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Methotrexate Drug Interaction — SCE Rheumatology MCQ

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HardRheumatology PharmacologyMethotrexate Drug InteractionSCE Rheumatology

A patient taking methotrexate 20 mg weekly is prescribed trimethoprim for a urinary infection. Ten days later she has oral ulceration, fever and bruising despite having stopped the antibiotic. Which interpretation is safest?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — Withhold methotrexate and urgently investigate additive antifolate marrow toxicity

Explanation lettering: B = shown as A · A = shown as B

B is correct. Trimethoprim and co-trimoxazole add antifolate marrow toxicity to methotrexate, and clinically important cytopenia may be delayed. Mucositis, fever and bruising require immediate interruption and urgent assessment. A misunderstands delayed toxicity. C uses the same hazardous trimethoprim component. D ignores a classic toxicity constellation. E would increase exposure and harm.

Reference: NHS Specialist Pharmacy Service: managing interactions with methotrexate: https://www.sps.nhs.uk/articles/managing-interactions-with-methotrexate/