skip to main content

Warfarin and trimethoprim — GPhC CRA MCQ

Instant feedback + full explanation. One question, done properly.

HardDrug InteractionsWarfarin and trimethoprimGPhC CRA

A 55-year-old woman taking warfarin for a mechanical mitral valve presents with new trimethoprim 200 mg twice daily for UTI. Her last INR 4 days ago was 2.8 and she reports no bleeding. What is the most important interaction to flag?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: B — Trimethoprim may increase INR and bleeding risk

Trimethoprim can potentiate warfarin and increase INR, particularly in older people or those on longer courses. The pharmacist should contact the prescriber or anticoagulation clinic to agree management and INR monitoring. The patient should also be counselled to report bruising, nosebleeds, melaena or haematuria.

Reference: BNF