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Methotrexate and trimethoprim — GPhC CRA MCQ

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HardDrug InteractionsMethotrexate and trimethoprimGPhC CRA

A 58-year-old woman taking methotrexate 15mg once weekly for rheumatoid arthritis presents a prescription for trimethoprim 200mg twice daily for 3 days. She also takes folic acid 5mg weekly and has a recent neutrophil count of 2.0 x 10^9/L. What is the most important interaction to flag?

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Correct answer: E — trimethoprim can increase risk of methotrexate-related bone marrow toxicity

Trimethoprim and methotrexate both affect folate pathways and the combination increases the risk of severe bone marrow suppression. The risk is clinically important even with short antibiotic courses, particularly when baseline counts are borderline. The pharmacist should contact the prescriber for an alternative antibiotic where appropriate. Folic acid does not remove this interaction risk.

Reference: BNF