A sulfur-containing amino acid, which is an intermediate in the metabolism of methionine, another sulfur-containing amino acid.  Homocysteine appears to be a nerve and vessel toxin, promoting mortality and cardiovascular disease as well as strokeAlzheimer's disease, birth defects, recurrent pregnancy loss, and eye disorders.

Elevated homocysteine levels in the blood have been associated with increased risk of cardiovascular disease. Most research indicates that a blood plasma homocysteine level of less than 10 micromoles/liter is associated with a lower risk of cardiovascular disease and a reasonable treatment goal for individuals at high risk. However, it is not yet clear whether lowering homocysteine levels will reduce cardiovascular disease risk.

The metabolism of homocysteine provides an example of the interrelationships among nutrients necessary for optimum physiological function and health: the amount of homocysteine in the blood is regulated by three vitamins: vitamin B9, vitamin B12, and vitamin B6.