They'll have to wait a bit longer as Biogen says U.S. officials need more time to review the drug, known as BG-12.

The U.S. Food and Drug Administration is extending its review time by three months past the original deadline date of Dec. 28, Biogen says. The company says only that the agency needs more time to review the application and the FDA is not asking for additional studies. That makes a first-quarter launch of the drug unlikely, but the extension itself doesn't necessarily mean bad news. 

So far, there are no signals that the drug won't be approved.

Shares of Biogen were down 1.42% in trading Thursday. The stock has been on a tear this year, rising 37%, which is huge for a company with a $36 billion market cap.

Just last week, Biogen reported more data showing that BG-12, a pill, appears to be effective and generally safe for patients. That news further fueled investor excitement about the drug.

"BG-12 is poised for blockbuster potential," Leerink Swann analyst Marko Kozul says in a recent note. What's more, doctors' feedback “leads us to believe the Street's apparent enthusiasm for BG-12 is well founded."

If approved, Biogen's pill would go up against Novartis' (NVS) Gilenya, the first oral medicine approved for MS. Historically, drugs for the disease were injected or infused. Gilenya was introduced to the market in late 2010. Despite Novartis' head start, analysts expect Biogen's drug will take market share from Gilenya. Biogen's drug study data has been very good.  

MS is a chronic disease of the central nervous system. It's considered an autoimmune disease, which means the body's immune system attacks healthy tissue. Usually diagnosed between the ages of 20 and 40, the disorder affects women more than men. About 400,000 Americans have MS, according to estimates from the National Multiple Sclerosis Society.

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