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Related Experiment Videos

Warfarin and acetaminophen interaction.

Markus G Gebauer1, Karin Nyfort-Hansen, Philip J Henschke

  • 1Pharmacy Department Repatriation General Hospital, Adelaide, South Australia, Australia. Markus.Gebauer@rgh.sa.gov.au

Pharmacotherapy
|January 14, 2003
PubMed
Summary

Acetaminophen significantly increases warfarin

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Area of Science:

  • Pharmacology
  • Drug Interactions
  • Clinical Medicine

Background:

  • Warfarin is a common anticoagulant for atrial fibrillation.
  • Maintaining a stable International Normalized Ratio (INR) is crucial for efficacy and safety.

Observation:

  • A patient on stable warfarin therapy experienced a sharp INR increase after taking acetaminophen.
  • This interaction occurred without changes in warfarin levels, suggesting a pharmacodynamic mechanism.

Findings:

  • Acetaminophen (1g, 4 times/day for 3 days) raised INR from 2.3 to 6.4.
  • Factor VII and X activities decreased, indicating enhanced warfarin effect.
  • Warfarin levels remained unchanged, pointing to a pharmacodynamic interaction.

Implications:

  • Acetaminophen may augment warfarin's anticoagulant effect via vitamin K antagonism.
  • Patients on warfarin should monitor INR closely when starting acetaminophen (>2g/day).
  • This interaction highlights the need for careful medication review in anticoagulated patients.

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