Metronidazole interaction with warfarin — ORE Part 1 MCQ

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HardDental Pharmacology and TherapeuticsMetronidazole interaction with warfarinORE Part 1

A 72-year-old man on warfarin is given metronidazole for a spreading dental infection. Three days later he develops gingival bleeding and bruising. His INR has risen from 2.4 to 5.8, with no change in diet or warfarin dose. What is the most likely cause?

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Correct answer: A — Potentiation of warfarin anticoagulation by metronidazole

Metronidazole potentiates warfarin anticoagulation by inhibiting its hepatic metabolism. UK prescribing guidance (eMC Marevan SmPC) explicitly identifies metronidazole as a drug that potentiates warfarin effect. The stereoselective interaction occurs because metronidazole, like disulfiram, inhibits CYP2C9 and reduces clearance of the active S-enantiomer of warfarin. The clinical presentation—rapid INR rise to 5.8 with gingival bleeding after 3 days of metronidazole—is consistent with potentiation, not reduced absorption. Distractor A reverses the mechanism. Distractors C (adrenaline toxicity) and D (chlorhexidine allergy) would not cause selective INR elevation. Distractor E (analgesic tolerance) is unrelated to anticoagulation.

Reference: UK eMC: Marevan 0.5 mg Tablets – Summary of Product Characteristics. https://www.medicines.org.uk/emc/product/2803/smpc; Hassall CH, Gander RM. The stereoselective interaction of warfarin and metronidazole in man. J Clin Pharmacol. 1976;16(8–9):434–440.