Hyperhomocysteinemia is a medical condition characterized by an abnormally high level of total homocysteine (that is, including homocystine and homocysteine-cysteine disulfide) in the blood, conventionally described as above 15 μmol/L.

[4] Hyperhomocysteinemia is a risk factor for cardiovascular disease; supplements of these vitamins may slightly reduce stroke outcome but not myocardial infarction, death from any cause or adverse events.

In proteins, homocysteine permanently degrades cysteine disulfide bridges and lysine amino acid residues,[9] affecting structure and function.

Homocysteine auto-oxidizes and reacts with reactive oxygen intermediates, damaging endothelial cells and increasing the risk of thrombus formation.

[2] Vitamin B12 acts as a cofactor for the enzyme methionine synthase (which forms part of the S-adenosylmethionine (SAM) biosynthesis and regeneration cycle).

[citation needed] A blood test can be performed to quantify total homocysteine concentration in the plasma, of which approximately 80% is generally protein-bound.

[27] Vitamins B6, B9, or B12 supplements (alone or combined) lower homocysteine level and might slightly reduce the risk of stroke but not of myocardial infarction compared to standard care or placebo in clinical trials.

[5] When combined with medicine to reduce blood pressure (antihypertensive drugs), it is not clear if treatments that lower homocysteine can help prevent a stroke in some people.