Antibiotic Interactions with Warfarin

Mechanism of Interactions

The American College of Cardiology notes that all antibiotics can potentially alter the gut microbiome, which is a source of vitamin K, thereby potentiating warfarin's anticoagulant effects 1
Many antibiotics interact through inhibition of specific cytochrome P450 enzymes that metabolize warfarin 1
The anticoagulant effect of warfarin is augmented by second- and third-generation cephalosporins, which inhibit the cyclic interconversion of vitamin K 2, 3

Metronidazole requires approximately 33% warfarin dose reduction when co-administered 1

Monitor INR closely whenever any antibiotic is initiated in patients on warfarin therapy 1
Consider pre-emptive warfarin dose reductions of 25-33% when prescribing high-risk antibiotics 1
Higher antibiotic doses increase the risk of interaction 1

Don't assume that all antibiotics within the same class have identical interaction profiles with warfarin 1
Topical antifungals, like miconazole oral gel, can also interact with warfarin 1
The full effect of enzyme induction with some antibiotics can take 2-4 weeks to develop and persist for 2-4 weeks after discontinuation 1