Basic Information
ID DDInter1202 and DDInter98
Interaction Coadministration with metronidazole may increase the plasma concentrations and hypoprothrombinemic effect of warfarin. The proposed mechanism is metronidazole inhibition of CYP450 2C9, the isoenzyme responsible for the metabolic clearance of the more active S(-) enantiomer of warfarin.
Management Given the potential for interaction and the high degree of interpatient variability with respect to warfarin metabolism, patients should be closely monitored during concomitant therapy with metronidazole. The INR should be checked frequently and warfarin dosage adjusted accordingly, particularly following initiation or discontinuation of metronidazole in patients who are stabilized on their warfarin regimen. The same precaution may be applicable during therapy with other oral anticoagulants, although clinical data are lacking. Patients should be advised to promptly report any signs of bleeding to their physician, including pain, swelling, headache, dizziness, weakness, prolonged bleeding from cuts, increased menstrual flow, vaginal bleeding, nosebleeds, bleeding of gums from brushing, unusual bleeding or bruising, red or brown urine, or red or black stools.
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Alternative for Metronidazole A02B
Misoprostol Bismuth subcitrate potassium Famotidine Nizatidine Ranitidine Vonoprazan

P01A
Quinacrine Secnidazole Atovaquone Furazolidone Nitazoxanide
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Alternative for Anisindione -
Potential Metabolism Interactions
Substrate-Substrate Interaction：If more than one drug is metabolized by the same CYP, it is possible that its metabolism is inhibited because of the competition between the drugs. That means, it can be useful to lower the dosage of the drugs in the drug-cocktail because they remain longer in the organism than in monotherapy.
Inhibitor-Inhibitor Interaction：Combining two or more inhibitors of one CYP, should be compensated by lowering the dosage of these drugs because the metabolism is reduced and the drugs remain longer in the organism than in monotherapy. Not adapting the dosage bears the risk of even more side effects.
Inhibitor-Substrate Interaction：Combining drugs that have inhibitory effect and are substrates of one particular CYP, should be compensated by lowering the dosage. They rest longer in the organism than in monotherapy. Not adapting the dosage bears the risk of even more side effects.