Amoxicillin × Warfarin

Amoxicillin suppresses vitamin K-producing gut flora and enhances warfarin anticoagulation. Effect is unpredictable: INR may stay flat or rise by 1–2.

Gum bleeding, epistaxis, bruising without trauma, black or tarry stools, haematuria. Higher risk in older patients, prior peptic ulcer disease, and chronic kidney disease.

For amoxicillin courses of 5–7 days, check INR 3–5 days after start and one week after stopping. If INR drifts outside target, adjust warfarin by 10–20%. Do not stop the antibiotic — complete the full course for the infection.