skip to main content

Lithium-ACE Inhibitor Interaction — MRCPsych Paper B MCQ

Instant feedback + full explanation. One question, done properly.

ModerateMood DisordersLithium-ACE Inhibitor InteractionMRCPsych Paper B

A 55-year-old man with bipolar disorder has stable renal function and a stable 12-hour plasma lithium concentration while taking lithium carbonate. Ramipril is started for hypertension. What is the most likely pharmacokinetic effect on lithium?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: C — Increased plasma lithium concentration from reduced renal excretion

Ramipril is an ACE inhibitor and can reduce the renal excretion of lithium, increasing its plasma concentration and the risk of toxicity. Lithium has a narrow therapeutic index, so its level and renal function should be monitored when an interacting medicine is introduced, with lithium dose adjustment if required. Lithium is eliminated almost entirely through the kidneys rather than hepatic metabolism, making D incorrect. ACE inhibition does not increase lithium excretion, so C is incorrect, and this interaction is not mediated by reduced intestinal absorption, excluding E. A is unsafe because a clinically important rise in lithium concentration can occur, although the magnitude varies between patients.

Reference: Lithium Carbonate Essential Pharma 250 mg film-coated tablets, Summary of Product Characteristics, section 4.5: Interaction with other medicinal products, revised 26 November 2024. https://www.medicines.org.uk/emc/product/10828/smpc