A patient maintained on high-dose morphine for cancer pain is given nalbuphine for breakthrough pain. Within minutes he becomes agitated, tachycardic and complains of diffuse pain. The most likely mechanism is:

• A Nalbuphine's mu-antagonist activity displacing morphine from receptors and precipitating withdrawal ✓
• B Allergic reaction to nalbuphine causing catecholamine release
• C Additive respiratory depression leading to hypercapnic encephalopathy
• D Nalbuphine-induced histamine release causing a pseudo-withdrawal syndrome

Explanation

Nalbuphine is a mixed agonist-antagonist: it is a kappa agonist and a competitive mu-receptor antagonist. In a physically dependent patient its mu antagonism displaces morphine from receptors and abruptly precipitates an abstinence syndrome with agitation, tachycardia and pain. Mixed agonist-antagonists such as nalbuphine, pentazocine and butorphanol must be avoided in opioid-dependent patients for this reason. Histamine release and allergy do not reproduce the specific withdrawal picture.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.