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

Correct answer: C. Hyperventilation

• A. SEDATION and drowsiness are dose-related central effects and one of the reasons patients must not drive after opioid administration.
• B. DIAPHORESIS is a well-documented opioid effect, occurring both as a direct pharmacological action and prominently during withdrawal.
• C. CORRECT — this is the EXCEPTION. Opioids produce RESPIRATORY DEPRESSION, the exact opposite of hyperventilation. They blunt the sensitivity of the medullary respiratory centre to rising carbon dioxide, so respiratory rate and tidal volume fall. This is the mechanism of death in opioid overdose and the reason naloxone is the reversal agent.
• D. CONSTIPATION is the classic opioid effect that does NOT undergo tolerance. Activation of mu receptors in the myenteric plexus reduces gut motility, and patients on chronic therapy require a scheduled bowel regimen indefinitely.

Note: The classic opioid effect cluster: miosis (pinpoint pupils), respiratory depression, sedation, constipation, nausea/vomiting, urinary retention, pruritus (histamine release) and biliary spasm. The overdose triad tested most often is pinpoint pupils + respiratory depression + coma.