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Lithium-NSAID Interaction — MRCPsych Paper B MCQ

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EasyMood DisordersLithium-NSAID InteractionMRCPsych Paper B

A 42-year-old woman with bipolar disorder is stable on lithium carbonate 1000 mg daily. Her renal function is normal and her most recent serum lithium concentration was within the therapeutic range. She begins taking over-the-counter ibuprofen regularly for a viral sore throat. Which pharmacokinetic interaction is most concerning?

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Reveal the answer and explanation

Correct answer: A — Reduced renal clearance of lithium, raising serum lithium and toxicity risk

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

C is correct. Lithium is eliminated almost entirely through the kidneys. NSAIDs such as ibuprofen inhibit renal prostaglandin synthesis and can reduce renal lithium clearance, increasing serum lithium concentrations and the risk of toxicity. NICE therefore advises patients taking lithium not to use over-the-counter NSAIDs; if an NSAID is unavoidable, lithium concentrations require closer monitoring. Lithium is not significantly metabolised, so CYP1A2 inhibition is irrelevant, excluding B. Ibuprofen does not characteristically reduce lithium absorption or increase its renal elimination, excluding A and D. Lithium is not appreciably bound to plasma proteins, so protein-binding displacement does not explain the interaction, excluding E.

Reference: Lithium Carbonate Essential Pharma 250 mg film-coated tablets, Summary of Product Characteristics, sections 4.2 and 4.5, revised 26 November 2024. https://www.medicines.org.uk/emc/product/10828/smpc