Causes of hyperhomocysteinaemia
| Male sex |
| Age |
| Menopause |
| Lifestyle factors including smoking, heavy intake of coffee, and alcohol |
| Genetic |
| Cystathionine β synthase deficiency (the cause of classic homocystinuria) |
| 5,10-Methylenetetrahydrofolate reductase deficiency (rare) or thermolability (common) |
| Methionine synthase deficiency (rare) |
| Nutritional disorders |
| Folate deficiency |
| Vitamin B12 deficiency |
| Vitamin B6 deficiency |
| Systemic disease |
| Renal failure |
| Malignant diseases |
| Psoriasis |
| Hypothyroidism |
| Drugs |
| Cholestyramine and colestipol (decrease cobalamin and folate absorption) |
| Methotrexate (inhibits dihydrofolate reductase) |
| Phenytoin and carbamazepine (antagonise folate) |
| Nitrous oxide (inactivates methionine synthase) |
| Niacin and theophylline (decrease pyridoxal kinase) |
| Androgens (increase muscle mass and serum creatinine) |
| Cyclosporin A (decreases renal function) |
| Metformin (may decrease cobalamin absorption) |
| Fibric acid derivatives (may interfere with renal function) |