Lithium Dehydration Risk — MFDS Part 1 MCQ

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ModerateHuman DiseaseLithium Dehydration RiskMFDS Part 1

At a dental appointment, a patient taking maintenance lithium reports 24 hours of vomiting and markedly reduced fluid intake and appears dehydrated. Which effect on lithium is most likely?

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Reveal the answer and explanation

Correct answer: E — Increased serum lithium due to reduced renal clearance

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

The correct answer is A. Lithium has a narrow therapeutic index and is eliminated almost entirely by the kidneys. Dehydration and associated sodium depletion promote renal conservation of sodium and lithium, reducing lithium clearance and potentially increasing its serum concentration to toxic levels. Warning features include worsening tremor, gastrointestinal disturbance, ataxia, dysarthria, confusion and drowsiness. Vomiting and poor fluid intake are therefore clinically important findings in a dental patient taking lithium and warrant urgent medical advice. Dehydration does not increase lithium clearance or cause rapid elimination, so B and E are incorrect. Lithium is not primarily hepatically eliminated, excluding C. Any apparent increase in effect during dehydration may represent toxicity rather than improved therapeutic efficacy, excluding D.

Reference: NHS Specialist Pharmacy Service. Supporting lithium safety across the system: Patient education and toxicity sections. 2026. https://sps.nhs.uk/articles/supporting-lithium-safety-across-the-system/