Interaction of Doxycycline and Warfarin

An Enhanced Anticoagulant Effect

Abstract

Purpose: 

To highlight the importance of recognizing the increased anticoagulant effect of warfarin associated with use of doxycycline.

Design: 

Case report.

Methods: 

Clinical review describing initial presentation, course, and outcome of one patient who received oral doxycycline for corneal melt while on warfarin in a multi-specialty clinic.

Results: 

Concomitant use of doxycycline and warfarin resulted in prominent ecchymosis of the upper extremities associated with a marked elevation in the international normalized ratio (INR = 6.5). Discontinuation of doxycycline resulted in reduction of INR to therapeutic levels (INR = 2.3).

Conclusions: 

Doxycycline can enhance the anticoagulant effect of warfarin. This is likely due to competitive interaction for albumin binding and possibly inhibition of the cytochrome P-450 system. An increase in plasma levels of free warfarin may result in severe bleeding. It is important to recognize this potential interaction and obtain a complete medication profile prior to initiating doxycycline.