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Trimethoprim and methotrexate — GPhC CRA MCQ

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HardDrug InteractionsTrimethoprim and methotrexateGPhC CRA

A 50-year-old man presents a prescription for trimethoprim 200mg twice daily for UTI. His repeat medicines include methotrexate 15mg once weekly for psoriasis and folic acid 5mg once weekly. His last FBC was two months ago. What is the most important interaction to flag?

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Correct answer: C — Trimethoprim increases risk of methotrexate-related bone marrow toxicity

Trimethoprim and methotrexate both affect folate pathways and can increase risk of serious marrow suppression. The weekly methotrexate and infection context make the interaction clinically significant. The pharmacist should contact the prescriber for an alternative and monitoring advice.

Reference: BNF; MHRA