Major
Co-trimoxazole × Warfarin
Sulfonamide-trimethoprim combination×Vitamin K antagonist anticoagulants
Mechanism
Trimethoprim in co-trimoxazole inhibits CYP2C9. INR rises 50-100% within 3-5 days. High bleeding risk in elderly.
Symptoms
Gum and nose bleeding, petechiae, haematuria within 5-10 days.
Management
If possible – alternative antibiotic. If co-trimoxazole needed – cut warfarin by 30% and check INR on days 3 and 7.
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.