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Possible Nafcillin-Warfarin Interaction

Greta M. Heilker, PharmD; John W. Fowler Jr, MD; Timothy H. Self, PharmD
Arch Intern Med. 1994;154(7):822-824. doi:10.1001/archinte.1994.00420070152023.
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There are multiple reports of drug interactions with warfarin in the literature. Many of these result in resistance to warfarin therapy due to enzyme inducers such as barbiturates, carbamazepine, and rifampin, with resulting decreased warfarin effect and prothrombin time (PT).1-3 Nafcillin is not well documented as an inducer of drug metabolizing enzymes, yet it appears to have this effect on warfarin.4 We recently observed a possible interaction between nafcillin and warfarin.

Report of a Case.  A 71-year-old woman was admitted with a 3-month history of left lower extremity swelling and a 3-week history of pain, tenderness, and edema. On admission, an attempted left lower leg ultrasound was unsuccessful due to swelling of the lower leg. Therefore, her admitting diagnosis was rule-out deep vein thrombosis vs cellulitis of the lower leg. Her medicines prior to admission included levothyroxine (0.125 μg daily), extended-release nifedipine (60 mg twice daily), extended-release potassium


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