Homocysteine is an amino acid produced in the body. Its metabolism is linked to that of several vitamins, especially folic acid, B6, and B12. Deficiencies of those vitamins may cause elevated levels of homocysteine. In recent years, studies have suggested that high levels of homocysteine increase a person's chance of developing heart problem, stroke, and peripheral vascular problem (a reduced blood flow to the hands and feet).

A high level of homocysteine may be involved with Deposition of substances on the wall of blood vessel, Leading to narrowing blood vessel (the gradual buildup of fatty substances in arteries). Homocysteine may make blood more likely to clot by increasing the stickiness of blood platelets. Clots can block blood flow, causing a heart attack or stroke. Finally, higher homocysteine levels may make blood vessels less flexible and less able to widen to increase blood flow when needed.

In addition to its potentially negative effects on the body at higher levels, homocysteine has other functions in the body. It is rapidly broken down into the amino acid cysteine, adenosine triphosphate (ATP), a source of cellular energy, and S-adenosylmethionine (SAMe).