Shares of Biogen (NASDAQ:BIIB) hardly moved following the release of second-quarter earnings results that beat Wall Street's expectations on the top and bottom lines.  

Revenue rose 1.7% year over year to $3.7 billion, exceeding the average analyst estimate by $240 million. Adjusted earnings rose 12% to $10.26 per share, $2.25 per share higher than expected.

A doctor holding a dollar sign.

Image source: Getty Images.

Despite that outperformance, the stock was close to flat in mid-afternoon trading Wednesday, down 0.3% as of 2:15 p.m. EDT, because investors aren't expecting double-digit-percentage earnings growth to continue much longer. Sales of the company's multiple sclerosis drugs still provide 58% of its revenue, and they fell a combined 3.5% year over year to $2.1 billion.

Biogen also receives royalties from Roche (OTC:RHHB.Y) for sales of Ocrevus, a biannual injection that's putting pressure on sales of Biogen's multiple sclerosis therapies. Those royalties rose 14% to $208 million in the quarter, but they can't carry Biogen forward on their own.

Spinraza, a lifelong treatment for spinal muscular atrophy (SMA) was Biogen's lead growth driver before a once-and-done gene therapy from Novartis (NYSE:NVS) called Zolgensma became the second SMA treatment to earn approval last year. In the second quarter, sales of Spinraza rose just 1% to $495 million.

Earlier this month, Biogen submitted an application to the FDA for its experimental Alzheimer's disease treatment, aducanumab. As potentially the first disease-modifying therapy for Alzheimer's disease, sales of aducanumab could solve all of Biogen's problems, but its approval is far from guaranteed.