amoxicillin × warfarin

Broad-spectrum antibiotics eliminate gut flora that synthesize vitamin K2 – reducing vitamin K availability for hepatic γ-carboxylation. INR rises 3–7 days after antibiotic initiation. The effect is most pronounced in patients on a low-vitamin-K diet.

Check INR on days 3 and 7 of the antibiotic course, and again 7–10 days after completion. For courses ≥ 7 days, pre-emptively reduce the warfarin dose by 10–20 % and titrate by INR.