Drug Interaction — ORE Part 1 MCQ

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ModeratePharmacology & TherapeuticsDrug InteractionORE Part 1

A 50-year-old man taking long-term warfarin presents with an acute dental abscess and requires an antibiotic because of spreading infection. He has a documented history of anaphylaxis to penicillin. Which antibiotic should be avoided if a suitable alternative is available because it has a clinically important interaction that can potentiate warfarin?

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Correct answer: A — Metronidazole

A. Metronidazole should be avoided where a suitable alternative exists because it can enhance the effect of warfarin, raising the INR and bleeding risk. The UK metronidazole SmPC states that warfarin-type oral anticoagulant therapy may be potentiated; it advises monitoring prothrombin time and notes that warfarin dose reduction may be required. SDCEP similarly identifies metronidazole as a drug that may enhance warfarin anticoagulation. Phenoxymethylpenicillin is excluded by the patient's previous anaphylaxis, not by a warfarin interaction. Cefalexin is a poor choice following immediate penicillin hypersensitivity. Azithromycin and doxycycline can occasionally affect anticoagulant control, so INR awareness is prudent, but neither is as classically and directly associated with clinically important warfarin potentiation as metronidazole in this option set.

Reference: Electronic Medicines Compendium. Metronidazole 200 mg Tablets – Summary of Product Characteristics, section 4.5 (Interactions), 2024. https://www.medicines.org.uk/emc/product/12348/smpc