Determination of the Lack of a Drug Interaction Between Azithromycin and Warfarin

Study Objective. To determine the effect on the international normalized ratio (INR) of adding azithromycin to patients receiving stable dosages of warfarin. Design. Retrospective chart review. Setting. Outpatient clinic. Patients. Ambulatory patients receiving warfarin and azithromycin concurrently who had a documented therapeutic INR value before the start of azithromycin therapy (pre‐INR) and a documented INR value within 30 days after the start of azithromycin therapy (post‐INR). Measurements and Main Results. Patients given felodipine during long‐term warfarin therapy formed a comparative control group. Patient demographics were similar in both treatment groups. Mean age of the azithromycin group (17 patients) was 59 ± 13 years and of the control group (20 patients) 65 ± 12 years. All 17 patients in the azithromycin group and 16 of the controls were women. Mean change from pre‐INR to post‐INR in the azithromycin and control groups, respectively, was 0.14 ± 0.64 (pre‐INR 2.46, post‐INR 2.61) and 0.19 ± 0.54 (pre‐INR 2.46, post‐INR 2.66) (p=0.74). A post hoc power analysis based on a pooled standard deviation of 0.60 revealed that the study had 68% power to detect a 0.5 change in the INR value. Conclusion. No interaction between azithromycin and warfarin was observed in ambulatory patients with therapeutic baseline INR values.