Tysabri isn't Biogen Idec's (NASDAQ:BIIB) biggest seller, but it was the focus of much of the company's conference call today -- and rightfully so. The multiple sclerosis drug is growing at a 21% year-over-year clip compared with Avonex, its top revenue-generator, which increased just 1% year over year.

Biogen and marketing partner Elan (NYSE:ELN) added nearly 3,000 new Tysabri patients during the quarter, but the rate of new additions per week slowed compared with the second quarter, which Biogen blamed on vacations in Europe. I guess that's better than blaming it on the price of tea in China.

Increasing the number of patients on the drug may only get harder, though. On the conference call, management said that Elan and Biogen now believe that the risk of developing progressive multifocal leukoencephalopathy (PML), a potentially deadly brain infection, increases with the number of infusions a patient receives. Biogen is in discussions with the Food and Drug Administration to update the label to reflect this new belief.

The change could result in patients taking breaks from Tysabri, perhaps going onto other medications and then returning later. Just realize that drug holidays for patients result in revenue holidays for Biogen and Elan, so they're definitely not a reason to celebrate with friends and families.

That's not a foregone conclusion, however. Keep in mind that Tysabri reduces flareups better than other multiple sclerosis drugs such as Teva Pharmaceutical's (NASDAQ:TEVA) Copaxone, Novartis' (NYSE:NVS) Extavia, and Rebif from Pfizer (NYSE:PFE) and EMD Serono. It's possible that patients will take their chances with the rare (but potentially lethal) brain infection in exchange for a better prognosis.

I continue to think that Biogen and Elan need to focus on that "potentially" part of the dangerous equation. Increased sales will come if the companies can lower the likelihood of infection by identifying patients who are more likely to contract PML. They're also working on developing a treatment for PML, but data from the trial isn't expected until the end of next year.

If you're an investor, stay tuned, because this story isn't over yet.

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Fool contributor Dr. Brian Orelli doesn't own shares of any company mentioned in this article. Pfizer is an Inside Value recommendation. Novartis is a Global Gains pick. The Fool has a disclosure policy.