Homocysteine is a homologue of the amino acid cysteine. It is biosynthesized from methionine and can be recycled back into methionine or converted into cysteine by Vit-B.

A higher levels of homocysteine is a possible risk factor for coronary artery disease. Coronary artery disease occurs when an atherosclerotic plaque blocks blood flow to the coronary arteries, which supply the heart with oxygenated blood. Hyperhomocysteinemia has been correlated with the occurrence of blood clots, heart attacks, and strokes, although it is unclear whether hyperhomocysteinemia is an independent risk factor for these conditions.

Higher levels are also been associated with early-term spontaneous abortions and with neural tube defects.