To the Editor: Warfarin is an orally administered vitamin K antagonist and has many well described interactions with commonly prescribed medications.1 However, only a handful of case reports worldwide have shown that flucloxacillin, a widely used antibiotic, may affect warfarin therapy by reducing its anticoagulant effect — in one instance contributing to the development of ischaemic stroke.2,3 There are several reports describing potential mech-anisms of interaction for flucloxacillin with warfarin.4,5
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- 1 St George Hospital, Sydney, NSW.
- 2 Queen Street Clinic, Grafton, NSW.
- 1. Juurlink DN. Drug interactions with warfarin: what clinicians need to know. CMAJ 2007; 177: 369-371.
- 2. Merwick A, Hannon N, Kelly PJ, O’Rourke K. Warfarin-flucloxacillin interaction presenting as cardioembolic ischemic stroke. Eur J Clin Pharmacol 2010; 66: 643-644.
- 3. Garg A, Mohammed M. Decreased INR response secondary to warfarin-flucloxacillin interaction. Ann Pharmacother 2009; 43: 1374-1375.
- 4. Huwyler J, Wright MB, Gutmann H, Drewe J. Induction of cytochrome P450 3A4 and P-glycoprotein by the isoxazolyl-penicillin antibiotic flucloxacillin. Curr Drug Metab 2006; 7: 119-126.
- 5. Tang W, Stearns RA. Heterotropic cooperativity of cytochrome P450 3A4 and potential drug-drug interactions. Curr Drug Metab 2001; 2: 185-198.