Interaction between acetaminophen and warfarin in adults receiving long-term oral anticoagulants: a randomized controlled trial.

Purpose: We investigated whether acetaminophen, given at 2 g/day and 3 g/day might potentiate the anticoagulant effect of warfarin.

Methods: Forty-five patients on stable warfarin therapy, enrolled in this prospective, randomized, parallel (three arms), placebo-controlled study, received a 10-day regimen of acetaminophen (2 g/day or 3 g/day) or placebo.

Results: The mean maximal INR increase was 0.70 ± 0.49 and 0.67 ± 0.62 in patients receiving acetaminophen at 2 g/day and 3 g/day, respectively (P=0.01 for the respective comparisons versus placebo). The INR increase became significant on day 3 and was independently and significantly predicted by a maximal decrease in factor II (R(2)=0.36, P<0.0001), factor VII (R (2)=0.46, P<0.0001) and a maximal increase in acetaminophen plasma concentrations (R(2)=0.563, P<0.0001).

Conclusion: Acetaminophen, at 2 g/day or 3 g/day, enhanced the anticoagulant effect of warfarin in stable patients, thus requiring close INR monitoring in the clinical setting.