A patient stabilized on chronic morphine therapy takes pentazocine for breakthrough pain and within minutes develops lacrimation, diarrhea, abdominal cramps, and piloerection. The most likely explanation is:

• A Pentazocine converts morphine into normeperidine in the gut wall
• B Pentazocine accelerates hepatic clearance of morphine through enzyme induction
• C Pentazocine acts as a mu-antagonist and displaces morphine from receptors, precipitating withdrawal ✓
• D Pentazocine blocks morphine reuptake at sympathetic nerve terminals

Explanation

Pentazocine is an agonist-antagonist opioid: it stimulates kappa receptors while exerting weak antagonist or partial agonist activity at mu receptors. In a physically dependent patient this mu-antagonist component abruptly removes full agonist occupancy and precipitates an acute abstinence syndrome. Enzyme induction takes days to weeks and cannot act within minutes, killing option B. The same property limits pentazocine's abuse potential but makes it unsuitable alongside full agonists.

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

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Written and medically reviewed by the StethoPrep medical team.