	24.3		40.0		64.3		24.1		35.4		59.5
Total Interferon	132.8		94.7		227.5		132.7		93.6		226.3
TYSABRI	241.8		199.7		441.5		200.8		180.7		381.5
FAMPYRA(1)	—		—		—		—		0.3		0.3
Subtotal: Multiple Sclerosis	559.4		398.1		957.5		490.4		462.6		953.0

Rare Disease:
SPINRAZA	142.2		231.8		374.0		154.4		269.5		423.9
SKYCLARYS	71.8		78.9		150.7		69.1		54.8		123.9
QALSODY	10.5		22.0		32.5		7.5		8.0		15.5
Subtotal: Rare Disease	224.5		332.7		557.2		231.0		332.3		563.3

Biosimilars:
BENEPALI	—		122.1		122.1		—		111.3		111.3
IMRALDI	—		49.6		49.6		—		47.4		47.4
FLIXABI	—		10.5		10.5		—		13.1		13.1
BYOOVIZ(2)	—		—		—		4.2		4.7		8.9
TOFIDENCE(2)	—		—		—		0.1		—		0.1
Subtotal: Biosimilars	—		182.2		182.2		4.3		176.5		180.8

Other:
ZURZUVAE	55.3		0.1		55.4		27.7		—		27.7


Other(3)	—		—		—		0.4		1.3		1.7
Subtotal: Other	55.3		0.1		55.4		28.1		1.3		29.4
Total product revenue, net	$	839.2	$	913.1	$	1,752.3	$	753.8	$	972.7	$	1,726.5

(1) Effective January 1, 2025, our collaboration and license agreement for FAMPYRA global commercialization rights was terminated.

(2) In 2025 we completed the sale of our rights to TOFIDENCE and BYOOVIZ.

(3) Other includes FUMADERM and ADUHELM.

Analysis of change In reserves

An analysis of the change in reserves for discounts and allowances is summarized as follows:


(In millions)	Discounts		Contractual Adjustments		Returns		Total
Balance, December 31, 2025	$	115.7	$	1,046.1	$	49.0	$	1,210.8
Current provisions relating to sales in current year	214.0		683.7		7.3		905.0
Adjustments relating to prior years	0.8		(28.4)		10.4		(17.2)
Payments/credits relating to sales in current year	(133.4)		(255.3)		(0.1)		(388.8)
Payments/credits relating to sales in prior years	(92.3)		(426.0)		(14.9)		(533.2)
Balance, March 31, 2026	$	104.8	$	1,020.1	$	51.7	$	1,176.6

Total reserves included in consolidated balance sheets

The total reserves above, which are included in our condensed consolidated balance sheets, are summarized as follows:


(In millions)	As of March 31, 2026		As of December 31, 2025
Component of accrued expense and other	$	961.1	$	1,000.4
Reduction of accounts receivable	215.5		210.4
Total revenue-related reserves	$	1,176.6	$	1,210.8

Revenues from anti-CD20 therapeutic programs

Revenue from anti-CD20 therapeutic programs is summarized in the table below. For the purposes of this footnote, we refer to RITUXAN and RITUXAN HYCELA collectively as RITUXAN.


	For the Three Months Ended March 31,
(In millions)	2026		2025
Royalty revenue on sales of OCREVUS	$	317.2	$	288.8
Biogen’s share of pre-tax profits in the U.S. for RITUXAN, GAZYVA and LUNSUMIO	94.7		83.7
Other revenue from anti-CD20 therapeutic programs	7.2		5.7
Total revenue from anti-CD20 therapeutic programs	$	419.1	$	378.2

Other revenues

Contract manufacturing, royalty and other revenue is summarized as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Contract manufacturing revenue	$	237.2	$	282.3
Royalty and other revenue	9.7		11.0
Total contract manufacturing, royalty and other revenue	$	246.9	$	293.3

Derivative Instruments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Derivatives [Line Items]
Foreign currency cash flow hedge gain (loss) to be reclassified during next 15 months	$ 14.7
Foreign currency cash flow hedge gain (loss) to be reclassified during next 12 months	16.4
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments	18.8	$ (7.7)
Not designated as hedging instrument
Derivatives [Line Items]
Derivative, notional amount	$ 1,241.1		$ 1,193.7
Minimum
Derivatives [Line Items]
Derivative, remaining maturity	1 month		1 month
Maximum
Derivatives [Line Items]
Derivative, remaining maturity	18 months		21 months

Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Inventory - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2026	Mar. 31, 2025	Sep. 26, 2023
Inventory [Line Items]
Net income	$ 319.5	$ 240.5
Cost of sales, excluding amortization and impairment of acquired intangible assets	661.0	629.3
Reata Pharmaceuticals, Inc
Inventory [Line Items]
Inventory			$ 1,300.0
Reata Pharmaceuticals, Inc \| Business Combination, Pro Forma Information, Nonrecurring Adjustment, Inventory, Fair Value Adjustment
Inventory [Line Items]
Net income	107.6	$ 51.4
Reata Pharmaceuticals, Inc \| Business Combination, Pro Forma Information, Nonrecurring Adjustment, Inventory, Fair Value Adjustment \| Clinical Inventory
Inventory [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets	$ 56.8

- Definition

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

+ References + Details

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

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+ Details

Segment Information - Additional Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Segment Reporting, Asset Reconciling Item [Line Items]
Total revenue	$ 2,477.8	$ 2,431.0
Cost of sales, excluding amortization and impairment of acquired intangible assets	661.0	629.3
Research and development:	539.0	434.1
Acquired in-process research and development, upfront and milestone expense	34.0	200.7
Selling, general and administrative	607.3	572.5
Net income attributable to Biogen Inc.	319.5	240.5
Product revenue, net
Segment Reporting, Asset Reconciling Item [Line Items]
Total revenue	1,752.3	1,726.5
Royalty and other revenue
Segment Reporting, Asset Reconciling Item [Line Items]
Total revenue	9.7	11.0
Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total revenue	2,477.8	2,431.0
Other segment expense	317.0	353.9
Net income attributable to Biogen Inc.	319.5	240.5
Reportable Segment \| Research and discovery
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	40.5	44.3
Reportable Segment \| Early stage programs
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	69.0	72.4
Reportable Segment \| Late stage programs
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	102.5	49.4
Reportable Segment \| Marketed products
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	154.2	89.5
Reportable Segment \| Other research and development costs
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	172.8	178.5
Reportable Segment \| Product revenue, net
Segment Reporting, Asset Reconciling Item [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets	478.7	462.2
Reportable Segment \| Royalty and other revenue
Segment Reporting, Asset Reconciling Item [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets	$ 182.3	$ 167.1

Financial Instruments - Summary of Contractual Maturities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Estimated Fair Value
Due in one year or less	$ 900.0	$ 807.2
Due after one year through five years	454.7	419.5
Due after five years	10.9	12.4
Total marketable debt securities	1,365.6	1,239.1
Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Due in one year or less	900.8	807.2
Due after one year through five years	455.9	419.3
Due after five years	10.9	12.4
Total marketable debt securities	$ 1,367.6	$ 1,238.9

Other Consolidated Financial Statement - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Collaborative arrangements and non-collaborative arrangement transactions
Net unrealized (gains) losses recognized on equity securities	$ (19.1)	$ 41.0
Prepaid Taxes	661.6		$ 693.6
Other liabilities, noncurrent	787.1		748.5
Accrued income taxes	174.2		$ 166.4
Sage and Denali
Collaborative arrangements and non-collaborative arrangement transactions
Net unrealized (gains) losses recognized on equity securities	$ 19.2
Sage and Sangamo
Collaborative arrangements and non-collaborative arrangement transactions
Net unrealized (gains) losses recognized on equity securities		48.5
Denali Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Net unrealized (gains) losses recognized on equity securities		$ 15.7

Collaborative and Other Relationships - Stoke Therapeutics, Inc. (Details) - USD ($) $ in Millions	1 Months Ended
	Feb. 28, 2025	Mar. 31, 2026
Zorevunersen
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of future development costs	30.00%
Stoke Therapeutics Inc
Collaborative arrangements and non-collaborative arrangement transactions
Upfront and milestone paymentsmade to collaborative partner	$ 165.0
Stoke Therapeutics Inc \| Zorevunersen
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of future development costs	70.00%
Stoke Therapeutics Inc \| Milestone Payment, Developmental
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones		$ 50.0
Stoke Therapeutics Inc \| Milestone Payment, Commercial
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones		$ 335.0

Collaborative and Other Relationships - Summary of Activity Related to Sage Therapeutics (Details) - Sage Therapeutics - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Research and Development Expense [Member]
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration	$ 1.2	$ 2.4
Expense reflected within statements of income	0.6	1.2
Selling, General and Administrative Expenses [Member]
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration	47.5	43.5
Expense reflected within statements of income	$ 23.7	$ 21.7

Subsequent Events (Details) - Subsequent Event $ in Millions	1 Months Ended
	Apr. 29, 2026 USD ($)
TJ BIOPHARMA
Subsequent Event [Line Items]
Upfront and milestone paymentsmade to collaborative partner	$ 100.0
TJ BIOPHARMA \| Felzartamab \| Milestone Payment, Commercial
Subsequent Event [Line Items]
Estimated additional payments upon achievement of development and commercial milestones	20.0
TJ BIOPHARMA \| Felzartamab \| Milestone Payment, Sales-Based
Subsequent Event [Line Items]
Estimated additional payments upon achievement of development and commercial milestones	$ 730.0

Earnings per Share

3 Months Ended

Mar. 31, 2026

Earnings Per Share [Abstract]

Earnings per Share


Note 14:			Earnings per Share

Basic and diluted shares outstanding used in our earnings per share calculation are calculated as follows:


	For the Three Months Ended March 31,
(In millions)	2026		2025
Numerator:
Net income attributable to Biogen Inc.	$	319.5	$	240.5
Denominator:
Weighted average number of common shares outstanding	147.2		146.1
Effect of dilutive securities:

Time-vested restricted stock units	1.0		0.5

Performance stock units settled in stock	0.2		—
Dilutive potential common shares	1.2		0.5
Shares used in calculating diluted earnings per share	148.4		146.6

Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.