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.

We and certain current and former officers are defendants in four securities actions pending in the District Court, one filed in November 2020 (Nadia Shash was named lead plaintiff and named in an amended complaint along with named plaintiff Amjad Khan in April 2021), 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, one filed in May 2024 (Francis Clarity Stokes was named lead plaintiff and named along with Thomas Allen Gray in an amended complaint filed in December 2024), which relates to statements about LEQEMBI, TECFIDERA and VUMERITY, and one filed in August 2023 (Ray Peleckas and the Michigan Laborers’ Pension Fund were appointed lead plaintiffs and named in an amended complaint filed in October 2023), which relates to statements made by Apellis Pharmaceuticals, Inc. about SYFOVRE. All allege violations of federal securities laws under 15 U.S.C. §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5c 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 appealed. In June 2026 Thomas Allen Gray requested voluntary dismissal of his appeal but the appeal by lead plaintiff Frances Clarity Stokes continues. 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 and the District Court preliminarily approved the settlement in June 2026 and set a hearing on final approval for September 29, 2026. In March 2025 the District Court dismissed the action brought by Michigan Laborers' Pension Fund and Ray Peleckas. They appealed and the appeal is pending.

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.

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, and has also threatened litigation in Switzerland. 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 and alleging damages of approximately €19.1 million plus interest and costs. Biogen disputes infringement and challenges the validity of the patent. Trial was held in June 2026 and a decision is pending.

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, and a hearing on validity is set for February 2027.

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 making 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.

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.

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 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 seeks monetary, declaratory and equitable relief and attorneys' fees and costs.

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

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 for damages based on injunctions we obtained prior to patent revocation. Such claims have been filed in the Danish Maritime and Commercial High Court by Viatris ApS in January 2024, Glenmark Pharmaceuticals Nordic AB in July 2025, Sandoz A/S in October 2025, Neuraxpharm Sweden AB and Neuraxpharm Pharmaceuticals S.L. in February 2026 and in the Prague Municipal Court, Czechia, by Glenmark Pharmaceuticals s.r.o., Glenmark Pharmaceuticals Distribution s.r.o. and Zakłady Farmaceutyczne Polpharma S.A. in May 2026.

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. In January 2026 we filed an amended complaint against Sandoz Inc. alleging infringement of U.S. Patent Nos. 10,233,245 and U.S. 11,292,845 and seeking damages. Trial against Sandoz Inc. is scheduled for April 2027.

In May and June 2026, Biogen filed patent infringement proceedings relating to SPINRAZA Orange-Book listed patents (U.S. Patent Nos. 9,926,559, 10,436,802 and 12,013,403) pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984 (the Hatch-Waxman Act) in the U.S. District Court for the District of Delaware against Somerset Therapeutics, LLC, Somerset Pharma, LLC, Somerset Therapeutics Private Limited f/k/a Somerset Therapeutics Limited and Odin Pharmaceuticals, LLC (collectively, Somerset) and Cipla Ltd. and Cipla USA, Inc.

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. A hearing is set for May 2027.