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Methotrexate and trimethoprim — GPhC CRA MCQ

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HardDrug InteractionsMethotrexate and trimethoprimGPhC CRA

A patient taking weekly methotrexate for psoriasis is prescribed trimethoprim for UTI. He asks whether it is safe to start today. What is the most important interaction to flag?

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Correct answer: A — Increased risk of bone marrow suppression

Trimethoprim and methotrexate can have additive antifolate effects. This increases the risk of serious bone marrow suppression, especially in older patients or renal impairment. The pharmacist should contact the prescriber for an alternative and advise the patient not to start without confirmation. Warning symptoms include sore throat, mouth ulcers, bruising and fever.

Reference: BNF; SPS interactions