A recent study published in JAMA found an association between treatment with dicloxacillin and a decrease in international normalized ratio (INR) levels among patients taking vitamin K antagonists (VKAs) such as warfarin or phenprocoumon.

Anton Pottegard, MscPharm, PhD, of the University of Southern Denmark, and colleagues identified 7,400 VKA-treated patients in Thrombobase, an anticoagulant database. They included 236 patients who filled a prescription for dicloxacillin while receiving warfarin therapy between March 1998 and November 2012. They compared the last INR measurement before dicloxacillin exposure with the first measurement within 2–4 weeks after dicloxacillin exposure.

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The average INR level prior to dicloxacillin exposure was 2.6 compared with 2.0 at 2–4 weeks after dicloxacillin exposure, an average decrease of 0.6. In total, 61% (n=144) experienced sub-therapeutic INR levels (<2.0) within 2–4 weeks after dicloxacillin treatment. For patients taking phenprocoumon (n=64), an average decrease of 0.3 occurred after dicloxacillin exposure. Sub-therapeutic INR levels after dicloxacillin exposure was seen in 41% (n=26) of patients.

Study findings suggest that physicians should be aware that dicloxacillin treatment may cause a significant decrease in INR levels among patients taking VKAs.

For more information visit JAMAnetwork.com.