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Methotrexate and trimethoprim — GPhC CRA MCQ

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HardDrug InteractionsMethotrexate and trimethoprimGPhC CRA

A patient taking methotrexate weekly for rheumatoid arthritis presents a prescription for trimethoprim for cystitis. She also takes folic acid once weekly. She is clinically well today. What is the most important interaction to flag?

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Correct answer: B — The combination increases risk of bone marrow suppression

Trimethoprim and methotrexate both affect folate pathways and the combination can increase risk of serious bone marrow suppression. The prescription should be queried and an alternative antibiotic considered. Folic acid does not reliably prevent this interaction. Being clinically well at presentation does not remove the risk during treatment.

Reference: BNF; MHRA Drug Safety Update