The Effects of Acetaminophen on Pharmacokinetics and Pharmacodynamics of Warfarin

Authors

  • Debora Kwan BScPhm, MSc,

    1. Departments of Pharmacy, Sunnybrook Health Science Centre, North York, Ontario, and the Toronto Rehabilitation Institute.
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  • William R. Bartle PharmD,

    Corresponding author
    1. Departments of Pharmacy, Sunnybrook Health Science Centre, North York, Ontario, and the Toronto Rehabilitation Institute.
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  • Scott E. Walker MScPhm

    1. Departments of Pharmacy, Sunnybrook Health Science Centre, North York, Ontario, and the Toronto Rehabilitation Institute.
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Department of Pharmacy, Sunnybrook Health Science Centre, 2075 Bayview Ave., North York, Ontario, Canada M5N 3M5.

Abstract

The oral anticoagulant warfarin is clinically administered as a racemic mixture of two enantiomers, (R) and (S). Many relevant drug interactions with warfarin have been attributed to the specific metabolic inhibition of the elimination of the more pharmacologically active (S)-enantiomer. To investigate reports that acetaminophen can potentiate the anticoagulant effect of warfarin, 20 healthy male volunteers were each given single oral 20 mg doses of racemic warfarin on three separate occasions: (1) alone, (2) after 1 day of acetaminophen (4 g/d), and (3) after 2 weeks of acetaminophen (4 g/d). The urinary excretion pattern of acetaminophen and its metabolites was not significantly altered over its course of administration. The (R)- and (S)-enantiomers of warfarin exhibited significantly different pharmacokinetic properties. However, acetaminophen did not alter the disposition of either (R)- or (S)-warfarin. All subjects exhibited a pharmacodynamic response to racemic warfarin. The response was not significantly altered in the presence of acute or chronic acetaminophen dosing, as assessed by prothrombin time and factor VII concentrations.

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