The Potential Effects of Dicloxacillin on Warfarin Management

Christina A Clarke, Anita D Patel

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Objective: To illustrate the effect that dicloxacillin may have on the anticoagulant properties of warfarin.

Case Summary: A 69-year-old white male presented to the emergency department following a motor vehicle accident. He had been receiving a stable dose of warfarin of approximately 35-37.5 mg/wk for more than 2 years for atrial fibrillation. Computed tomography and magnetic resonance imaging revealed a 1 × 1.3 cm lesion in the anterior aspect of the pancreatic body. During a subsequent hospital stay for a distal pancreatectomy and splenectomy, the patient began a 6-week course of dicloxacillin to treat osteomyelitis of the elbow. During the course of treatment, the patient’s warfarin dose was increased by 114% to reach a therapeutic international normalized ratio (INR) approximately 11 days after the antibiotic was discontinued.

Discussion: Compared with previously published cases documenting the interaction between dicloxacillin and warfarin, to our knowledge, this is the first case to comprehensively assess and document other factors that potentially influenced the patient’s INR. At every clinic visit, the patient was assessed for several factors that could have contributed to fluctuations in INR, none of which proved to have a significant effect.

CONCLUSIONS: The addition of dicloxacillin to our patient's stable warfarin regimen resulted in a significant interaction that placed the patient at risk for thromboembolic complications. Careful monitoring of INR from the start of dicloxacillin therapy until 2 weeks after its discontinuation should be considered in patients who are taking warfarin and require dicloxacillin therapy.

J Pharm Technol 2013;29:139-42