Methotrexate and trimethoprim — GPhC CRA MCQ
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Correct answer: E — Trimethoprim increases risk of methotrexate-related bone marrow toxicity
Trimethoprim and methotrexate both affect folate pathways and can increase risk of serious bone marrow suppression. Borderline neutrophils and reduced eGFR make the interaction more concerning. The prescriber should be contacted for an alternative antibiotic and monitoring advice. Folic acid does not reliably prevent this interaction.
Reference: BNF Methotrexate interactions; NICE CKS UTI