Free radicals are chemical species possessing an unpaired electron that can be considered as fragments of molecules and which are very reactive. They are produced continuously in cells either as accidental by-products of metabolism or during phagocytosis. Oxidation reactions can produce free radicals, which start chain reactions that damage cells. Antioxidants terminate these chain reactions by removing free radical intermediates, and inhibit other oxidation reactions by being oxidized themselves. Antioxidants are also widely used as ingredients in dietary supplements in the hope of maintaining health and preventing diseases such as cancer and coronary heart disease. The important antioxidants are superoxide dismutase, catalase and glutathione.