A patient on chronic high-dose morphine therapy for cancer pain requires increasing doses for the same analgesic effect. This tolerance does NOT develop equally to all morphine effects. Which opioid effect shows the LEAST tolerance?
- A Analgesia
- B Constipation and miosis ✓
- C Euphoria and sedation
- D Respiratory depression
Correct answer: B. Constipation and miosis
Explanation
Tolerance to opioids develops at different rates for different effects. Tolerance develops rapidly to analgesia, euphoria, sedation, and respiratory depression, necessitating dose escalation. However, tolerance to constipation (mediated by mu receptors in the myenteric plexus) and miosis (due to direct action on Edinger-Westphal nucleus) develops very slowly or minimally. This explains why opioid-dependent patients remain constipated and miotic even after years of use, while needing increasing doses for pain control.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.