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Methotrexate and trimethoprim — GPhC CRA MCQ

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HardDrug InteractionsMethotrexate and trimethoprimGPhC CRA

A 57-year-old woman with rheumatoid arthritis takes methotrexate 15mg once weekly and folic acid 5mg once weekly on a different day. She is prescribed trimethoprim for UTI by an out-of-hours doctor. What is the most important interaction to flag?

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Correct answer: B — Increased risk of methotrexate toxicity and bone marrow suppression.

Trimethoprim has antifolate effects and can increase the risk of methotrexate toxicity, including serious bone marrow suppression. Methotrexate is a high-risk medicine requiring careful interaction screening. The prescription should be queried and an alternative considered.

Reference: BNF; MHRA Drug Safety Update