A patient stabilised on high-dose oral morphine for cancer pain is given nalbuphine 10 mg intravenously for breakthrough pain. Within 20 minutes he develops sweating, abdominal cramps, diarrhoea, and piloerection. What is the mechanism?
- A Nalbuphine accelerates morphine glucuronidation, dropping plasma morphine levels abruptly
- B Nalbuphine induces serotonin release, producing a serotonergic withdrawal picture
- C Nalbuphine competitively displaces morphine from mu receptors while having lower intrinsic activity there ✓
- D Nalbuphine blocks kappa receptors that were mediating morphine's analgesia
Correct answer: C. Nalbuphine competitively displaces morphine from mu receptors while having lower intrinsic activity there
Explanation
Nalbuphine is a kappa agonist and a competitive antagonist (partial agonist) at mu receptors. In a physically dependent patient it displaces full agonists such as morphine from mu receptors but delivers far lower intrinsic activity, precipitating an acute abstinence syndrome. This same logic underlies buprenorphine precipitation of withdrawal. Kappa receptors mediate part of nalbuphine's own analgesia and are not blocked, eliminating option D.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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Written and medically reviewed by the StethoPrep medical team.