POTENTIAL INTERACTION BETWEEN WARFARIN AND DICLOXACILLIN

OBJECTIVE: To report a case and retrospective review of seven patients who experienced a decrease in prothrombin time during concomitant adm inistration of warfarin and dicloxacillin. CASE SUMMARY: A 41-year-old man receiving warfarin 22 mg/wk with a final baseline prothrombin tim e (PT) of 20.7 sec was prescribed dicloxacillin 500 mg qid for 10 days . Plasma collected for PT determinations was also used to measure trou gh warfarin R- and S- enantiomer concentrations. The PT and S- and R-w arfarin concentrations decreased 17%, 25%, and 20%, respectively, on d ay 5 after initiation of dicloxacillin. For the retrospective review, the mean PT decreased 17.0% (range 10.5-25.9%) as soon as 4 days after the initiation of dicloxacillin. DISCUSSION: Our observations, which are consistent with those ui two previously published reports, suggest a close temporal relationship between the administration of dicloxaci llin and a decreased anticoagulant effect of warfarin. Limited data fr om our patient further suggest that this may result from declines in s ystemic warfarin concentrations. The time course of the fall of PTs ap pears to occur within 4-5 days; return of the PT to baseline after dic loxacillin administration is stopped appears to take up to 3 weeks. Un til further controlled studies are conducted to confirm this interacti on, clinicians should be aware that patients may bt at risk for a decr eased anticoagulant effect of warfarin when dicloxacillin is given con comitantly. CONCLUSIONS: Careful monitoring of international normalize d ratios and titration of the warfarin dosage is recommended on initia tion and for 3 weeks after discontinuation of dicloxacillin in patient s receiving warfarin.