Biotin, also known as vitamin H or coenzyme R, is a water-soluble B-vitamin (vitamin B7). It is composed of a ureido (tetrahydroimidizalone) ring fused with a tetrahydrothiophene ring. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring. Biotin is a coenzyme for carboxylase enzymes, involved in the synthesis of fatty acids, isoleucine, and valine, and in gluconeogenesis. The only human health condition for which there is strong evidence of biotin's potential benefit as a treatment is biotin deficiency.