A patient receives morphine for severe postoperative pain and develops respiratory depression. Naloxone is administered. Within 2 minutes the patient is breathing normally but becomes agitated and in severe pain. What phenomenon explains why repeat naloxone doses will likely be needed?

• A Naloxone forms an irreversible bond with opioid receptors that reverses after 30 minutes
• B Morphine redistributes from peripheral tissues back to the CNS after naloxone clears
• C Naloxone causes tachyphylaxis at opioid receptors with repeat dosing
• D Naloxone has a shorter half-life (60-90 min) than morphine (4-6 hours), so renarcotization occurs as naloxone is eliminated ✓

Explanation

Naloxone has a plasma half-life of only 60-90 minutes, which is considerably shorter than most opioids. After initial reversal, as naloxone is metabolized and its receptor occupancy falls, free opioid receptors become accessible to residual morphine (or other opioids), causing recurrent respiratory depression ('renarcotization'). This is why clinical protocols for naloxone administration recommend continuous IV infusion (2/3 of the reversal dose per hour) rather than single bolus dosing when reversing long-acting opioids, or repeat boluses every 45-60 minutes with vigilant monitoring for at least 4-6 hours.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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