Interaction between warfarin and nafcillin: case report and review of the literature.
A 39-year-old man with a history of deep vein thrombosis and septic arthritis of the left knee was treated with warfarin and cefazolin. Therapeutic prothrombin times and international normalized ratios (INRs) were maintained with warfarin 32 mg/week for approximately 1 month. When the patient's antibiotic regimen was changed from cefazolin to nafcillin 2 g every 4 hours, his INR declined significantly. His warfarin dosage had to be increased to a maximum of 88 mg/week to achieve a therapeutic INR. After completion of antibiotic therapy with nafcillin, the patient's warfarin requirements slowly declined over several weeks. A maintenance dosage of warfarin 42-48 mg/week was necessary after nafcillin discontinuation. Hepatic cytochrome P450 isoenzyme induction by nafcillin is likely the mechanism of a warfarin-nafcillin interaction. The usual onset of effect is within 1 week after starting nafcillin, and the offset of the effect is usually evident within 4 weeks after nafcillin discontinuation. In patients taking warfarin who are prescribed nafcillin, a 2-4-fold increase in the warfarin dose may be necessary, and clinicians should closely monitor INRs.
Kim
Karissa Y
KY
James L. Winkle College of Pharmacy, University of Cincinnati, Cincinnati, Ohio 45267, USA. karissa.kim@uc.edu
Frey
R J
RJ
Epplen
Kelly
K
Foruhari
Farzin
F
D000328:Adult; D000900:Anti-Bacterial Agents / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use; D000925:Anticoagulants / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use; D004347:Drug Interactions; D006801:Humans; D019934:International Normalized Ratio; D008297:Male; D009254:Nafcillin / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use*; D020246:Venous Thrombosis / Q000097:blood / Q000188:drug therapy; D014859:Warfarin / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use*
D000900:Anti-Bacterial Agents; D000925:Anticoagulants; D009254:Nafcillin; D014859:Warfarin