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Published on: October 12, 2012

Interaction between warfarin and nafcillin: case report and review of the literature

Karissa Y Kim1, R J Frey, Kelly Epplen

  • 1James L. Winkle College of Pharmacy, University of Cincinnati, Cincinnati, Ohio 45267, USA. karissa.kim@uc.edu

Pharmacotherapy
|September 28, 2007
PubMed

Insights

Nafcillin significantly reduces warfarin effectiveness, requiring a 2-4 fold dose increase to maintain therapeutic international normalized ratios (INRs). Warfarin dosage adjustments are crucial when switching antibiotics, with effects reversing after nafcillin discontinuation.

Area of Science:

  • Pharmacology
  • Drug Interactions
  • Clinical Medicine

Background:

  • Warfarin is a common anticoagulant used for deep vein thrombosis (DVT) and other conditions.
  • Maintaining therapeutic international normalized ratios (INRs) is critical for warfarin efficacy and safety.
  • Antibiotic therapy can influence warfarin metabolism and efficacy.

Observation:

  • A patient on stable warfarin therapy experienced a significant INR decline when cefazolin was switched to nafcillin.
  • Warfarin dosage increased two-fold to maintain therapeutic INRs during nafcillin treatment.
  • INR levels gradually returned to baseline after nafcillin discontinuation, necessitating a dosage reduction.

Findings:

  • Nafcillin significantly reduces warfarin's anticoagulant effect, likely through hepatic cytochrome P450 isoenzyme induction.
  • A 2-4 fold increase in warfarin dosage may be required when co-administered with nafcillin.
  • The interaction's onset is within one week of nafcillin initiation, with offset typically observed within four weeks of discontinuation.

Implications:

  • Clinicians must closely monitor INRs in patients receiving both warfarin and nafcillin.
  • Dosage adjustments for warfarin are essential during and after nafcillin therapy.
  • Understanding drug-drug interactions is vital for optimizing anticoagulant therapy and patient safety.

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