Biogen Inc.

Biogen Inc. is a global biopharmaceutical company that discovers, develops, and delivers therapies for people living with serious neurological, rare, and immunological diseases, generating $9,891 million in revenue in fiscal 2025. Founded in 1978 by Nobel laureates Walter Gilbert and Phillip Sharp alongside Charles Weissmann and Kenneth Murray, Biogen is one of the original biotechnology companies and remains defined by neuroscience — it brought the first disease-modifying therapies to multiple sclerosis, the first treatment for spinal muscular atrophy, and now co-commercializes one of the first disease-modifying Alzheimer's therapies.

This is a story about a pharmaceutical franchise in transition. Biogen's legacy multiple sclerosis business, which still contributes 41% of revenue, is in structural decline as generics and biosimilars erode TECFIDERA and TYSABRI. The future rests on a widening portfolio of rare disease drugs, an Alzheimer's collaboration with Eisai, a newly acquired immunology platform from Apellis Pharmaceuticals, and a pipeline of mid- to late-stage assets addressing large unmet needs. The company has spent the past three years restructuring under a CEO who arrived in late 2022 — cutting costs, pruning the pipeline, and deploying capital toward acquisitions that broaden the revenue base beyond MS.

The file turns on a single question: whether the rare disease, Alzheimer's, and immunology businesses can grow fast enough to outrun the erosion of the MS franchise before the capital structure — now carrying $8.1 billion in debt after the Apellis acquisition — becomes a source of pressure rather than a bridge to a more diversified future.