Potential interaction between azithromycin and warfarin.

Azithromycin is considered unlikely to interact with warfarin. Unlike other macrolide antibiotics, it is not hepatically metabolized and did not produce an interaction with warfarin in a single-dose study. A 71-year-old woman with a prosthetic heart valve, stabilized with warfarin, had international normalized ratios (INRs) maintained between 2.5 and 3.5. Six days after she received a prescription for a 5-day course of azithromycin, her INR was 15.16. Phytonadione 10 mg was administered subcutaneously, and warfarin was held for 3 days until her INR fell to 2.10. She then was restabilized with warfarin. Until more information is known about the safety of warfarin and azithromycin, caution is advised when the agents are given together. Close monitoring of INR is recommended, and warfarin dosage adjustment may be necessary.