Homocysteine is an amino acid derived from methionine (an essential amino acid). Amino acids are the building blocks of proteins. Elevated homocysteine levels (hyperhomocysteinemia) have been found to increase the risk of venous thrombosis up to fourfold. According to one multicenter study, high blood levels of homocysteine are associated with early onset of vascular disease and venous thrombosis (10–25%) and with recurrence (19%) of deep venous thrombosis after 2 years.

High homocysteine levels in people who have no symptoms can be normalized with supplements of folic acid, vitamin B₆, and vitamin B₁₂ (although the benefit is not proved).

Patients who have had deep venous thrombosis should have anticoagulation therapy with warfarin or low molecular weight heparin. The physician determines the duration of this treatment. Lifelong anticoagulation may be considered for patients with a high risk for recurrent deep venous thrombosis.