Lithium-Safe Antihypertensives — MRCPsych Paper B MCQ

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HardMood DisordersLithium-Safe AntihypertensivesMRCPsych Paper B

A 52-year-old man with bipolar I disorder has remained stable on lithium, with a therapeutic 12-hour serum lithium concentration and normal renal function. Pharmacological treatment is required for uncomplicated primary hypertension. Which of the following antihypertensive classes is least likely to increase his serum lithium concentration by reducing renal lithium clearance?

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Reveal the answer and explanation

Correct answer: B — Calcium-channel blockers, such as amlodipine

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

B is correct. ACE inhibitors, angiotensin II receptor blockers and thiazide-type diuretics can increase serum lithium concentrations and therefore increase the risk of toxicity. Calcium-channel blockers are less likely to produce this pharmacokinetic interaction and are the best answer among the listed classes. They are not completely interaction-free: the lithium SmPC reports that calcium-channel blockers may precipitate neurotoxicity, including ataxia, confusion or somnolence, even when the lithium concentration is therapeutic. Consequently, clinical vigilance and appropriate lithium monitoring remain necessary when treatment is changed. Option E is incorrect because the interaction profiles differ substantially. The original wording, asking which class was simply “safest”, obscured the important distinction between increased lithium concentration and possible pharmacodynamic neurotoxicity.

Reference: Essential Pharma, Lithium Carbonate Essential Pharma 250 mg film-coated tablets, Summary of Product Characteristics, section 4.5 “Interaction with other medicinal products and other forms of interaction”, updated 2025. https://www.medicines.org.uk/emc/product/10828/smpc