skip to main content

MTX Toxicity Drug Interaction — RACP Adult Medicine MCQ

Instant feedback + full explanation. One question, done properly.

HardRheumatologyMTX Toxicity Drug InteractionRACP Adult Medicine

A 60-year-old taking methotrexate 20 mg weekly develops oral ulceration, neutropenia and acute kidney injury five days after trimethoprim was prescribed. What best explains the deterioration?

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

Reveal the answer and explanation

Correct answer: D — Additive antifolate toxicity plus reduced renal methotrexate clearance

The best answer is “Additive antifolate toxicity plus reduced renal methotrexate clearance”. Trimethoprim adds antifolate activity and can interact with low-dose methotrexate, while kidney impairment reduces methotrexate elimination. The combination explains mucositis and marrow suppression and requires both drugs to be withheld, urgent assessment, folinic-acid rescue when indicated and treatment of infection. Rheumatoid arthritis itself does not account for this acute temporal syndrome.

Reference: Australian Prescriber: Managing the drug treatment of rheumatoid arthritis: https://australianprescriber.tg.org.au/articles/managing-the-drug-treatment-of-rheumatoid-arthritis.html