Even after recently buying back $1 billion worth of shares, Biogen still had $2.2 billion in cash, cash equivalents, and marketable securities at the end of the first quarter.

Of course, Biogen will be able to buy more shares today than it could yesterday, since shares are down after this morning's report of rather poor performance in the first quarter. Both revenue and adjusted earnings rose year over year, but not as much as analysts were expecting. The biggest culprit is multiple sclerosis drug Tysabri, still saddled with decelerating growth.

Biogen and marketing partner Elan  added 1,500 new Tysabri patients in the first quarter, compared to adding 2,600 patients in the fourth quarter of last year. This marks the third quarter in a row of declining patient growth.

Tysabri's label was recently changed to indicate that the longer patients remain on the drug, the greater their risk of a potentially deadly brain infection called progressive multifocal leukoencephalopathy (PML). Even though Tysabri is a more effective drug, patients are comfortable with inferior but safer options such as Teva Pharmaceutical's Copaxone, Novartis'  Extavia, and Rebif from Pfizer  and EMD Serono.

There's some indication that doctors and patients may be shrugging off the data after digesting it for a while, however. Biogen said that it saw about 190 new patients start therapy in each week of March, which would work out to about 2,500 patients per quarter.