Drug-Drug Interaction of Warfarin and Doxycycline Leading to a Large Rectus Sheath Hematoma

In recent years, there has been a significant reduction in the use of warfarin, as many patients have transitioned to direct oral anticoagulants (DOACs) for the management of atrial fibrillation. However, a considerable number of patients continue to rely on warfarin due to financial limitations and the specific requirement for its use in individuals with mechanical heart valves, given the insufficient data on the effectiveness of DOACs in these scenarios. While warfarin is recognized for its high efficacy, it possesses a narrow therapeutic window, necessitating careful monitoring to avoid excessive bleeding. Consequently, regular assessments of the international normalized ratio (INR) are essential to ensure that anticoagulation remains within the therapeutic range, guiding appropriate dosage adjustments. This report details a case involving a 69-year-old male with a history of atrial fibrillation who was on warfarin therapy and subsequently developed a significant rectus sheath and extraperitoneal hematoma after receiving doxycycline for pneumonia. His INR was recorded at over 8 (facility laboratory limit highest at the time of this case). He was initially managed with vitamin K and prothrombin complex concentrate, followed by interventional radiology consultation for embolization of the right inferior epigastric artery due to the presence of an expanding rectus sheath hematoma observed on CT imaging. Doxycycline may potentiate the anticoagulant effects of warfarin by competing for albumin binding, which may elevate the risk of severe bleeding complications. Therefore, it is imperative to exercise caution when prescribing doxycycline to patients undergoing warfarin therapy.