Regeneron (NASDAQ:REGN) is today's big loser in the biotech space because of a new set of FDA concerns over its PCSK9 drug. The FDA is now concerned that this class of drugs, aimed at lowering "bad" cholesterol in the bloodstream, could have a number of neurocognitive adverse effects in patients. Amgen (NASDAQ:AMGN) has a drug in this class, as well, that is considered a critical product for the company; it also saw shares fall today.

In this video from Friday's Market Checkup, Motley Fool health-care analyst David Williamson discusses this potentially multi-billion dollar blockbuster drug class, and how likely these FDA concerns are to derail these drugs.