An antioxidant is a molecule that inhibits the oxidation of other molecules. Oxidation is a chemical reaction that can produce free radicals, leading to chain reactions that may damage cells. Antioxidants such as thiols or ascorbic acid (vitamin C) terminate these chain reactions.

To balance the oxidative state, plants and animals maintain complex systems of overlapping antioxidants, such as glutathione and enzymes (e.g., catalase and superoxide dismutase) produced internally or vitamin C, vitamin A and vitamin E obtained by ingestion.

Diets containing foods high in antioxidants have been shown to improve health. However, in supplement form, the prevention of diseases such as cancer or coronary heart disease and the general promotion of health has not been confirmed experimentally. Trials including supplements of beta-carotene, vitamin A, and vitamin E singly or in different combinations found no effect on mortality or might increase it.

Randomized clinical trials of taking antioxidants including beta-carotene, vitamin E, vitamin C and selenium have shown no effect on cancer risk or have increased cancer risk.

Supplementation with selenium or vitamin E does not reduce the risk of cardiovascular disease.

Oxidative stress can be considered as either a cause or consequence of some diseases, stimulating drug development for potential antioxidant compounds for use as treatments.