Safety or saturation? Last November, Tecfidera's safety record also took a hit when the FDA added information concerning a reported case of PML in an MS patient treated with the drug. It was noted that the patient had a very low number of lymphocytes, a type of white blood cell, that could have contributed to the development of PML. The chart displayed earlier in this article shows that the deceleration in Biogen's revenue coincides with the report. Tecfidera may have simply reached a saturation point in the U.S. market, or maybe physicians are exhibiting extra caution.