A dietary supplement is a manufactured product intended to supplement the diet when taken by mouth as a pill, capsule, tablet, or liquid. A supplement can provide nutrients either extracted from food sources or synthetic, individually or in combination, in order to increase the quantity of their consumption. The class of nutrient compounds includes vitamins, minerals, fiber, fatty acids and amino acids. Dietary supplements can also contain substances that have not been confirmed as being essential to life, but are marketed as having a beneficial biological effect, such as plant pigments or polyphenols. Animals can also be a source of supplement ingredients, as for example collagen from chickens or fish. These are also sold individually and in combination, and may be combined with nutrient ingredients.

The scope of their use is to deliver nutrients that humans may not consume in sufficient quantities. People following the modern way of living, supplement their diets for reasons as uncertainty about the nutrient sufficiency of their diets, a desire for an upgraded health status, or following medical consultation, and desire to treat themselves for an illness. However, in order human body functions to be in balance only a certain amount of each nutrient is needed and some substances may have adverse effects, and become harmful at high doses. The use of dietary supplements is probably fostered by their wide availability, aggressive marketing, and media reports on studies suggesting that supplements may help to prevent or treat common health problems. The use of dietary or nutritional supplements now days, is extensive and noticeably increasing, as they are available in different doses and combinations. For safeguarding consumers' health, supplements can therefore only be legally sold with an appropriate daily dose recommendation, and a warning statement not to exceed that dose. In the United States and Canada, dietary supplements are considered a subset of foods, and are regulated accordingly. The European Commission has also established harmonized rules to help insure that food supplements are safe and properly labeled and the EU's food safety authority, EFSA, is currently reviewing health claims for the various nutrients.