Any drugs which are intended to cure, treat, or prevent disease for purchase in the United States require FDA approval. However, nutritional supplements require only the manufacturer to be responsible for the labeling and safety of the product. Because companies do not have to prove their products safety, efficacy, or accuracy of ingredients before hitting the market, the product could contain any or none of the ingredients listed.
An organization called the United States Pharmacopeia (USP) is a not for profit dedicated to setting public standards for quality of health care products including supplements. The USP-verified mark (see picture) is used only on supplements that have been through the extensive verification process required by the USP.
Mke sure to look for the USP logo on any nutritional supplement you may be considering purchasing!!