[T]he main law on dietary supplements gives the F.D.A. jurisdiction only after the products go on the market. Rather than reviewing the supplements and approving them for sale, as the agency does with drugs, the F.D.A. is limited to spot-checking manufacturers and distributors, and testing products already on store shelves. Even the F.D.A. acknowledges there may be hundreds of other drug-contaminated weight-loss supplements for sale that the agency does not have the resources to identify.
But even when the agency identifies contaminated products, it does not have the ability to remove the pills from stores, because it is initially up to companies to issue a recall. Eventually, though, if contaminated products stay on the market, the F.D.A. can seek injunctions, seize products or file criminal charges.