Biogen's Growth Transition Still in Progress, Legacy Drag Persists

What happened

Biogen's multiple sclerosis franchise continues to erode, but the company is banking on Leqembi, Skyclarys, and Zurzuvae to stabilize revenue. While these launch products saw 67% year-over-year growth in Q3 2025, they are not yet large enough to offset the decline in MS sales, which still account for roughly 45% of product revenue. Management's 2025 guidance calls for flat to slightly positive revenue at constant currency, implying that the transition has not yet inflected to clear growth. The stock has rerated sharply from its 2025 lows to around $184, but the DeepValue report assigns a WAIT rating with a base case of $185, suggesting limited upside at current levels. Investors now need to see 2026 guidance and sustained launch momentum to confirm that Biogen can generate durable mid-single-digit revenue growth.

Implication

Long-term investors face a balanced risk-reward: if Leqembi and rare-disease drugs sustain >25% growth while MS declines moderate, the stock could re-rate toward $215. However, the risk of further EPS cuts from pipeline investments and Alzheimer's safety scrutiny remains. The DeepValue report's fair value range of $160-$215 brackets current price, with the base case at $185. With limited margin of safety, patience is warranted until clearer evidence of revenue growth emerges. Until then, the stock is fairly priced for a flat-to-slight-growth trajectory.

Thesis delta

The news reinforces the existing thesis that Biogen's launch products are growing but have not yet offset legacy headwinds; no fundamental shift in view. The rerating already prices in a successful transition, so upside requires acceleration beyond current expectations. The waiting stance remains appropriate until 2026 guidance and several more quarters of data confirm inflection.