Metronidazole causes an unexpected rise in INR in anticoagulated patients even after warfarin has been stopped

In the 1970s an interaction between metronidazole and warfarin was described in healthy volunteers. Subsequently, there have been few reports describing the interaction during the concurrent administration of warfarin and metronidazole. A critical analysis of warfarin interactions concluded that the metronidazole-warfarin interaction is “highly probable”, with metronidazole potentiating the anticoagulant effect of warfarin. We propose mechanisms for this interaction, based on three patients in a surgical emergency unit whose international normalised ratios (INR) rose despite the cessation of warfarin some days previously. Informed patient consent has been obtained for publication of these studies.