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Lithium Toxicity — RACP Adult Medicine MCQ

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ModerateClinical PharmacologyLithium ToxicityRACP Adult Medicine

A 35-year-old man with known bipolar disorder on lithium presents with tremor, polyuria, and polydipsia. His serum lithium level is 1.8 mmol/L (therapeutic range 0.6–0.8 mmol/L for maintenance). He recently commenced ibuprofen for back pain. What is the most likely mechanism of lithium toxicity?

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Correct answer: E — Ibuprofen reduces renal lithium clearance via prostaglandin inhibition

NSAIDs reduce renal lithium clearance by inhibiting prostaglandin-mediated renal blood flow, reducing GFR, and enhancing proximal tubular lithium reabsorption. This predictably elevates lithium levels by 25–50%. All NSAIDs (except low-dose aspirin and sulindac at some doses) cause this interaction. Patients on lithium should avoid NSAIDs or have lithium levels monitored closely.

Reference: AMH – 2025 – Drug Interactions; eTG – 2025 – Psychotropic