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Methotrexate and trimethoprim — GPhC CRA MCQ

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HardDrug InteractionsMethotrexate and trimethoprimGPhC CRA

A 57-year-old woman takes methotrexate 15mg once weekly for rheumatoid arthritis. She is prescribed trimethoprim 200mg twice daily for UTI. Her latest FBC was normal. What is the most important interaction to flag?

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Correct answer: A — Trimethoprim increases risk of serious methotrexate haematological toxicity

Trimethoprim can increase the risk of methotrexate bone marrow suppression, which can be severe and sometimes fatal. The prescription should be queried and an alternative antibiotic considered. The patient should be counselled to report mouth ulcers, sore throat, bruising or fever.

Reference: BNF; SPS