warfarin and azithromycin

MajorAvoid this combination where possible.

Mechanism

Azithromycin may enhance the anticoagulant effect of warfarin. The exact mechanism is not fully established; it may involve inhibition of warfarin metabolism via CYP2C9 or CYP3A4, or reduction in vitamin K synthesis by gut flora having an additive effect.

What it means clinically

Patients may experience a significant rise in INR and an increased risk of bleeding, which can be severe. The effect may be delayed, occurring several days after starting azithromycin, and may persist for days after stopping.

What to do

If co-administration is necessary, monitor INR frequently (e.g., within 3-5 days of starting or stopping azithromycin) and adjust warfarin dose as needed. Consider alternative antibiotics with lower interaction potential. Advise patients to report signs of bleeding.

Evidence

Documented in case reports and small studies; clinically relevant with macrolides, with azithromycin having a weaker but established interaction. The interaction is well recognised in the literature.

The two drugs

warfarinVitamin K antagonistRead the monograph azithromycinmacrolide antibioticRead the monograph

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Other interactions involving warfarin or azithromycin

Educational reference for students and clinicians in training. It does not replace professional judgement, local protocols or current prescribing information.