Major
Metronidazole × Warfarin
Imidazole derivatives (antiprotozoal / antibacterial)×Vitamin K antagonist anticoagulants
Mechanism
Metronidazole inhibits CYP2C9 – S-warfarin (more active isomer) accumulates. INR doubles within 5-7 days.
Symptoms
Gum and nose bleeding, petechiae, haematuria within 5-10 days.
Management
Reduce warfarin by 25-50% from day 1. Check INR on days 3 and 7. After metronidazole withdrawal – retitrate up.
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.