BIOGEN INC. (BIIB) stock intrinsic value fair value estimate & owner earnings analysis

Biogen is a neuroscience drug company. Its earnings come from four places: an aging multiple sclerosis franchise (TYSABRI, VUMERITY, TECFIDERA, AVONEX and PLEGRIDY, about $4.0 billion of 2025 sales and shrinking); rare-disease drugs (SPINRAZA for spinal muscular atrophy at $1.5 billion, SKYCLARYS for Friedreich's ataxia at $521 million and growing, QALSODY); a highly profitable royalty and profit share from Genentech on OCREVUS, RITUXAN, GAZYVA and LUNSUMIO ($1.86 billion in 2025, of which the OCREVUS royalty was $1.41 billion); and newer launches such as ZURZUVAE for postpartum depression ($195 million) and the LEQEMBI Alzheimer's partnership with Eisai. European biosimilars add about $0.7 billion. Total 2025 revenue was $9.9 billion. Earnings. We measure the cash the business produces after stock-based pay, after the capital spending needed to stay in business, and with year-to-year swings in receivables, inventory and tax payments removed. On that basis Biogen produced about $33 per share at the 2019 peak of TECFIDERA, then fell to about $11-12 per share in 2022-2023 when TECFIDERA generics arrived, a $900 million legal settlement was paid and the Reata acquisition costs ran through cash flow. It has since recovered to $17.72 (2024) and $19.21 (2025). Biogen's cash flow in 2025 looked weaker than this because it paid about $960 million more in income taxes than it expensed, including a one-time $234 million repatriation tax; we treat that as timing, not lost earning power. Large non-cash charges for amortising acquired drugs (about $780 million of depreciation and amortisation in 2025, mostly acquired intangibles) are added back, so the decay of those acquired products is charged instead in the falling earnings paths below. Before interest, the business generated roughly $2.8-3.1 billion a year in 2024-2025. We anchor on about $2.85 billion, the middle of the years since the TECFIDERA collapse once the 2022 settlement and 2023 deal costs are set aside; the pre-2021 years describe a different, TECFIDERA-driven company and are excluded. Owner earnings grew about 9% in 2025; every case starts at or below that pace. The dated cliff. The most important fact in the filings is the OCREVUS contract: Biogen's U.S. royalty, which steps up to 24% of sales above $900 million, is cut in half on the first FDA-approved OCREVUS biosimilar, and the 3% ex-U.S. royalty runs only 11 years from each country's launch (2017-2018). OCREVUS was approved in the U.S. in March 2017, so U.S. biosimilar entry becomes legally possible from 2029. We estimate this removes roughly $600 million a year of near-pure profit. Other dated exposures: TYSABRI already faces a biosimilar in Europe and its listed U.S. method patents run 2026-2034; SKYCLARYS U.S. regulatory exclusivity runs to 2030 (Europe 2034); SPINRAZA faces competing oral and gene therapies. Base case (about $154 per share). Business cash flow of about $2.83 billion in 2026 drifting down roughly 3% a year as multiple sclerosis erosion slightly outruns the growth of SKYCLARYS, ZURZUVAE and QALSODY; the OCREVUS royalty halves in 2030 (about -$600 million) and SKYCLARYS loses U.S. exclusivity in 2031 (about -$250 million); afterwards earnings keep fading about 3% a year. Biogen's long record of producing or acquiring a new neurology franchise each decade (AVONEX, TYSABRI, TECFIDERA, SPINRAZA, SKYCLARYS) is why we let the business continue at a slowly declining level rather than ending it. No pipeline product (felzartamab, salanersen, LEQEMBI subcutaneous dosing, litifilimab) is credited because none is yet approved and earning. The business is worth about $24.1 billion; net of debt it is worth about $22.9 billion. Bear case (about $53 per share). Erosion of 7% a year through 2029, a U.S. TYSABRI biosimilar in 2027 (-$300 million), the OCREVUS royalty cut arriving at the earliest legal date in 2029 (-$650 million including the ex-U.S. royalty expiry) and SKYCLARYS generics in 2030, with earnings declining 6% a year thereafter. The model for this is Biogen's own TECFIDERA episode, when owner earnings fell about 70% between 2019 and 2023. The bear case also charges about $3 billion for value-destroying deals, reflecting the company's record (ADUHELM write-offs, roughly $7 billion paid for Reata for a drug now selling about $0.5 billion a year). The business is worth about $12.1 billion; after $1.2 billion of debt and the $3 billion deal charge, about $7.9 billion remains. Bull case (about $256 per share). The newer launches grow faster than the legacy erosion, lifting cash flow about 2% a year to 2030; the OCREVUS royalty halves in 2031, SKYCLARYS exclusivity holds until its European dates, and the business afterwards declines only about 1% a year as the research engine keeps replacing what expires. The business is worth about $39 billion; net of debt about $37.9 billion. Debt. Biogen owes $6.3 billion of senior notes with nothing due until May 2030, and holds about $4.2 billion of cash and marketable securities. Most of the debt is long-dated at low fixed rates (2.25% to 2030, 3.15% to 2050, 3.25% to 2051), which makes it materially cheaper than its face value over its life; adjusting for that and adding about $0.25 billion of HI-Bio milestone obligations, we count net debt at about $1.2 billion. Every case covers its interest bill comfortably. Margin-of-safety price. We are reasonably confident of the current earnings level: two years running at about $19 per share on a stable product base, with a strong first quarter of 2026. We are much less sure of its durability, because a large share of profit comes from a royalty whose halving is written into the contract and from rare-disease drugs with finite exclusivity. The worst plausible outcome is a repeat of the TECFIDERA collapse combined with another expensive acquisition that fails; the balance sheet would survive that, since there are no maturities before 2030 and the company is investment grade, but shareholders would be left with a shrinking business. If we are wrong about the timing of the OCREVUS biosimilar and the staying power of SKYCLARYS and ZURZUVAE, the shares are worth about the bear figure. Biogen pays no dividend, so the return comes only through the roughly $19 per share of cash the business generates each year and how management deploys it. At $68 per share, the business would be generating close to 28% of that price in cash each year before the OCREVUS cliff, and in the four years before 2030 would produce roughly as much cash per share as the whole bear value; from that price the base case implies roughly doubling within about four years, and the bear case implies a loss of about a fifth rather than most of it. That is the highest price at which we believe a 20% annual return is likely across the plausible range of outcomes. What changed. Our previous figures were built on a different earnings basis and are not comparable. On the business itself, 2025 confirmed that the post-TECFIDERA base is holding (owner earnings up about 9%), while the filings make the OCREVUS royalty reduction and SKYCLARYS exclusivity dates explicit, which we now model as dated cliffs rather than a smooth decline.