Acetaminophen interacts with Vitamin K antagonism

Warfarin and acetaminophen interaction in a 47-year-old woman | CMAJ - Acetaminophen interacts pharmacodynamically with the vitamin K antagonism effect of warfarin, leading to significant and under-recognized potential for supratherapeutic anticoagulation - Effect can be seen with even 2-4 grams (therapeutic dose) daily. A systematic review and meta-analysis of 7 randomized controlled trials (RCTs) looking at mean differences of maximum INR change when comparing acetaminophen (1.3–4 g/d) with placebo in patients taking vitamin K antagonists found that acetaminophen caused a dose-dependent increase in the INR of 0.17 per gram per day, with an average INR increase of 0.62 - No guidelines for concomitant usage, suggest more frequent INR checks while taking acetaminophen - Management: - stop acetaminophen - reverse coagulopathy if required - tight monitoring of INR - bridge with heparins if required