A dietary supplement is a product intended for ingestion that, among other requirements, contains a "dietary ingredient" intended to supplement the diet. The term "dietary ingredient" includes vitamins and minerals; herbs and other botanicals; amino acids; "dietary substances" that are part of the food supply, such as enzymes and live microbials (commonly referred to as "probiotics"); and concentrates, metabolites, constituents, extracts, or combinations of any dietary ingredient from the preceding categories.

Dietary supplements may be found in many forms, such as pills, tablets, capsules, gummies, softgels, liquids, and powders. They can also be in the same form as a conventional food category, such as teas or bars, but only if the product is not represented as a conventional food or as a "sole item of a meal or the diet."

To be a dietary supplement, a product must also be labeled as a dietary supplement; that is, the product label must include the term "dietary supplement" or equivalent (e.g., "iron supplement" or "herbal supplement"). DSHEA places dietary supplements in a special category under the general umbrella of "foods," unless the product meets the definition of a drug (e.g., because it is labeled to treat or mitigate a disease).

Generally, the dietary supplement category excludes articles approved as new drugs, licensed as biologics, or authorized for clinical investigation under an investigational new drug application (IND) that has gone into effect, unless the article was previously marketed as a dietary supplement or as a food.

In the case of articles authorized for clinical investigation under an IND, the exclusion from the dietary supplement definition applies only if "substantial clinical investigations" have been instituted and the existence of such investigations has been made public.