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Methotrexate and trimethoprim — GPhC CRA MCQ

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HardDrug InteractionsMethotrexate and trimethoprimGPhC CRA

A patient taking methotrexate once weekly for psoriasis presents a new prescription for trimethoprim. She has mouth ulcers and feels unusually tired. What is the most important interaction to flag?

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Correct answer: C — Additive antifolate toxicity causing marrow suppression

Methotrexate and trimethoprim both affect folate pathways and can increase the risk of blood dyscrasias. Mouth ulcers and fatigue may already suggest toxicity. The prescription should be queried urgently and the patient assessed.

Reference: BNF