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The Annals of Pharmacotherapy: Vol. 30, No. 12, pp. 1402-1407.
© 1996 Harvey Whitney Books Company.
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Research Articles

Potential interaction between warfarin and dicloxacillin

AT Mailloux, BE Gidal, and CA Sorkness

OBJECTIVE: To report a case and retrospective review of seven patients who experienced a decrease in prothrombin time during concomitant administration of warfarin and dicloxacillin. CASE SUMMARY: A 41-year-old man receiving warfarin 22 mg/wk with a final baseline prothrombin time (PT) of 20.7 sec was prescribed dicloxacillin 500 mg qid for 10 days. Plasma collected for PT determinations was also used to measure trough warfarin R- and S- enantiomer concentrations. The PT and S- and R-warfarin concentrations decreased 17%, 25%, and 20%, respectively, on day 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 of two previously published reports, suggest a close temporal relationship between the administration of dicloxacillin and a decreased anticoagulant effect of warfarin. Limited data from our patient further suggest that this may result from declines in systemic warfarin concentrations. The time course of the fall of PTs appears to occur within 4-5 days; return of the PT to baseline after dicloxacillin administration is stopped appears to take up to 3 weeks. Until further controlled studies are conducted to confirm this interaction, clinicians should be aware that patients may be at risk for a decreased anticoagulant effect of warfarin when dicloxacillin is given concomitantly. CONCLUSIONS: Careful monitoring of international normalized ratios and titration of the warfarin dosage is recommended on initiation and for 3 weeks after discontinuation of dicloxacillin in patients receiving warfarin.


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 Arch Intern MedHome page
A. M. Holbrook, J. A. Pereira, R. Labiris, H. McDonald, J. D. Douketis, M. Crowther, and P. S. Wells
Systematic Overview of Warfarin and Its Drug and Food Interactions
Arch Intern Med, May 23, 2005; 165(10): 1095 - 1106.
[Abstract] [Full Text] [PDF]

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 The Annals of PharmacotherapyHome page
C. S Lacey
Interaction of dicloxacillin with warfarin
Ann. Pharmacother., May 1, 2004; 38(5): 898 - 898.
[Full Text] [PDF]


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Copyright © 1996 by Harvey Whitney Books Company.