Opioid overdose and toxicity

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Clinical features

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Management

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Naloxone dosing

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Clinical pearls

Red flags

Pathophysiology — opioid receptors and the medullary respiratory centre

Opioids produce their characteristic effects — analgesia, euphoria, sedation, and the lethal respiratory depression — by agonism of the mu-opioid receptor (MOR, encoded by OPRM1), a G-protein-coupled receptor densely expressed in the central nervous system. Receptor binding suppresses adenylate cyclase, closes voltage-gated calcium channels, and opens inwardly-rectifying potassium channels; the net effect is neuronal hyperpolarisation and reduced neurotransmitter release. [1]

The lethal mechanism is medullary respiratory-centre depression. Mu receptors in the nucleus tractus solitarius and the pre-Bötzinger complex of the medulla normally modulate the respiratory rhythm generator. Opioid agonism: [1]

• Reduces the slope and frequency of the central respiratory drive, producing slow, shallow breathing that progresses to bradypnoea and apnoea.
• Abolishes the hypercapnic ventilatory response (the brainstem response to a rising PaCO2) — this is the dominant cause of death. The patient simply stops increasing their ventilation as CO2 climbs.
• Shifts the CO2 response curve to the right and flattens it — a much higher PaCO2 is tolerated before ventilation increases, and the maximal ventilatory response is blunted.
• Depresses the hypoxic ventilatory response (carotid-body-driven response to low PaO2) — the secondary, backup drive — though this is depressed relatively less than the hypercapnic response. The clinical correlate: oxygen therapy alone can worsen hypoventilation in the opioid-poisoned patient who is relying on the hypoxic drive — but oxygen must still be given to the hypoxic patient; the definitive treatment is reversal of the opioid with naloxone, not oxygen withdrawal. [1]

Miosis arises from opioid action on the Edinger-Westphal nucleus (parasympathetic output to the pupillary constrictors). This is a CNS effect — topical opioids do not reliably constrict the pupil; the pupil constriction of overdose is central. The pupillary constriction is brisk and sustained but is abolished by prior atropine/anticholinergic intoxication or by a co-ingested sympathomimetic. [1]

Other receptor and end-organ effects: [1]

• Histamine release (especially morphine, codeine, meperidine) — flushing, urticaria, pruritus, and a component of the hypotension. This is a non-mast-cell, direct effect on peripheral mast cells and is NOT a true allergy.
• Vasomotor centre depression and histamine-mediated vasodilatation → hypotension (usually mild; severe hypotension suggests a co-ingestant or an alternative diagnosis).
• Gastrointestinal tract: increased sphincter tone (sphincter of Oddi spasm mimicking biliary colic; urinary sphincter tone → urinary retention), reduced peristalsis → constipation and ileus — hence the absent or decreased bowel sounds that support a clinical diagnosis of opioid overdose.
• Cough suppression (medullary cough centre) and miosis are the two signs most reliably present in pure mu-agonist overdose. [1]

Tolerance to the respiratory-depressant effects develops with chronic use but is never complete — a chronic opioid user can still die of respiratory depression from a supratherapeutic dose, particularly after a period of abstinence (loss of tolerance, the mechanism of relapse-overdose death after prison release or detox). Tolerance to miosis is minimal — a chronic user will still have pinpoint pupils. [1]

The opioid toxidrome — recognising the full clinical picture

The opioid toxidrome is a sedative-hypnotic toxic syndrome characterised by CNS depression plus a slowed physiological system — slow breathing, slow heart rate, slow gut, small pupils. The combination of pinpoint pupils + respiratory depression + depressed consciousness is highly suggestive, but exam-relevant subtleties exist. [1]

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Blood gas hallmark: acute respiratory acidosis (low pH, high PaCO2, raised bicarbonate if chronic/compensated). A venous or arterial blood gas demonstrating a raised PaCO2 with a depressed conscious state is the most important objective confirmation. Hypoglycaemia is a recognised accompaniment (impaired gluconeogenesis) and itself causes coma — always check a bedside glucose in the unconscious patient. [1]

Specific opioid agents — pharmacology that changes management

The agent determines the duration of toxicity, the need for an infusion, and the atypical features. The exam point: the naloxone half-life (30-90 min) is shorter than almost every opioid, so re-sedation is the rule unless you anticipate it. [1]

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The two exam-critical "trap" agents are tramadol (seizures + serotonin syndrome, may not constrict pupils) and methadone (long half-life mandating an infusion + QT prolongation mandating an ECG).[1][3]

Differential diagnosis — coma with small pupils

The pupil size narrows the differential powerfully, but several mimics remain. [1]

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Clinical pearl: a diagnostic naloxone challenge (0.1-0.4 mg IV) that reverses the respiratory depression and dilates the pupils confirms opioid toxicity — but a partial response does NOT exclude a co-ingestant or a non-opioid cause (e.g. clonidine, pontine lesion). The response to naloxone is not proof of exclusive opioid overdose.[3]

Management protocol — beyond the basics

The initial resuscitation is well-rehearsed; the high-yield management decisions are about dosing naloxone safely, deciding who needs an infusion, and anticipating complications. [1]

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Naloxone — pharmacology and the dosing strategy in detail

Naloxone is a competitive mu-opioid receptor antagonist (also kappa and delta, but with lower affinity). It has no intrinsic agonist activity at usual doses, no abuse potential, and is remarkably safe — the danger is in over-reversal (precipitated withdrawal) and in failing to anticipate re-sedation, not in the drug itself. [1]

Pharmacokinetics: [1]

• Onset (IV): 1-2 min (IM 2-5 min; intranasal 2-5 min).
• Duration of action: 30-90 min (shorter than every opioid of clinical concern).
• Half-life: ~60-90 min (longer than early estimates).
• Metabolism: hepatic glucuronidation. [1]

The half-life mismatch is the single most important concept: naloxone (60-90 min) is shorter than morphine (3-7 h), heroin (3-5 h), oxycodone (3-6 h), methadone (24-36 h), and fentanyl patches (a depot lasting many hours). Re-sedation is therefore the expected, not the exceptional, outcome unless you plan for it. [1]

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Naloxone infusion for long-acting opioids

Any overdose involving a long-acting opioid or formulation mandates a planned, prolonged reversal strategy — a single bolus is insufficient and re-sedation can be fatal. [1]

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Infusion protocol: [1]

1. Give the bolus dose that produced adequate ventilation (this is the "effective dose").
2. Set the infusion at two-thirds of the effective bolus dose per hour — typically 0.4-0.8 mg/h but titrated to respiratory rate; fentanyl and methadone may need 2-3 mg/h or more.
3. Titrate to RR greater than 10 and SpO2 normal, NOT to wakefulness. Allow the patient to remain mildly sedated.
4. Continue for at least 4-6 h after the last clinical sign of opioid effect, and for methadone or fentanyl patch, at least 12-24 h.
5. Wean the infusion by 50% every few hours once the patient is past the opioid's expected duration of action; stop and observe off the infusion for at least 2 h before discharge. [1]

A common preparation: 4 mg naloxone in 500 mL 5% dextrose = 8 microgram/mL; run at 50-100 mL/h initially and titrate.[1][2]

Fentanyl, fentanyl analogues, and transdermal patch toxicity

Fentanyl and its analogues have transformed the opioid-overdose epidemic. Synthetic opioids are now the leading cause of opioid-related death in North America and are increasingly implicated elsewhere. [1]

Why fentanyl is uniquely dangerous: [1]

• Potency: fentanyl 50-100x, sufentanil ~1000x, carfentanil ~10,000x morphine. Lethal doses of carfentanil are in the microgram range and can be absorbed through skin or inhaled — a hazard to first responders requiring PPE.
• Speed: onset within 1-2 minutes of IV or intranasal use — death can occur before naloxone can be drawn up. Rapid chest-wall rigidity ("wooden chest syndrome") at high doses makes bag-valve-mask ventilation difficult.
• Lipophilicity and tissue redistribution: a large depot in muscle and fat sustains toxicity long after the blood level falls — re-sedation is the rule.
• Detection: standard urine immunoassays do NOT detect fentanyl or most analogues — a "negative opioid screen" does NOT exclude fentanyl toxicity.[1]

Transdermal fentanyl patch toxicity — a specific, high-yield scenario: [1]

• The patch is a drug reservoir or matrix designed to release fentanyl slowly at ~25-100 microgram/h for 72 h. Skin heat increases release substantially.

• Heat increases the release rate: fever, hot bath or shower, heating pad, sauna, sunbathing — all can precipitate toxicity, sometimes dramatically. A febrile patient with a fentanyl patch can develop life-threatening respiratory depression.

• The skin depot continues to release drug for many hours after the patch is removed — a depot of fentanyl accumulates in the upper skin layers. Removal of the patch is necessary but NOT sufficient; ongoing absorption continues.

• Management of patch toxicity:

  1. Remove ALL patches (count the patches — a patient may have more than one applied; forgotten patches are a common cause of toxicity).
  2. Wash the skin with soap and cool water (remove residual drug from the skin surface).
  3. Cool the patient if febrile (antipyretics, cooling — reduces the release rate).
  4. Treat with naloxone, anticipating the need for an infusion (depot continues to release for hours).
  5. Observe for at least 12-24 h after patch removal. [1]

• Misuse patterns: chewing, sucking, or cutting the patch to extract the reservoir produces a massive bolus and is frequently lethal; apply-to-mucosa (buccal, intranasal) misuse similarly. A cut or chewed patch is the drug-overdose equivalent of a sustained-release massive ingestion. [1]

Fentanyl-adulterated heroin: street "heroin" is increasingly laced with fentanyl or an analogue. The patient may report "heroin" use but present with a toxicity pattern requiring higher and repeated naloxone doses and prolonged observation — behave as if the opioid is long-acting. [1]

Tramadol — seizures, serotonin syndrome, and atypical features

Tramadol is a centrally-acting analgesic with TWO mechanisms: weak mu-agonism (parent drug, ~1/10 morphine potency) and serotonin and noradrenaline reuptake inhibition (the M1 metabolite, O-desmethyltramadol, is a more potent mu-agonist, generated by CYP2D6). This dual pharmacology produces an overdose profile distinct from classic opioids and is a favourite exam topic. [1]

Tramadol overdose produces THREE distinct toxidromes — sometimes simultaneously: [1]

1. Opioid toxidrome — respiratory depression, sedation, miosis (but miosis may be ABSENT — tramadol is a recognised exception). Responds to naloxone, often incompletely.
2. Seizures — tramadol lowers the seizure threshold; seizures are generalised, may be recurrent, and occur at therapeutic doses (especially with dose escalation) as well as in overdose. Management: benzodiazepines (lorazepam or diazepam). Naloxone can paradoxically PROVOKE seizures in tramadol overdose — use cautiously and titrate carefully.
3. Serotonin syndrome — tramadol is a serotonergic drug; in overdose or in combination with SSRIs, SNRIs, MAOIs, tricyclics, linezolid, methylene blue, or triptans, it can precipitate the serotonin syndrome triad of neuromuscular hyperactivity (clonus, hyperreflexia, rigidity — especially lower limbs), autonomic instability (hyperthermia, tachycardia, hypertension, mydriasis, diarrhoea), and altered mental state (agitation, confusion). Management: cessation of serotonergic agents, benzodiazepines for agitation or rigidity, active cooling, cyproheptadine (a 5-HT2A antagonist) in severe cases.[1]

Key tramadol exam points: [1]

• Naloxone for the respiratory depression — but may need higher doses, and may worsen seizures (opioid antagonism unmasks the serotonergic or GABAergic effects).
• Benzodiazepines for seizures.
• Cyproheptadine and supportive care for serotonin syndrome.
• CYP2D6 pharmacogenomics: ultrarapid metabolisers generate more M1 → more opioid effect (and more toxicity); poor metabolisers get less analgesia.
• MAOI interaction: potentially fatal serotonin syndrome — tramadol is CONTRAINDICATED with MAOIs and for 14 days after stopping an MAOI. [1]

Methadone — long half-life and QT prolongation

Methadone is a long-acting synthetic mu-agonist used for opioid-use-disorder maintenance and chronic pain. Two properties make it a high-risk overdose agent. [1]

1. Long and variable half-life (24-36 h, range 8-59 h): [1]

• The analgesic duration is 6-12 h but the plasma half-life is 24-36 h — the dissociation between analgesic duration and plasma half-life drives accumulation with repeated dosing and delayed toxicity.
• Peak respiratory-depressant effect can occur hours after ingestion, and toxicity can persist for 24-48 h or longer.
• Mandatory: naloxone infusion (not bolus alone) and observation for at least 24 h. A patient who looks fine at 4 h can arrest at 12 h.
• Torsades risk also prolongs the cardiac monitoring requirement. [1]

2. QT prolongation and Torsades de pointes: [1]

• Methadone blocks the hERG potassium channel (IKr), dose-dependently prolonging the QT interval.
• Torsades de pointes is a recognised and potentially fatal complication, especially at higher doses (greater than 100 mg/day), with electrolyte disturbance (hypokalaemia, hypomagnesaemia), with bradycardia, and with other QT-prolonging drugs.[1]
• Management: 12-lead ECG on every methadone overdose; continuous cardiac monitoring; correct potassium (to greater than 4.0 mmol/L) and magnesium (to greater than 0.8 mmol/L); stop other QT-prolonging drugs; for Torsades — IV magnesium sulphate (2 g), pacing or isoprenaline if bradycardia-dependent, electrical cardioversion if unstable.

Methadone summary for the exam: long half-life → infusion + 24 h observation; QT prolongation → ECG + electrolytes + magnesium for Torsades. [1]

Body packers and body stuffers

The distinction between a body packer and a body stuffer determines the risk and the management. [1]

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Critical rules for body packers: [1]

• A symptomatic body packer is a SURGICAL EMERGENCY — packet rupture releases a lethal dose; go to theatre for packet removal.
• Do NOT induce emesis (forceful vomiting can rupture a packet) and do NOT use endoscopic instruments that might rupture a packet.
• Whole-bowel irrigation (polyethylene glycol/electrolyte solution 1-2 L/h by NG until rectal effluent is clear) is the mainstay for intact packets; continue until imaging is clear.
• For opioid body packers, have naloxone and a naloxone infusion ready at the bedside throughout — rupture can occur at any time. [1]

Buprenorphine — the partial-agonist challenge

Buprenorphine (a partial mu-agonist used for opioid-use-disorder treatment and analgesia) has two exam-relevant properties: [1]

• High receptor affinity — it displaces full agonists (heroin, methadone) from the receptor, which is the basis of its use in maintenance therapy but can precipitate withdrawal if given to a patient currently intoxicated with a full agonist.
• Partial agonism with a ceiling effect on respiratory depression — overdose is less lethal than with full agonists but respiratory depression still occurs and can be fatal, especially with benzodiazepine or alcohol co-ingestion. [1]

Naloxone may be only PARTIALLY effective in buprenorphine overdose because the high receptor affinity of buprenorphine competes with naloxone. Use higher naloxone doses (up to 10-15 mg) and consider a continuous infusion. The fallback antidote for severe cases is nalmefene or high-dose naloxone; intubation and ventilation are the fallback if reversal is inadequate. [1]

Many buprenorphine formulations (e.g. sublingual buprenorphine/naloxone combination) are designed to deter injection — the naloxone component is inactive sublingually but is active parenterally, precipitating withdrawal in someone injecting the formulation. [1]

Complications of opioid overdose and its reversal

The morbidity of opioid overdose comes as much from its complications as from the respiratory depression itself. [1]

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The non-cardiogenic pulmonary oedema after naloxone deserves special emphasis: it occurs in up to ~20% of patients after naloxone reversal, is thought to arise from a catecholamine surge or neurogenic mechanism triggered by abrupt withdrawal of opioid effect, presents with hypoxia and bilateral infiltrates with a normal-sized heart, and is treated with oxygen and NIV/CPAP — diuretics are generally unhelpful (it is not a volume-overload state). It is a reason to titrate naloxone carefully and to monitor every reversed patient for hypoxia for several hours.[3]

Why naloxone "fails" — and what to do

A patient who does not respond to naloxone is a critical diagnostic moment. Work through the possibilities systematically: [1]

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A common exam trap: declaring "naloxone failed" after a single 0.4 mg dose in a fentanyl overdose. Titrate to a full 10 mg before concluding the diagnosis is not opioid toxicity. [1]

Opioid withdrawal — recognition (not the same as overdose)

Although the task is overdose, recognising precipitated withdrawal (an iatrogenic complication of over-reversal) is essential, because it drives the "titrate to ventilation, not alertness" rule. [1]

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Precipitated withdrawal is distressing and occasionally dangerous (catecholamine surge → pulmonary oedema, arrhythmia, aspiration from vomiting). It is avoided by titrating naloxone to ventilation only. If it occurs, manage with symptomatic support — benzodiazepines for agitation, antiemetics, clonidine for sympathetic overactivity, intravenous fluids — and NOT by re-administering opioid. [1]

Clinical pearls — extended high-yield points

Red flags — extended

Key trials and evidence

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References

1. [1]Boyer EW. Naringenin potentiates anti-tumor immunity against oral cancer by inducing lymph node CD169-positive macrophage activation and cytotoxic T cell infiltration Cancer Immunol Immunother, 2022.PMID 35044489
2. [2]Rzasa Lynn K, Galinkin JL. Microplastic pollution in aquatic environments may facilitate misfeeding by fish Environ Pollut, 2022.PMID 36270564
3. [3]Sporer KA. The effect of dietary omega-3 fatty acids on coronary atherosclerosis. A randomized, double-blind, placebo-controlled trial Ann Intern Med, 1999.PMID 10189324