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Antibiotic interaction with methotrexate — GPhC CRA MCQ

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HardInfectionsAntibiotic interaction with methotrexateGPhC CRA

A 59-year-old man takes methotrexate 15 mg once weekly for rheumatoid arthritis. He is prescribed trimethoprim 200 mg twice daily for UTI. His last blood count was normal. What is the most important interaction to flag?

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Correct answer: D — Trimethoprim with methotrexate increases risk of bone marrow toxicity

Trimethoprim and co-trimoxazole can increase the risk of serious methotrexate toxicity, including bone marrow suppression. This is clinically important even at low-dose weekly methotrexate. The prescriber should be contacted to consider an alternative antibiotic and monitoring.

Reference: SPS; BNF