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Lithium-NSAID Interaction — MRCPsych Paper B MCQ

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ModerateMood DisordersLithium-NSAID InteractionMRCPsych Paper B

A 32-year-old woman with bipolar disorder is stable on lithium 1000 mg daily. Her most recent 12-hour plasma lithium level was 0.7 mmol/L, with stable renal function. She starts taking regular ibuprofen for back pain. One week later, she develops nausea, a worsening tremor and unsteadiness when walking. Which pharmacological interaction best explains this deterioration?

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Reveal the answer and explanation

Correct answer: B — Ibuprofen reduces renal lithium clearance and raises serum levels

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

B is correct. NSAIDs inhibit renal prostaglandin synthesis, which can reduce renal perfusion and lithium clearance. Serum lithium may therefore rise despite an unchanged lithium dose, producing toxicity such as nausea, worsening tremor and ataxia. A previously therapeutic level does not protect against a subsequent drug interaction. Ibuprofen does not reduce lithium absorption, and lithium is renally eliminated rather than metabolised, so C and D are incorrect. Ibuprofen is not a serotonergic drug, making serotonin syndrome unlikely. NICE advises avoiding NSAIDs in people taking lithium where possible. If an NSAID is necessary, lithium concentrations require additional monitoring; the magnitude of the interaction varies, so a universal percentage increase should not be stated.

Reference: Ibuprofen 600 mg film-coated tablets, Summary of Product Characteristics, sections 4.4 and 4.5, updated 1 July 2026. https://www.medicines.org.uk/emc/product/7021/smpc