NSAID-Lithium — MRCPsych Paper B MCQ

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ModerateMood DisordersNSAID-LithiumMRCPsych Paper B

A 46-year-old woman with bipolar I disorder has been stable on lithium with normal renal function. After starting one of the following medicines, her plasma lithium concentration rises despite no change in her lithium dose, fluid intake or dietary sodium. Which medicine is most likely to have reduced renal lithium clearance through inhibition of renal prostaglandin synthesis?

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Correct answer: A — Ibuprofen

Ibuprofen is the correct answer. As an NSAID, it inhibits cyclo-oxygenase and renal prostaglandin synthesis, which can reduce renal blood flow and lithium elimination. The resulting rise in plasma lithium may cause toxicity, so UK guidance recommends avoiding NSAIDs where possible and monitoring lithium concentrations more frequently if one is started or stopped. The magnitude of the increase is variable; a fixed 25–50% rise should not be assumed. Paracetamol is generally the preferred simple analgesic in a patient taking lithium and does not produce this characteristic interaction. Amlodipine, amoxicillin and omeprazole do not reduce lithium clearance through renal prostaglandin inhibition.

Reference: Ibuprofen 600 mg film-coated tablets, Summary of Product Characteristics, sections 4.4, 4.5 and 5.1, revised 26 June 2026. https://www.medicines.org.uk/emc/product/7021/smpc