Naloxone Mechanism — FRCA Primary MCQ
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Correct answer: D — It is a competitive antagonist at mu, delta and kappa opioid receptors, with greatest potency at mu receptors.
Explanation lettering: D = shown as B · B = shown as C · E = shown as D · C = shown as E
Naloxone competitively antagonises mu, delta and kappa opioid receptors and has greatest potency at the mu receptor. Displacement of opioid agonists from mu receptors accounts principally for reversal of respiratory depression, sedation, miosis and analgesia. It is neither receptor-selective nor irreversible, excluding B. Sigma, NMDA and GABA-A receptors are not its mechanism of opioid reversal, excluding A, C and D. Its clinical effect is often shorter than that of the causative opioid, so recurrent toxicity may require repeat doses or an infusion. Reversal is not invariably complete: respiratory depression caused by high-affinity or partial agonists such as buprenorphine may respond incompletely.
Reference: Martindale Pharma. Prenoxad 1 mg/ml Solution for Injection in a pre-filled syringe, Summary of Product Characteristics, section 5.1 Pharmacodynamic properties. Updated 2025. https://www.medicines.org.uk/emc/product/3054/smpc