A 30-year-old heroin addict is found unconscious with pinpoint pupils, bradycardia, and respiratory rate 6/min. After two doses of IV naloxone he wakes up, but 45 minutes later lapses back into coma. The BEST explanation for this recurrence is:

• A Heroin metabolises to 6-monoacetylmorphine which is naloxone-resistant
• B Naloxone undergoes rapid renal clearance and is excreted before opioid is metabolised
• C Tolerance to naloxone develops within 45 minutes in opioid-dependent individuals
• D Naloxone has shorter half-life (~30–90 minutes) than most opioids, requiring repeated dosing or infusion ✓

Explanation

Naloxone has a plasma half-life of approximately 30–90 minutes (shorter when given IV), which is considerably shorter than that of many opioids (heroin/morphine ~2–4 hours; methadone ~24–36 hours; buprenorphine ~24 hours). As naloxone is cleared while the opioid remains, re-narcotisation occurs. Management requires repeated IV naloxone boluses or a continuous IV infusion (typically two-thirds of the effective bolus dose per hour) and monitoring for ≥4 hours. 6-MAM is not naloxone-resistant.

Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.

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