ModerateXanthine oxidase inhibitors×Vitamin K antagonist anticoagulants
Updated · Methodology
Mechanism
Allopurinol weakly inhibits CYP2C9 and reduces metabolism of the more active S-warfarin enantiomer. INR may rise by 0.5–1.
Symptoms
Gum bleeding, epistaxis, bruising without trauma, black or tarry stools, haematuria. Higher risk in older patients, prior peptic ulcer disease, and chronic kidney disease.
Management
On allopurinol initiation or dose increase, check INR at 1 and 2 weeks. If outside target, reduce warfarin by 10%.
Check the full regimen, not just this pair
Opens the checker with these two drugs prefilled. Add the rest of the regimen and recompute additive risks.
Summary of a multi-drug list →
Medicine list: everything you take in one place →
Sources
- FDA: Allopurinol tablet (Dr. Reddy's Laboratories Limited) – Prescribing Information (2018)– ; DailyMed setid 19a138b8-d225-03e6-f762-abe71560204b
- AEMPS: Ficha técnica: Alopurinol (nº registro 63222), sección 4.5 (2025)– AEMPS, CIMA, nº registro 63222, revisión 08/2025, sección 4.5