Investigations into whether a supplement may react poorly in combination with certain drugs, and in certain people, are not required. Should numerous reports of a bad reaction surface once a product reaches the market, the FDA will intervene. Then it is up to the FDA to show that the product is unsafe. Often some scientific evidence supports a manufacturer's claims about a given supplement. Manufacturers are required to have substantiating evidence on hand in case the FDA chooses to investigate.
Manufacturers are responsible for ensuring that their product's ingredient list is accurate.
But some supplements have been shown to contain different amounts of the active ingredient from bottle to bottle within the same brand. The FDA specifically prohibits manufacturers from claiming that dietary supplements can treat or prevent actual disease.