All of the following are recognized adverse effects of opioid analgesics EXCEPT:

Correct answer: D. Diarrhea

• A. Respiratory depression is the effect that kills. Opioids blunt the medullary chemoreceptor response to rising CO₂, and the risk multiplies when combined with benzodiazepines or alcohol.
• B. Constipation IS a classic opioid effect — see the correct answer.
• C. Nausea and vomiting are very common early effects, mediated at the chemoreceptor trigger zone, and usually subside with continued use.
• D. CORRECT as the EXCEPTION. Opioids cause CONSTIPATION, not diarrhea. Mu-receptor stimulation in the myenteric plexus reduces propulsive peristalsis and increases sphincter tone and water absorption. This is the one opioid effect to which tolerance essentially never develops, which is why every patient on chronic opioids needs a bowel regimen from day one.

Note: Opioid effect set to memorize: analgesia, sedation, euphoria, respiratory depression, MIOSIS (pinpoint pupils), constipation, nausea, urinary retention, pruritus and cough suppression. Tolerance develops to most — but not to constipation or miosis.