Ephedra, a plant product promoted for weight loss, was taken off the market by the FDA a few years ago after a study showed many adverse effects related to its use. Similarly, an amino acid, L-tryptophan, was removed after its use led to permanent disability and even death. It turned out that the amino acid itself was safe, but a contaminant introduced in tiny amounts by a new method of manufacture was intensely toxic. That's a risk with any product, I suppose, but more likely when there's no requirement for testing.