In the body, homocysteine can be recycled into methionine or converted into cysteine with the aid of vitamin B6, B9, and B12.

[3] High levels of homocysteine in the blood (hyperhomocysteinemia) is regarded as a marker of cardiovascular disease, likely working through atherogenesis, which can result in ischemic injury.

Cystathionine γ-lyase then converts this double amino acid to cysteine, ammonia, and α-ketobutyrate.

[13] It has been proposed that both homocysteine and its thiolactone may have played a significant role in the appearance of life on the early Earth.

Abnormally high levels of homocysteine in the serum, above 15 μmol/L, are a medical condition called hyperhomocysteinemia.

[23] This has been claimed to be a significant risk factor for the development of a wide range of diseases, in total more than 100[24] including thrombosis,[25] neuropsychiatric illness,[26][27][28][29] in particular dementia[30] and fractures.

[31][32] It also is found to be associated with microalbuminuria (moderately increased albuminuria), which is a strong indicator of the risk of future cardiovascular disease and renal dysfunction.

[33] Vitamin B12 deficiency, even when coupled with high serum folate levels, has been found to increase overall homocysteine concentrations as well.