Biogen shares dropped over 7% in premarket trading.
The stock has soared since the FDA gave its stamp of approval for an Alzheimer's drug from Biogen earlier this month. As of Wednesday close, the stock was up over 50% this year, compared with the S&P 500's roughly 13% advance.
The drug would be the first new treatment for the disease in decades. But the FDA approval has been mired in controversy, with the agency's own statisticians saying there was insufficient evidence to support approval.