Supplements are extra sources of dietary components that are available in pill, powder or liquid form. They are intended to be taken by mouth and nutritionally supplement the diet. They may include vitamins, minerals, herbs or other substances (e.g., amino acids, enzymes, probiotics, hormones). Tens of thousands of products in the United States are marketed as dietary supplements. Multivitamins and minerals make up the largest group of these products. 

The use of supplements may ensure that people with inadequate nutrition receive adequate amounts of essential nutrientsessential nutrients to promote good health and performance. Supplements may be recommended for people who receive insufficient nutrients (due to a poor diet or too few foods consumed), have certain medical conditions (e.g., food intolerance), lifestyle choices (e.g., excessive drinking, smoking) or at certain stages of life (e.g., during pregnancy, menopause, older adults).

Most responsible nutrition organizations recommend that people receive their daily nutritional requirements through food instead of using supplements. Supplements can interact with medications (both prescription and over-the-counter), other supplements and certain medical conditions. Side effects may range from bothersome to deadly.

Receiving needed nutrients from food is less likely to lead to a toxic buildup of certain substances within the body. In addition, multiple varieties of nutrients may be available in a single food source, and additional health benefits can be obtained from foods (e.g., phytochemicals) that may not necessarily be provided in a supplement.

Supplements are not intended to treat diseases or medical conditions, replace prescription medication or make up for poor dietary choices.

Manufacturers or distributors of supplements are responsible for determining that their products are safe and their product claims are substantiated by adequate evidence to show they are not false or misleading. Some supplements have convincing scientific evidence supporting their claims. In most cases, however, there is a lack of independent and well-designed scientific studies on supplements.

The U.S. Food and Drug Administration (FDA) is responsible for overseeing labeling requirements for supplements. Certain information is required on supplement labels and only certain types of product claims can be made. The advertising of supplement products is regulated by the U.S. Federal Trade Commission.

There are certain things to look for when purchasing supplements. These include reading product labels carefully, using supplements appropriately and avoiding megadoses (more than 100 percent of the daily value) of any one dietary ingredient. Recommended daily levels are available for certain vitamins and minerals, although not for herbal supplements or other non-nutrient supplements.