Naltrexone Mechanism — MRCPsych Paper B MCQ

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EasySubstance MisuseNaltrexone MechanismMRCPsych Paper B

A 55-year-old man with alcohol dependence has completed assisted withdrawal and remains abstinent, but continues to experience strong cravings. He is prescribed oral naltrexone 50 mg daily. What is the primary pharmacological action of naltrexone?

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Correct answer: D — Competitive antagonism of opioid receptors

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

Naltrexone is a long-acting, specific opioid receptor antagonist that binds competitively to central and peripheral opioid receptors (B). Alcohol consumption stimulates the endogenous opioid system, contributing to its rewarding and reinforcing effects; by blocking opioid receptor activation, naltrexone reduces craving and the risk that a lapse escalates into full relapse. Note that it blocks receptor activation rather than preventing endogenous opioid release, and it is non-aversive, causing no disulfiram-like reaction. Distractors: positive allosteric modulation of GABA-A receptors (A) describes benzodiazepines used in assisted withdrawal; serotonin reuptake inhibition (C) describes SSRIs, which have no specific licensed role in relapse prevention; aldehyde dehydrogenase inhibition (D) is the aversive mechanism of disulfiram; modulation of glutamatergic NMDA-related activity (E) is attributed to acamprosate. NICE CG115 recommends acamprosate or oral naltrexone (after specialist initiation) for relapse prevention following assisted withdrawal in moderate-to-severe alcohol dependence.

Reference: Adepend 50 mg film-coated tablets (naltrexone hydrochloride), Summary of Product Characteristics, section 5.1 Pharmacodynamic properties, eMC, updated 2020. https://www.medicines.org.uk/emc/product/3559/smpc