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Trimethoprim methotrexate interaction — GPhC CRA MCQ

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HardInfections and AntimicrobialsTrimethoprim methotrexate interactionGPhC CRA

A patient taking weekly methotrexate for rheumatoid arthritis presents trimethoprim for UTI. Their last FBC was normal. What is the most important interaction to flag?

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

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Correct answer: D — Increased bone marrow suppression risk

Trimethoprim can increase antifolate toxicity with methotrexate and raise risk of serious blood dyscrasias. The prescription should be queried before supply. In this scenario, the correct option is increased bone marrow suppression risk. The other options are plausible but do not address the key clinical, legal or safety issue as directly. This aligns with UK pharmacy practice guidance cited in the reference field.

Reference: BNF; Stockley's Drug Interactions