BIIB Leqembi: $184M Q2, Donanemab Has 55% Share

Leqembi received traditional FDA approval on July 6, 2023. That was three years and one month ago. In that time, Biogen and Eisai have built the first commercial anti-amyloid Alzheimer’s franchise from scratch: they signed up infusion centers, navigated the CMS coverage gauntlet, launched a subcutaneous initiation dose (Leqembi IQLIK, approved July 2025), and generated $184 million in Q2 2026 global in-market sales. That is 15% year-over-year growth and 9% sequential growth from Q1’s $168 million: the best quarter since launch. The ramp is real. Eisai now guides for ¥143.5 billion (roughly $905 million) in fiscal 2026 Leqembi revenue, implying the drug is on track to cross the $1B+ revenue threshold this year or next.

The problem is that Eli Lilly’s donanemab (Kisunla) is growing faster. Lilly reported $167 million in Q2 2026 Kisunla revenue, up from $124 million in Q1: a 35% sequential jump. Lilly’s Q2 earnings presentation put Kisunla at 54.9% unit market share. That means Leqembi has roughly 45% of a growing but still-modest market. Two drugs, one pie, and Lilly is taking the bigger slice. For context on how the anti-amyloid market is developing, see the GLP-1 and Alzheimer’s pipeline analysis for the broader competitive picture that includes Novo Nordisk’s semaglutide Alzheimer’s program.

Biogen closed at $207.94 with a $30.7 billion market cap. The full Q2 earnings breakdown: Biogen’s Growth Portfolio surpassed its Legacy MS franchise for the second straight quarter. Q2 2026 total revenue was $2.74 billion, up 3% year-over-year, beating the consensus estimate of $2.50 billion. The Growth Portfolio (Leqembi, Skyclarys, Zurzuvae, Spinraza, Qalsody, Syfovre, Empaveli, Vumerity) generated $1.06 billion, surpassing the Legacy MS portfolio ($767 million, down 14%) for the second straight quarter. Non-GAAP EPS was $3.60, crushing the $3.04 consensus. Biogen raised 2026 guidance to mid-single-digit revenue growth (up from a mid-single-digit decline) and non-GAAP EPS of $12 to $13.

Here is the math the bulls are relying on. Eisai guided for ¥143.5 billion (roughly $905 million) in fiscal 2026 Leqembi revenue. At a $26,500 annual list price, that implies roughly 34,000 patients globally by year-end. A real-world persistence study presented at AD/PD 2026 tracked 10,763 Leqembi patients and found 78.4% remained on treatment at 18 months and 71.7% at 20 months. Those are solid adherence numbers for an infused biologic with a five-MRI monitoring burden in year one. The registrational Clarity AD trial, which supported the July 2023 traditional approval, showed a 27% slowing of cognitive decline on CDR-SB (0.45-point difference, p<0.001) versus placebo at 18 months (NEJM, Clarity AD). That effect size is real but modest: patients still decline, just 27% slower. If Leqembi reaches $5 billion in peak sales, Biogen’s 50% share contributes $2.5 billion in annual collaboration revenue. At $10 billion, that is $5 billion. Biogen’s total revenue run rate is $10.9 billion annualized; Leqembi at the high end would be 46% of revenue.

ARIA monitoring remains the structural cap on how many patients will start treatment. Every Leqembi patient needs at least five brain MRIs in the first year to screen for amyloid-related imaging abnormalities. ARIA-E (edema) occurs in roughly 12% of patients and ARIA-H (hemorrhage) in roughly 17%, though the vast majority are asymptomatic. Primary care physicians are not going to manage a five-MRI protocol for a drug that slows decline by 27% on CDR-SB. The subQ initiation dose, Leqembi IQLIK, was approved by the FDA in July 2025 as an at-home initiation option. It helps with administration logistics (no infusion center needed for dose one), but it does not remove the MRI burden.

Donanemab compounds the problem with two structural advantages. Its limited-duration dosing gives physicians a defined treatment endpoint: patients stop after amyloid clearance, typically 12 to 18 months. Leqembi’s indefinite maintenance dosing does not offer that off-ramp. And Lilly’s commercial infrastructure for Alzheimer’s is built on top of a $23 billion quarterly revenue machine: Mounjaro, Zepbound, and Foundayo generate the cash flow to outspend Biogen on DTC advertising and neurology sales reps. Lilly’s total Q2 revenue was $22.97 billion, more than eight times Biogen’s $2.74 billion. Lilly can afford to lose money on Kisunla for five years while it builds the market. Biogen cannot.

Biogen’s balance sheet adds another layer of risk. The company exited Q2 with $1.3 billion in cash and $6.8 billion in net debt, largely from the Reata ($7.3 billion, closed September 2023) and Apellis acquisitions. The Apellis deal closed May 14, 2026, and contributed $128 million in combined Syfovre and Empaveli revenue in the partial quarter. Those are good assets: Syfovre at $162 million full-quarter sales (up 8% YoY) leads the geographic atrophy market, and Empaveli at $46 million (up 123% YoY) is growing fast in PNH. But they were bought with debt that needs servicing, and Biogen’s GAAP net income was $97.5 million in Q2, heavily compressed by amortization of the acquired intangible assets from both deals. The company is profitable on a non-GAAP basis ($536 million), but GAAP profitability matters when you are carrying $6.8 billion in net debt at a time when interest rates are not zero. A single disappointing quarter from any of the acquired assets and the debt service math gets uncomfortable.

The pipeline is the offset. Diranersen, Biogen’s tau-targeting antisense oligonucleotide, reported Phase 2 CELIA data at AAIC 2026. The tau hypothesis is the next frontier after amyloid: if diranersen slows tau pathology, Biogen has a second Alzheimer’s mechanism that does not compete with Leqembi. For a deeper look at why most Alzheimer’s drugs fail and the structural reasons behind the 99% attrition rate, see the why Alzheimer’s drug trials fail analysis. BIIB080, the antisense tau program, is earlier. Beyond Alzheimer’s, Biogen’s litifilimab (lupus, Phase 3 TOPAZ-1 and TOPAZ-2 readouts in Q4 2026) and the FSGS program address large markets. The pipeline is deeper than it was three years ago, when Biogen was essentially a one-drug (Leqembi) story with a declining MS franchise.

At $207.94 and a $30.7 billion market cap, Biogen trades at roughly 2.8 times annualized revenue and 16 times the midpoint of 2026 non-GAAP EPS guidance of $12.50. Compare to Gilead at 3.2 times revenue and 10 times forward earnings, or Bristol Myers Squibb at 2.6 times revenue and 9 times forward earnings. Biogen’s multiple is not cheap on a GAAP basis, where the trailing P/E sits north of 300 from acquisition amortization charges. But the non-GAAP multiple of 16 times is higher than both GILD and BMY despite Biogen having a smaller, less diversified revenue base. The market is paying a premium for the Alzheimer’s franchise narrative; if Leqembi growth decelerates, that premium evaporates.

Here is the verdict: Leqembi is working. $184 million in quarterly sales with 15% growth is real commercial traction. But donanemab has 55% market share, Lilly has a $23 billion quarterly revenue war chest, and the ARIA monitoring burden has not been solved. At $30.7 billion and 16 times non-GAAP earnings, Biogen is pricing in Leqembi at $5 billion to $10 billion peak sales. If donanemab continues to take share and Leqembi plateaus at $1.5 billion to $2 billion, the stock is overvalued. If the subQ launch and expanded infusion capacity push Leqembi past $3 billion with 50%+ share, the stock is fairly valued. Hold BIIB at $208. The Growth Portfolio transition is real, but the market has already paid for the Leqembi thesis. Wait for the Q3 2026 Leqembi sales print to see whether the subQ launch is accelerating the ramp or just treading water against donanemab. The Q3 report, due in late October, is the next real catalyst for the Leqembi thesis.