MAJOR — Use With Caution
FDA-documented interaction. INR monitoring required. Warfarin dose reduction usually needed during metronidazole course.
Evidence level: STRONG
The antibiotic metronidazole (Flagyl) increases warfarin blood-thinning effect, raising your risk of serious bleeding. It does this by slowing how your body breaks down warfarin AND by reducing vitamin K made by gut bacteria.
Metronidazole inhibits CYP2C9, increasing warfarin S-enantiomer levels. Also may inhibit vitamin K production by gut flora. FDA warfarin label lists metronidazole as increasing anticoagulant effect.
If prescribed metronidazole while on warfarin, get your INR checked within a few days. Your warfarin dose may need temporary reduction for the duration of the antibiotic course.
Space Warfarin at least 2 hours from Metronidazole. Follow antibiotic-specific food instructions carefully. Complete the full antibiotic course even if symptoms improve.
Higher risk with: older age (65+), history of GI bleeding, concurrent use of multiple blood-thinning agents, liver disease, heavy alcohol use, recent surgery.
Easy bruising, prolonged bleeding from cuts, blood in urine or stool, nosebleeds, black stools
Request INR monitoring during and after metronidazole course. Discuss whether an alternative antibiotic could be used. Plan warfarin dose adjustment.
This medication combination requires careful medical oversight. Your doctor or anticoagulation clinic can adjust doses or select alternatives with fewer bleeding risks. Frequent INR monitoring is essential.
The antibiotic metronidazole (Flagyl) increases warfarin blood-thinning effect, raising your risk of serious bleeding. It does this by slowing how your body breaks down warfarin AND by reducing vitamin K made by gut bacteria.
Space Warfarin at least 2 hours from Metronidazole. Follow antibiotic-specific food instructions carefully. Complete the full antibiotic course even if symptoms improve.
Easy bruising, prolonged bleeding from cuts, blood in urine or stool, nosebleeds, black stools
This medication combination requires careful medical oversight. Your doctor or anticoagulation clinic can adjust doses or select alternatives with fewer bleeding risks. Frequent INR monitoring is essential.
Request INR monitoring during and after metronidazole course. Discuss whether an alternative antibiotic could be used. Plan warfarin dose adjustment.
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