A dietary supplement, also known as food supplement or nutritional supplement, is a preparation intended to provide nutrients, such as vitamins, minerals, fiber, fatty acids or amino acids, that are missing or are not consumed in sufficient quantity in a person's diet. Some countries define dietary supplements as foods, while in others they are defined as drugs.

Supplements containing vitamins or dietary minerals are included in the Codex Alimentarius Commission, a guidebook on food safety sponsored by the United Nations.

In the United States, a dietary supplement is defined under the Dietary Supplement Health and Education Act of 1994 (DSHEA) as a product that is intended to supplement the diet and contains any of the following dietary ingredients:

• a vitamin
• a mineral
• an herb or other botanical (excluding tobacco)
• an amino acid
• a concentrate, metabolite, constituent, extract, or combination of any of the above

Furthermore, it must also conform to the following criteria:

• intended for ingestion in pill, capsule, tablet, powder or liquid form
• not represented for use as a conventional food or as the sole item of a meal or diet
• labeled as a "dietary supplement"

The hormones DHEA (a steroid), pregnenolone (also a steroid) and the pineal hormone melatonin are marketed as dietary supplements in the US.

Dietary supplements are permitted to make structure/function claims. These are broad claims that the product can support the structure or function of the body (e.g., "glucosamine helps support healthy joints", "the hormone melatonin helps establish normal sleep patterns"). The FDA must be notified of these claims within 30 days of their first use, and there is a requirement that these claims be substantiated. In reality, misleading claims about supplements are common, particularly on poorly-regulated commercial websites. For example, the compound hydrazine sulfate is sold as a dietary supplement in the USA and promoted as a treatment for cancer, despite little evidence that it is either safe or effective.

Other claims that required approval from FDA include health claims and qualified health claims. Health claims are permitted to be made if they meet the requirements for the claims found in the applicable regulations. Qualified health claims can be made through a petition process, including scientific information, if FDA has not approved a prior petition.

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