warfarin and amoxicillin

ModerateUsually manageable, but it must be recognised and monitored.

Mechanism

Amoxicillin may reduce the production of vitamin K by intestinal flora, leading to decreased synthesis of clotting factors and enhanced anticoagulant effect of warfarin. Additionally, some penicillin antibiotics can interfere with the metabolism of warfarin or have an additive effect on haemostasis.

What it means clinically

Increased INR and risk of bleeding, which can occur several days after starting amoxicillin. Patients may experience bruising, epistaxis, or more serious haemorrhage.

What to do

Monitor INR closely, particularly within the first week of concurrent therapy. Consider more frequent INR checks and adjust warfarin dose as needed. Advise patient to report signs of bleeding.

Evidence

Well-established interaction; numerous case reports and clinical studies. The effect is modest in most patients but can be clinically significant, especially in those with high baseline INR or other risk factors for bleeding.

The two drugs

warfarinVitamin K antagonistRead the monograph amoxicillinPenicillin antibioticsRead the monograph

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Other interactions involving warfarin or amoxicillin

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