Riboflavin exists in a reduced and an oxidized form, from which two coenzymes are formed: flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). vitamin B 2 occurs free and in a protein-bound form in the diet, milk being the best source. Riboflavin-dependent enzymes (flavoproteins, flavoenzymes) catalyze hydroxylations, decarboxylations, dioxygenations, and reduction of oxygen to hydrogen peroxide. They play a major role in many aspects of electron transfer. FMN and FAD are involved in the metabolism of many substances, for example, glucose, fatty acids, amino acids, drugs, and vitamins K and D. The FAD-dependent glutathione reductase plays a major role in the endogenous antioxidant system. Riboflavin deficiency starts with nonspecific signs such as weakness, stomatitis, and itching eyes that later develop into cheilosis, seborrheic dermatitis, opacity of the cornea, and others. Deficiency may be due to insufficient intake, hormonal problems, or drugs, alcohol, or chelators reducing its bioavailability. Supplementation is regarded as safe even in high doses.