Pharmacology · Opioid Pharmacology

Adverse Effects, Tolerance, and Opioid Overdose

Organ system effects, tolerance patterns, overdose triad, and naloxone reversal

Opioid adverse effects map directly onto mu receptor distribution: sedation and euphoria (CNS), respiratory depression (pre-Bötzinger complex), constipation (enteric neurons), nausea (area postrema), pruritus (spinal cord), and urinary retention (urethral sphincter).
Tolerance develops to analgesia, euphoria, sedation, nausea, and respiratory depression — but never to constipation or miosis; these two effects persist throughout the entire course of chronic opioid therapy.
The opioid overdose triad is coma, pinpoint pupils, and respiratory depression — respiratory depression is the primary cause of death, and airway management takes priority over pharmacological reversal.
Naloxone is a pure competitive antagonist at all three opioid receptors with a duration of only 30–90 minutes — shorter than most opioids it reverses — making resedation a real risk with long-acting or extended-release formulations.
Opioid-induced hyperalgesia is distinguished from tolerance by a paradoxical response: dose escalation worsens pain, and dose reduction improves it — the opposite of tolerance.

Adverse Effects by System

System	Effect	Mechanism	Clinical Note
Central nervous system	Sedation	Mu activation in reticular activating system	Partial tolerance develops; worst at initiation
Central nervous system	Miosis	Mu activation at Edinger-Westphal nucleus	Tolerance does NOT develop; persists in chronic users
Central nervous system	Euphoria	Mu activation in nucleus accumbens	Tolerance develops rapidly; drives dose escalation in opioid use disorder
Respiratory	Respiratory depression	Mu activation in pre-Bötzinger complex	Primary cause of death; blunts carbon dioxide drive
Gastrointestinal	Constipation	Peripheral mu activation in enteric neurons	Tolerance does NOT develop; requires prophylactic bowel regimen
Gastrointestinal	Nausea and vomiting	Mu activation at area postrema	Partial tolerance develops within days to weeks
Urinary	Urinary retention	Peripheral mu activation; urethral sphincter tone	More common with neuraxial opioids; more common in men
Skin	Pruritus	Central mu activation at spinal cord	Central mechanism, not histamine; antihistamines only partially effective

The Opioid Overdose Triad

Sign 1

Coma

Unresponsive to stimulation
Mu activation in reticular activating system
Low Glasgow Coma Scale score

Sign 2

Miosis

Pinpoint pupils (1–2 mm)
Edinger-Westphal nucleus activation
Present even in chronic users (no tolerance)
Absent miosis: consider co-ingestion

Sign 3

Respiratory Depression

Slow, shallow, or absent breathing
Pre-Bötzinger complex suppression
Primary cause of death
Airway and ventilation: first priority

Naloxone: Reversal Agent

Pharmacology

Mechanism and Kinetics

Pure competitive mu, kappa, and delta antagonist
No intrinsic agonist activity
Onset (intravenous): 1–5 minutes
Duration: 30–90 minutes
Shorter acting than most opioids it reverses
Routes: intravenous, intramuscular, intranasal spray

Clinical Risks

Resedation and Withdrawal

Resedation: naloxone wears off while opioid still active
Highest risk: methadone, extended-release opioids, transdermal fentanyl
Repeat doses or infusion may be needed
In dependent patients: precipitates acute withdrawal
Goal: restore breathing, not full reversal

Tolerance: What Tolerizes and What Does Not

Tolerance develops to: analgesia, euphoria, sedation, respiratory depression, nausea.

Tolerance does NOT develop to: constipation, miosis. These persist throughout the course of chronic opioid therapy regardless of duration or dose.

Opioid-induced hyperalgesia: dose escalation worsens pain rather than improving it. Paradoxical improvement with dose reduction distinguishes it from tolerance.

Suggested References

Author / Organization	Title	Source
Katzung BG (ed)	Basic and Clinical Pharmacology, 15th ed. Chapter 31: Opioid Analgesics and Antagonists	McGraw-Hill, 2021
Brunton LL, Knollmann BC (eds)	Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed. Chapter 20: Opioids, Analgesia, and Pain Management	McGraw-Hill, 2023
Pattinson KT	Opioids and the control of respiration	Br J Anaesth. 2008;100(6):747–758
Swegle JM, Logemann C	Management of common opioid-induced adverse effects	Am Fam Physician. 2006;74(8):1347–1354
Lee M, Silverman SM, Hansen H, Patel VB, Manchikanti L	A comprehensive review of opioid-induced hyperalgesia	Pain Physician. 2011;14(2):145–161
Boyer EW	Management of opioid analgesic overdose	N Engl J Med. 2012;367(2):146–155
Moss RB, Carlo DJ	Higher doses of naloxone are needed in the synthetic opioid era	Subst Abuse Treat Prev Policy. 2019;14(1):6
Dahan A, Aarts L, Smith TW	Incidence, reversal, and prevention of opioid-induced respiratory depression	Anesthesiology. 2010;112(1):226–238
Chu LF, Angst MS, Clark D	Opioid-induced hyperalgesia in humans: molecular mechanisms and clinical considerations	Clin J Pain. 2008;24(6):479–496