Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals and chain reactions that may damage the cells of organisms. Antioxidants such as thiols or ascorbic acid (vitamin C) may act to inhibit these reactions. To balance oxidative stress, organisms contain and produce antioxidants, such as glutathione, mycothiol or bacillithiol.

The only dietary antioxidants are vitamins A, C, and E. The term antioxidant is also used for industrial chemicals added during manufacturing to prevent oxidation in synthetic rubber, plastics, and fuels, or as preservatives in food and cosmetics.

While fruits and vegetables are rich sources of antioxidant vitamins and can be part of a healthy diet, there is no clear evidence to indicate that plant-food consumption confers health benefits specifically because of antioxidant vitamins in such foods. Dietary supplements marketed as antioxidants have not been shown to improve health or prevent disease in humans. According to some studies, supplements of beta-carotene, vitamin A, and vitamin E have no positive effect on mortality rate or cancer risk. Additionally, supplementation with selenium or vitamin E does not reduce the risk of cardiovascular disease.