Enhanced anticoagulant effect of coumarin derivatives induced by doxycycline coadministration.

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.