Azithromycin and warfarin interaction

A 57-year-old Caucasian woman came to the clinic with symptoms of an upper respiratory tract infection. She was treated with a 5-day course of oral azithromycin 500 mg on day 1, then 250 mg/day for 4 days. During this period, the patient decreased her cigarette smoking from I pack/day to I pack every 3 days. No additional confounding variables were present. Two days after the completion of therapy, her international normalized ratio (INR) was 8.32. Six case reports documented in the literature have suggested an azithromycin-warfarin interaction with a resultant increase in INR. Many confounding variables existed in each of these cases, such as hepatic dysfunction, poor appetite, and concomitant drugs that resulted in an increased anticoagulant response. We report a case that involved only one potential confounding variable. Continued documentation of azithromycin-warfarin interactions is valuable considering no mention of this drug interaction exists in most tertiary references and in the package insert for azithromycin, the demonstration that no drug interaction occurred in a retrospective review of 52 cases, and the widespread use of azithromycin in the community. Clinicians should be mindful when prescribing azithromycin in combination with warfarin, and INR values should be monitored.