Moderate
Azithromycin × Warfarin
Macrolides×Vitamin K antagonist anticoagulants
Mechanism
Azithromycin suppresses vitamin K-producing gut flora, raising INR. Bleeding cases have been reported.
Symptoms
Gum bleeding, epistaxis, bruising without trauma, black or tarry stools. Higher risk in older patients and prior peptic ulcer disease.
Management
For short azithromycin courses (3–5 days), check INR 3–5 days after start and 5 days after stopping the antibiotic. If INR rises above target, reduce warfarin by 10–20%. Alternative antibiotics: a cephalosporin (minimal gut flora effect).