A patient maintained on high-dose morphine for cancer pain is given pentazocine for breakthrough pain. Within minutes he becomes agitated, sweats profusely, and complains of diffuse pain. What explains this response?

• A Pentazocine blocks kappa receptors that were mediating all of the analgesia
• B Pentazocine accelerates hepatic clearance of morphine through enzyme induction
• C Pentazocine converts morphine into normeperidine at the receptor level
• D Pentazocine displaces morphine from mu receptors while behaving as a weak partial agonist, precipitating withdrawal ✓

Explanation

Pentazocine is a kappa agonist with weak partial antagonist activity at the mu receptor. Because it binds mu receptors with appreciable affinity but low intrinsic activity, it competes off full agonists like morphine and precipitates an acute withdrawal syndrome. This is why agonist-antagonist opioids such as pentazocine, nalbuphine, and butorphanol are avoided in patients dependent on pure mu agonists.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.