A prescribed dietary supplement is intended to supply nutrients (vitamins, minerals, fatty acids or amino acids) that are missing or not consumed in sufficient quantity in a person's diet. This may include herbal supplements which have a history of claims that they cure or prevent certain diseases. The medical utility and regulatory status of dietary supplements is controversial.

The FDA regulates dietary supplements as foods, and not as drugs. The FDA does not pre-approve dietary supplements on their safety and efficacy, unlike drugs.

If a dietary supplement claims in any way to cure, mitigate, or treat a disease, it would be considered to be an unauthorized new drug and in violation of the applicable regulations and statutes. As the FDA states it in a response to this question in a FAQ:

Is it legal to market a dietary supplement product as a treatment or cure for a specific disease or condition?
No, a product sold as a dietary supplement and promoted on its label or in labeling* as a treatment, prevention or cure for a specific disease or condition would be considered an unapproved--and thus illegal--drug. To maintain the product's status as a dietary supplement, the label and labeling must be consistent with the provisions in the Dietary Supplement Health and Education Act (DSHEA) of 1994.
*Labeling refers to the label as well as accompanying material that is used by a manufacturer to promote and market a specific product.