Enhanced Anticoagulant Effect of Coumarin Derivatives Induced by Doxycycline Coadministration

Abstract

OBJECTIVE: To report two cases of enhanced oral anticoagulant effect induced by doxycycline coadministration. DESIGN: Case report. SETTING: University teaching hospital. PATIENTS: Two patients on chronic oral anticoagulation therapy who presented with severe bleeding and marked impairment in blood coagulation tests shortly after the initiation of doxycycline therapy. DISCUSSION: The literature concerning the possible effects of tetracyclines on hemostasis with or without antecedent anticoagulation therapy is reviewed and the speculated mechanisms for such an interaction are discussed. CONCLUSIONS: The administration of tetracyclines such as doxycycline to patients on chronic oral anticoagulation therapy may be associated with a marked enhancement in anticoagulant effect. In such patients the prothrombin ratio should be closely monitored and the anticoagulant dosage adjusted accordingly.