If you've got an "M" and a "D" at the end of your name, Biogen Idec (NASDAQ:BIIB) thinks you're special. The company is willing to hook medical doctors up with an automatic monthly update of the number of progressive multifocal leukoencephalopathy (PML) cases for people taking it and Elan's (NYSE:ELN) multiple sclerosis drug, Tysabri.

The company was posting an update on the PML cases every week, but stopped in July when we hit the third anniversary of Tyasbri's relaunch. Biogen will give medical doctors access to monthly updates through a password-protected website according to The Wall Street Journal, but the rest of us will have to ask for it each and every month, from Biogen's investor relations department. Still, I guess it's better than not having access at all.

Doctors and investors are interested in the rate of occurrence for PML, a kind of brain infection, in Tysabri patients because the side effect is potentially deadly. The rate of PML has been on the rise -- probably because the likelihood of developing PML increases with the amount of time patients are on Tysabri. Biogen plans to break out the cases into categories based on the amount of time the patients have been on Tysabri.

While Tysabri hit blockbuster status last year, it still has a lot of competition from drugs like Biogen's Avonex, Teva Pharmaceutical's (NASDAQ:TEVA) Copaxone, Novartis' (NYSE:NVS) Extavia, and Rebif from Pfizer (NYSE:PFE) and EMD Serono. They may not work as well as Tysabri, but they don't come with Tysabri's PML risk either.

Whether Biogen and Elan can reaccelerate sales growth depends a lot on how many new cases of PML occur. At least investors should be able to track how well that's going.

Jeff Fisher runs down four reasons to sell a stock; depending on how things progress, PML and Biogen might fit into number two.

Pfizer is a Motley Fool Inside Value recommendation. Elan is a Rule Breakers selection. Novartis is a Global Gains pick. Try any of our Foolish newsletters today, free for 30 days.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The side effect of the Fool's disclosure policy is your peace of mind.