A dietary supplement is a component found in foods (such as meats, grains, fruits, or vegetables) that can be singled out, extracted, and converted into another form, such as a pill or liquid.

Dietary supplements may be sold to the public without testing and approval from the U.S. Food and Drug Administration (FDA). Because of this, their effectiveness, purity, and quality are not known. For example, it is not required for manufacturers to detail how much of a product is the labeled supplement, such as melatonin, and how much of it is other substances or "fillers."