There's nothing like the disclosure of a side effect to ruin a perfectly good earnings report. Despite meeting analyst expectations on revenue, and beating on the earnings-per-share side, Biogen Idec (BIIB 0.51%) traded down more than 10% at one point today after announcing that a patient taking its oral multiple sclerosis drug Tecfidera contracted progressive multifocal leukoencephalopathy, or PML.

Source: Biogen Idec.

PML is a potentially lethal brain infection caused by a rather benign virus that can become active in patients with weakened immune systems. The patient actually died from complications from pneumonia; but that doesn't change the fact that the patient had PML.

The patient was treated with Tecfidera for 4.5 years as part of one of the clinical trials, and came down with severe lymphopenia, or low lymphocytes, the white blood cells responsible for fighting infections.This isn't particularly surprising; in the clinical trials, Tecfidera lowered lymphocytes by approximately 30% during the first year of treatment. There's a warning on its label about lymphopenia, which is known to be a risk factor for developing PML and other infections.

PML was responsible for another one of Biogen Idec's multiple sclerosis drugs, Tysabri, being pulled off the market. The FDA eventually decided the benefits of the drug outweighed its risk, and let Tysabri back on the market.

There's now a test for the virus that causes PML, which can make doctors and patients feel comfortable using the product if the test is negative. Some patients accept the risk even if they test positive, because Tysabri works so well at controlling multiple sclerosis symptoms. The risk, however, has certainly muted sales of Tysabri.

If patients started making the same kinds of choices with Tecfidera, it would be disastrous for Biogen Idec. The new medication has not only become the top-selling oral multiple sclerosis drug, but sales have surpassed Tysabri and Biogen Idec's bellwether, Avonex.

At this point, I doubt that would happen. It's only one case of PML, and there's a plausible explanation for why it occurred. The PML case will make doctors more conscious of testing for lymphopenia -- something they're supposed to be doing anyway -- and might cause discontinuations in patients that develop it, especially if that's what the FDA recommends. Six percent of patients taking Tecfidera had lymphocyte counts less than about half of the normal level, so discontinuations because of lymphopenia could cut into revenue, but wouldn't be disastrous.

With that negative news out of the way...
It really was a nice quarter. Revenue increased 37%, thanks to a solid quarter for its multiple sclerosis drugs. Tecfidera sales were up 12% quarter over quarter, capturing a third of new patient starts, and more than 40% of patients switched medications. Biogen Idec's new hemophilia franchise is off to a good start with $25 million in sales of Alprolix, up from $10 million last quarter, and Eloctate sales of $21 million in its first quarter on the market.

Adjusted earnings per share increased 61% year over year, although that was partially due to flat spending on research and development, which isn't a long-term positive. But the increase allowed Biogen Idec to increase non-GAAP guidance to between $13.45 and $13.55 for the year.