A 22-year-old is brought unconscious with miosis, bradycardia, coma, and profound respiratory depression following injection of an unknown drug. Naloxone produces dramatic reversal of all symptoms within minutes. What is the mechanism of naloxone's action in opioid overdose?
- A Reversal of GABA-A mediated respiratory depression
- B Stimulation of the carotid chemoreceptors to increase respiratory drive
- C Competitive antagonist at mu (μ), kappa (κ), and delta (δ) opioid receptors ✓
- D Enzymatic degradation of opioids to inactive metabolites
Explanation
Naloxone is a pure competitive opioid receptor antagonist with highest affinity at mu (μ) receptors, which mediate analgesia, euphoria, and respiratory depression — the most lethal effects of opioid overdose. By competitively displacing opioids from mu receptors, naloxone reverses miosis, respiratory depression, and coma within 1–3 minutes of IV administration. It also acts at kappa and delta receptors. Its half-life (60–90 minutes) is shorter than most opioids, so repeat dosing or infusion may be needed to prevent re-narcotisation. It has NO agonist activity ("pure" antagonist).
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
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Written and medically reviewed by the StethoPrep medical team.