skip to main content

Lamotrigine-Valproate — MRCPsych Paper B MCQ

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

HardMood DisordersLamotrigine-ValproateMRCPsych Paper B

A 58-year-old man with bipolar I disorder and predominantly depressive recurrences is established on valproate. Following specialist review, valproate is to be continued and lamotrigine is to be initiated for prevention of further depressive episodes. He takes no other medicines that induce or inhibit lamotrigine glucuronidation, and his renal and hepatic function are normal. Which lamotrigine dose-escalation regimen is most appropriate?

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

Reveal the answer and explanation

Correct answer: D — Weeks 1–2: 25 mg on alternate days; weeks 3–4: 25 mg daily; week 5: 50 mg daily; week 6: 100 mg daily

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

B is correct. Valproate inhibits lamotrigine glucuronidation, reducing its metabolism and approximately doubling its mean half-life. Lamotrigine must therefore be introduced using the valproate-specific regimen: 25 mg on alternate days in weeks 1–2, 25 mg daily in weeks 3–4, 50 mg daily in week 5 and usually 100 mg daily from week 6. The initial dose and escalation rate should not be exceeded because rapid titration and concomitant valproate increase the risk of serious rash. Option A is the standard schedule when neither valproate nor an enzyme inducer is present. Option C resembles the higher-dose schedule used with glucuronidation inducers in the absence of valproate. Option D omits essential titration. Valproate does not need to be stopped solely to initiate lamotrigine, so E is inappropriate.

Reference: electronic Medicines Compendium, Lamotrigine 50 mg tablets, Summary of Product Characteristics, sections 4.2 and 4.5, 2026. https://www.medicines.org.uk/emc/product/4737/smpc