Biogen Idec (Nasdaq: BIIB) may be in a tight spot with its multiple sclerosis drugs facing competition from a new oral drug from Novartis (NYSE: NVS), but its last competition-free quarter sure looked good. 

Sales of Avonex were up a stellar 11%. Sure, much of that came from a price increase, but the number of patients taking the drug has also increased a little, so Avonex is holding its own against the likes of Teva Pharmaceuticals' (Nasdaq: TEVA) Copaxone, Merck KGaA and Pfizer's (NYSE: PFE) Rebif, and Bayer's Betaseron.

Biogen's other multiple sclerosis drug, Tysabri, which it sells with Elan (NYSE: ELN), saw sales increase 7%. That's not bad, but it's not exactly the double-digit growth we've seen previously. Sales growth is slipping because the number of new patients starting the treatment is slowing. With an increasing number of patients getting progressive multifocal leukoencephalopathy (PML) -- a potentially deadly brain infection -- after taking the medication, who can blame them?

In order to reaccelerate that growth, Biogen and Elan need to figure out which patients get PML and then convince the patients without the risk factor that the drug's benefits outweigh the now-lower risks.

Tysabri doesn't actually cause the PML, but it tunes down the immune system, which gives the JC virus an opportunity to infect the brain. About 50% to 60% of patients don't have the JC virus in their systems, but 20 out of 20 patients that developed PML had the virus, so Biogen believes the presence of the JC virus is a risk factor for developing PML.

The company is in talks to get the new information on to Tysabri's label and also to get its JC virus test commercially available. An approval of the test in the U.S. may take some time -- management wasn't willing to give a timeframe -- but investors should hope it's sooner rather than later. Novarits is sure to make a big push with Gilenya starting in the fourth quarter.

Anand Chokkavelu has 5 stocks sitting on a market-thrashing opportunity.

Pfizer is a Motley Fool Inside Value recommendation. Elan is a Motley Fool Rule Breakers selection. Novartis is a Motley Fool Global Gains pick. The Fool owns shares of Teva Pharmaceutical. Try any of our Foolish newsletter services free for 30 days.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool has a disclosure policy.