MajorImidazole derivatives (antiprotozoal / antibacterial)×Vitamin K antagonist anticoagulants
Updated · Methodology
Mechanism
Metronidazole stereospecifically inhibits CYP2C9, the clearance route for S-warfarin. INR may double within 3–7 days of co-administration.
Symptoms
Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.
Management
For short metronidazole courses, empirically reduce warfarin by 20–30% and check INR 3–4 days after starting and 7 days after stopping the antibiotic. For prolonged therapy, titrate warfarin to INR.
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
- Lexicomp: Lexicomp Drug Interactions (2024)– Wolters Kluwer Clinical Drug Information, Inc. Lexi-Interact Online, 2024
- Pharmaceutical Press: Stockley's Drug Interactions, 12th edition (2024)– Preston CL (ed.). Stockley's Drug Interactions. 12th ed. London: Pharmaceutical Press; 2024
- FDA: Drug Development and Drug Interactions: Table of Substrates, Inhibitors and Inducers (2024)– FDA, Clinical Drug Interaction Studies, table of CYP substrates, inhibitors and inducers, 2024
- FDA: Flagyl (metronidazole) prescribing information (2020)– ; DailyMed setid a2883ca1-5a9a-4259-9d80-46ab67274384
- DailyMed: Warfarin: FDA prescribing information, Drug Interactions section (DailyMed) (2024)
- AEMPS: Ficha técnica: Warfarina (Aldocumar 5 mg) (nº registro 63064), sección 4.5 (2023)– AEMPS, CIMA, nº registro 63064, revisión 01/2023, sección 4.5