Aminoguanidine was discovered more than 100 years ago. Aminoguanidine is one of many derivatives of guanidine but it has many properties in common with hydrazines and is therefore often classified as a hydrazine. Important biological effects of aminoguanidine have been discovered in recent years. The first discovered effect was diamine oxidase inhibition. This enzyme catalyzes the degradation of biologically active diamines such as histamine and putrescine. Secondly, aminoguanidine inhibits nitric oxide synthase particularly the inducible nitric oxide synthase isoform. The inhibition of highly reactive advanced glycosylation end products is another important biological effect of aminoguanidine. Glycation is the reaction between a sugar and the free amino group of proteins and it is referred to as cross-linking. The linking of glycosylation byproducts to proteins ends up with the development of large, cross-linked molecules that inhibit the ability of the cell to function normally. Advanced glycosylation end products are increased in diabetes and play an important role in the development of diabetic complications. Furthermore, aminoguanidine was shown to act as an antioxidant, preventing reactive oxygen species formation and lipid peroxidation in cells and tissues. The antiaging and the protective effects of aminoguanidine in conditions such as cardiovascular diasease, diabetes, cataract and atherosclerosis have been reported. Animal studies indicate that aminoguanidine has very low toxicity. However, some studies indicate its hepatotoxicity.