Trimethoprim-methotrexate interaction — GPhC CRA MCQ
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Correct answer: D — Contact the prescriber as trimethoprim can increase methotrexate toxicity by inhibiting folate metabolism and reducing renal excretion
Trimethoprim inhibits dihydrofolate reductase and can enhance the antifolate effect of methotrexate, increasing the risk of bone marrow suppression and pancytopenia. Additionally, trimethoprim can reduce renal excretion of methotrexate. The BNF lists this as a potentially serious interaction. The prescriber should be contacted to discuss an alternative antibiotic such as nitrofurantoin or cefalexin. The patient should not adjust methotrexate independently.
Reference: BNF Drug Interactions