Moderate
Allopurinol × Warfarin
Xanthine oxidase inhibitors×Vitamin K antagonist anticoagulants
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%.