Thioctic acid also known as alpha-lipoic acid is a dietary supplement, which is a common ingredient in OTC (over-the-counter) multivitamin formulas and anti-aging supplements. Thioctic acid exists in both R- and S-enantiomeric forms, however, only R-form is essential as a cofactor in biological systems (the acid is coupled via an amide linkage to a lysine of several multienzyme complexes, such as the pyruvate dehydrogenase complex, the alpha-ketoglutarate dehydrogenase complex, the glycine cleavage system and the branched-chain oxo acid dehydrogenase complex). Most commercially available thioctic acid supplements are a mixture of both R and S enantiomers or R-form alone. Several studies have shown that the acid has beneficial effect on diabetes complications, cancer, glaucome, liver disease, etc. The mechanisms of thioctic acid is related to its antioxidant properties, metal chelator properties, however, those mechanisms need futher confirmation.