Why this poison is interesting

Opioid poisoning is mechanistically the simplest thing in Band A and clinically among the most consequential. There is no bioactivation to describe for most of the class, no organ destroyed, no metabolic acidosis, no latent phase and no lasting injury in anyone who is ventilated in time. A µ-opioid receptor agonist reaches a population of brainstem neurons and they stop driving breathing. Everything else is a consequence of hypoxia.

Three things make it worth a mechanism page anyway.

It is the only poison in Band A whose antidote is given to reverse a poisoning that is killing the patient. Flumazenil is an equally genuine competitive antagonist, and oxygen displaces carbon monoxide from haem by mass action — but see benzodiazepines for why an analogous antidote is usually withheld. The difference is not the pharmacology; it is what the poisoning is doing.
The antidote's kinetics create the next problem. Naloxone's short half-life against a long-acting agonist produces renarcotisation, and this is a purely pharmacokinetic phenomenon that follows from two half-lives failing to match.¹
Two members of the class are pharmacogenetic poisons. Codeine and tramadol are prodrugs requiring CYP2D6, and the UK labels contraindicate codeine outright in known ultra-rapid metabolisers.³ That is a minor-pathway story hidden inside a class that otherwise has none.

And a fourth, which is why this page exists now rather than in a later band: the drugs have changed. Nitazenes — 2-benzylbenzimidazole opioids originally developed in the 1950s and abandoned because of their high toxicity and unfavourable therapeutic index — have reappeared in illicit supply, and their pharmacology raises real questions about how long a reversed patient should be watched.²

A poison is a drug whose kinetics have escaped its pharmacology.

The toxic principle

The poison is whatever molecule occupies the µ-opioid receptor, and for most of the class that is the drug as taken. The clinically important complications are the cases where it is not.

Direct agonists: Morphine, diamorphine (heroin), oxycodone, methadone, buprenorphine (a partial agonist), fentanyl and its analogues, and the nitazenes. The parent is the poison. Established
Prodrugs requiring CYP2D6: Codeine is metabolised by CYP2D6 into morphine, its active metabolite.³ Tramadol behaves similarly for its O-desmethyl metabolite. In these two, the enzyme determines the exposure — see the metabolism section. Established
Active metabolites that accumulate: Morphine-6-glucuronide is a potent µ-agonist cleared renally. In renal impairment it accumulates while morphine itself appears to be handled normally, producing delayed and prolonged toxicity from an apparently modest dose. Inferred

What receptor occupancy actually does

µ-opioid receptors are inhibitory G-protein-coupled receptors. The lethal effect is not global CNS depression — it is the loss of a specific respiratory function. The PNAS work on this identifies µ-opioid-receptor-expressing neurons in the lateral parabrachial nucleus as critical: their activity is tightly correlated with respiratory rate, and that correlation is abolished following morphine injection. Chemogenetically silencing them reproduces opioid-induced respiratory depression in mice; activating them after morphine restores respiratory rhythm to baseline.¹ Established in the mouse model; the extrapolation to the human brainstem is Inferred

Toxicokinetics

Opioids — the class is heterogeneous, so read this as a set of variables rather than as one drug
Parameter	Therapeutic	In overdose	Why it changes
Absorption	Route decides everything: intravenous is seconds, insufflated or smoked is a minute or two, immediate-release oral is 30–60 minutes	Modified-release oral preparations and transdermal patches produce a rising concentration for many hours, and a fentanyl patch continues to release from a skin depot after removal	This is the single most important variable in disposition. A reversed intravenous heroin overdose and a reversed modified-release oxycodone overdose look identical at ten minutes and are entirely different problems at four hours. Naloxone's duration must be matched to the agonist's absorption, not to its own effect.
Protein binding	Variable across the class; low for morphine, high for methadone and fentanyl	Not the limiting factor in any clinically relevant scenario	Nothing here saturates in a way that changes the picture. Opioid poisoning is not a binding poisoning.
Volume of distribution	Wide range: morphine is modest; methadone and fentanyl are lipophilic with large volumes and long terminal phases	Unchanged	The large volume of the lipophilic opioids explains their long tail and, with fentanyl, the phenomenon of re-sedation as drug redistributes back out of fat. It also explains why extracorporeal removal is pointless for exactly the drugs in which prolonged toxicity occurs.
Half-life	Diamorphine and its active metabolites, minutes to hours; morphine 2–4 h; methadone very long and highly variable between individuals; fentanyl short after a single dose but context-sensitive	Prolonged by co-ingested sedatives, by hepatic impairment, and — for morphine-6-glucuronide — by renal impairment	The half-life that matters clinically is the mismatch with naloxone's. Naloxone is short-acting; methadone, modified-release oxycodone and a fentanyl depot are not. Renarcotisation is not an idiosyncrasy — it is arithmetic.
Elimination route	Hepatic metabolism, with renally excreted conjugates. Codeine requires CYP2D6 to become morphine³; morphine is glucuronidated to morphine-3- and morphine-6-glucuronide	CYP2D6 activity is genetically determined and varies widely between populations³; morphine-6-glucuronide accumulates in renal impairment	Codeine is the class's pharmacogenetic outlier: the same tablet is an inadequate analgesic in a poor metaboliser and a morphine overdose in an ultra-rapid one.³ See the metabolism section — the UK label carries the prevalence table.
Order of kinetics	First order	First order	No saturation. Opioid poisoning is dose-and-receptor, not kinetics-escaping-pharmacology — which is why this page's Why this poison is interesting section says so explicitly rather than manufacturing a pattern.
Dialysability	—	Not clinically relevant, and EXTRIP has published no opioid recommendation⁴	The absence is easy to justify. There is an effective antidote, ventilation is definitive supportive care, and the opioids that produce prolonged toxicity are precisely the lipophilic, large-volume ones a dialyser could not clear anyway — the same arithmetic that defeats removal in tricyclic antidepressant poisoning.

Metabolism and the metabolites

For most of the class this section would be short. Codeine makes it worth writing, because codeine is the clearest example in UK practice of a polymorphic enzyme deciding the dose.

Codeine — a prodrug whose activation rate is inherited

CodeineWeak µ-agonist in its own right; the analgesia is largely from the metabolite³
CYP2D6 O-demethylationMorphineThe active metabolite. A small percentage of the dose — but the percentage is genetically determined³
Glucuronidation and N-demethylationCodeine-6-glucuronide, norcodeineThe bulk of the dose. Not the source of the effect
Morphine
UGT2B7 glucuronidationMorphine-3-glucuronideNot a µ-agonist
UGT2B7 glucuronidationMorphine-6-glucuronideA potent µ-agonist, cleared renally — accumulates when the kidneys do not work

What the UK label says, and why it is a public-health statement

Two UK codeine phosphate SmPCs from different manufacturers were checked and agree.³ Both state that codeine is metabolised by CYP2D6 into morphine, that a deficient patient will not obtain an adequate effect — up to 7% of the Caucasian population may have this deficiency — and that an ultra-rapid metaboliser has an increased risk of opioid toxicity even at commonly prescribed doses, converting codeine to morphine rapidly and producing higher than expected serum morphine.³

Estimated prevalence of CYP2D6 ultra-rapid metabolisers, from the UK codeine SmPCs³
Population	Prevalence
African / Ethiopian	29%
Greek	6.0%
African American	3.4% to 6.5%
Caucasian	3.6% to 6.5%
Asian	1.2% to 2%
Hungarian	1.9%
Northern European	1% to 2%

What changes the answer

Loss of tolerance. The commonest fatal scenario in opioid dependence is a return to a previous dose after abstinence — release from prison, discharge from hospital, completion of detoxification. The receptor pharmacology has not changed; the person's adaptation to it has. Established
Co-ingested sedatives. Benzodiazepines, gabapentinoids, alcohol and sedating antihistamines each depress respiration weakly on their own and substantially in combination with an opioid. Most opioid deaths involve more than one drug — the nitazene review found all cases reviewed had several opioids and other sedating drugs in addition to nitazenes in their analytical workup.²
Renal impairment, through morphine-6-glucuronide accumulation. Inferred
Which opioid. The class spans four orders of magnitude of potency and an enormous range of duration. "Opioid overdose" is a syndrome, not a pharmacology.

Elimination and accumulation

There is no accumulation problem in a simple heroin overdose — the drug is cleared, the patient wakes up, and nothing has been damaged. The accumulation problems are all specific:

Methadone, whose half-life is long, highly variable between individuals, and longer than most people's estimate of it. A methadone overdose that has been reversed is a patient who will need reversing again.
Modified-release preparations, where absorption continues for hours after presentation — the same phenomenon as gastro-resistant aspirin in salicylate poisoning, and with the same consequence for observation periods.
Transdermal fentanyl, which continues to release from a cutaneous depot after the patch is removed.
Morphine-6-glucuronide in renal failure, producing toxicity that appears late and resolves slowly. Inferred
Body packing and stuffing, where the reservoir is not pharmacological at all and the risk is catastrophic release.

Target organs — and why those

Brainstem — the respiratory network

Targetµ-opioid receptors on neurons of the pontine and medullary respiratory network

Why hereBecause that is where the receptors that matter are, and because respiratory rhythm has no redundancy. Work in mice localises a critical population to the lateral parabrachial nucleus: the activity of µ-opioid-receptor-expressing neurons there is tightly correlated with respiratory rate, and morphine abolishes that correlation; silencing them reproduces the syndrome and reactivating them restores baseline breathing.¹ The chemoreceptor response to carbon dioxide is blunted at the same time, so the usual protective reflex — rising CO₂ driving harder breathing — is removed. That blunted hypercapnic response is established in human volunteers; it is not demonstrated by the mouse work cited here. Established in the mouse model; the extrapolation to the human brainstem is Inferred.

At the bedsideA falling respiratory rate in a patient who is not yet unconscious is the finding. By the time they are apnoeic the diagnosis is obvious and the hypoxic injury has started.

Brain — hypoxic injury

TargetEverything, by the ordinary mechanism

Why hereThis is not opioid pharmacology at all, and that is the point worth making: opioids do not damage the brain, hypoxia does. A patient ventilated from the moment of collapse has no lasting injury; a patient found some hours later may have profound hypoxic–ischaemic damage from a drug that was, pharmacologically, entirely reversible. The organ injury is a function of time, not of dose. Established

At the bedsideThis is why the intervention that saves brains is airway and ventilation rather than naloxone, and why bystander naloxone programmes save lives mainly by shortening the hypoxic interval.

Lungs

TargetThe alveolar–capillary membrane

Why hereNon-cardiogenic pulmonary oedema is a recognised complication of opioid overdose, classically after heroin and sometimes appearing or worsening after naloxone reversal. The mechanism is not settled — proposed contributions include hypoxic capillary injury, a surge in catecholamines and negative-pressure effects from attempted inspiration against a closed glottis. Inferred

At the bedsideA patient who is hypoxic after successful reversal, with an adequate respiratory rate, has a lung problem rather than a receptor problem, and further naloxone will not help.

Chest wall and larynx — fentanyl analogues

TargetCentral µ-receptor-mediated effects on muscle tone

Why hereRapid, high-potency µ-agonism can produce marked rigidity of the chest wall and vocal cords — the so-called wooden chest — which is a mechanical failure of ventilation rather than a failure of respiratory drive. It is described predominantly with fentanyl and its analogues, and predominantly with rapid high-dose exposure. Inferred

At the bedsideIt matters because it changes what fails first: bag-mask ventilation may be impossible in a patient whose problem naloxone would otherwise fix. Neuromuscular blockade and intubation may be needed for a poisoning that has an antidote.

Timeline of effects

Opioids — the course, and where the antidote inserts its own timeline

Time

What you seeWhat is happening

Seconds to hoursOnset
What you seeSedation, then a falling respiratory rate, then apnoea. Miosis in most but not all. Route decides the tempo entirely — seconds intravenously, hours for a modified-release tablet.
What is happeningµ-receptor occupancy in the brainstem respiratory network reduces respiratory rate and blunts the hypercapnic ventilatory response.¹ Nothing is being damaged yet: this stage is fully reversible and the patient is pharmacologically, not structurally, unwell.
Minutes after thatHypoxia
What you seeCyanosis, bradycardia, then hypoxic cardiac arrest. Where the patient is found late: aspiration, rhabdomyolysis from a prolonged period on the floor, and hypoxic brain injury.
What is happeningThis is where the harm is done, and it is not opioid harm. The drug is doing exactly what it was doing five minutes earlier; the difference is that oxygen delivery has now failed. Every intervention that shortens this interval — bystander naloxone, basic airway support, recovery position — acts on this line and not on the one above it.
0–2 min after naloxoneReversal
What you seeRespiratory rate recovers. In the dependent patient, acute withdrawal: agitation, vomiting, diarrhoea, pain, aggression. Occasionally acute pulmonary oedema.
What is happeningCompetitive displacement at the µ-receptor. The abruptness is the problem — a competitive antagonist given in excess does not just restore breathing, it strips receptor occupancy from a brain adapted to it, and the PNAS review notes naloxone's potential to induce a catecholamine surge at high doses, which can cause cardiopulmonary arrest.¹ Established
20–90 min after naloxoneRenarcotisation
What you seeThe patient, now awake and often wanting to leave, becomes drowsy again and stops breathing. There is nothing to see in the interval — they look well, and they may look well for an hour.
What is happeningPure pharmacokinetics. Naloxone's duration of action is shorter than that of most opioids taken in overdose, so as naloxone concentrations fall the agonist — still present, sometimes still being absorbed — reoccupies the receptor.¹ This is the closest thing opioid poisoning has to a latent phase, and it is created by the treatment.
Hours to daysProlonged toxicity
What you seeWith methadone, modified-release preparations, transdermal fentanyl, renal impairment or nitazenes: repeated deterioration requiring repeated or continuous reversal.
What is happeningThe agonist outlasts the antagonist by hours or days. In the nitazene review, six of thirty patients were treated with naloxone infusions — a ratio higher than that reflected in current clinical guidelines, in which a shorter observation time is deemed sufficient.² Inferred

What the mechanism predicts at the bedside

Why oxygen and ventilation come before naloxone

Because the organ injury is hypoxic and the drug effect is not injurious. A patient who is ventilated is not being harmed by the opioid at all, whatever their receptor occupancy; a patient given naloxone but not oxygenated is still hypoxic for the seconds to minutes it takes to work. The antidote treats the cause; ventilation treats the harm, and the harm is the time-critical one. Inferred

Why the goal of naloxone is a respiratory rate, not a conscious level

Because the receptor population that kills people is the one controlling respiratory rhythm,¹ and full arousal requires stripping receptor occupancy far beyond what breathing needs. Titrating to consciousness in a dependent patient converts a survivable poisoning into a withdrawal emergency, with vomiting and aspiration risk in someone with an unprotected airway — and, at high doses, a catecholamine surge that has itself caused cardiopulmonary arrest.¹ UK practice severity-bands the dose for exactly this reason; see the naloxone monograph.

Why the observation period belongs to the agonist

Renarcotisation is a mismatch of two half-lives, so the safe observation period is set by the opioid, not by the naloxone. Intravenous heroin, methadone, modified-release oxycodone and a fentanyl patch produce four different answers to the same question. A single naloxone bolus in a methadone overdose is a diagnostic test, not a treatment.

What the nitazene data actually show

This is where a mechanism page can correct an impression. Nitazenes are widely reported as being so potent that naloxone fails. The systematic review of 35 included articles found something more nuanced:

In vitro receptor affinity and potency often surpass those of both morphine and fentanyl — but real-world data indicate that in vivo potency is often lower than experimental findings, and post-mortem concentrations of many nitazenes are similar to those of fentanyl, indicating a similar potency.² Established
Naloxone works. "Case reports and clinical series indicate that naloxone remains an effective antidote for nitazene poisoning." A median parenteral dose of 1.20 mg reversed poisoning, with a median of 0.8 mg pre-hospital.² Those are not extraordinary doses.
The real difference is duration, not dose. Six of thirty patients required naloxone infusions because of the persistence of opioid effects — a higher proportion than current guidelines assume, and the review concludes that prolonged toxicity means extended monitoring and repeated naloxone dosing.²
Post-mortem concentrations overlap with those found in living patients, which complicates the establishment of lethal thresholds — so a concentration cannot be interpreted, even retrospectively.²

Why a normal pupil does not exclude opioid poisoning

Because miosis and respiratory depression are mediated at different sites, and only one of them is on the critical path. Co-ingested stimulants or anticholinergics, pethidine, and profound hypoxia itself can all leave the pupils unhelpful. The triad is a teaching device; the respiratory rate is the test.

Why polysubstance use changes the arithmetic

Benzodiazepines and gabapentinoids have a ceiling on respiratory depression when taken alone — see benzodiazepines — but that ceiling is a property of the drug acting on its own. Combined with a µ-agonist the two depress respiration at different points in the same network and the ceiling no longer holds. The nitazene review found that all reviewed cases involved several opioids and other sedating drugs.² Naloxone reverses only the opioid component, which is a mechanistic reason for an incomplete response that does not imply the diagnosis was wrong. Inferred

The antidote, from the poison's side

Naloxone is the only true antidote in Band A: a competitive antagonist at the receptor the poison occupies. Read from the poison's side, its three well-documented limitations are all direct consequences of that mechanism rather than defects in the drug.

Reappearance of respiratory depression because of its short half-life.¹ A competitive antagonist only works while it is present, and it is present for less time than the agonist. Established
Difficulty reversing very high-affinity agonists — the PNAS review names carfentanil and buprenorphine — because of naloxone's low binding affinity.¹ Competition is decided by relative affinity and concentration, so an agonist that binds far more tightly requires far more antagonist. Established
A catecholamine surge at high doses, which can cause cardiopulmonary arrest.¹ Abrupt total receptor blockade in an adapted brain is a physiological insult in its own right. Established

Critical appraisal

The neuroanatomy is mouse work, and the extrapolation is honest but real. Inferred The parabrachial findings are elegant — correlation with respiratory rate, abolition by morphine, reproduction by silencing, rescue by activation¹ — and they are chemogenetic experiments in mice. That opioid-induced respiratory depression in humans depends on the same population is a strong inference, not a demonstration. It does not change management, and it does change how confidently the mechanism should be taught.
"Nitazenes are naloxone-resistant" is closer to traditional teaching than to a finding. Inferred The systematic review reports a median reversal dose of 1.20 mg and states plainly that naloxone remains effective;² it also reports that in vivo potency is often lower than in vitro work suggests and that post-mortem concentrations resemble fentanyl's.² The genuine signal is a higher-than-expected rate of infusion requirement — 6 of 30 — which is a statement about duration. Downgrading this to traditional teaching is not justified: the claim is recent, plausible and only partly wrong, and a review that says "data on nitazene potency in humans are scarce" does not license certainty in either direction.
Six of thirty is a small numerator. Inferred The infusion figure that most changes practice comes from a subset of a review whose included case reports are subject to obvious reporting bias — the cases that get written up are the difficult ones. The direction is probably right; the proportion should not be quoted as an incidence.
The codeine pharmacogenetics are label-verified and the prevalence table should still be read carefully. Established Two UK SmPCs from different manufacturers were checked and agree.³ The population labels in that table are broad, self-reported-ancestry categories of the kind pharmacogenetics has been moving away from, and the ranges are wide. The clinical conclusion — that codeine's effective dose cannot be known in advance — does not depend on which row a patient is assigned to.
Non-cardiogenic pulmonary oedema after opioid overdose has no settled mechanism. Inferred Hypoxic capillary injury, catecholamine surge and negative-pressure effects are all proposed and none is established. The clinically actionable part — that persistent hypoxia after adequate reversal is a lung problem, not a dosing problem — holds regardless.
Chest-wall rigidity is described mainly in the anaesthetic literature at rapid high doses. Inferred Its frequency in illicit fentanyl-analogue exposure is genuinely unknown, and it is easy to over-teach because it is memorable. It belongs on the page because it changes what fails first, not because it is common.
This page treats a class as a poison, which is a compromise. Morphine, methadone, buprenorphine and carfentanil differ by orders of magnitude in potency, duration and receptor kinetics. The receptor mechanism genuinely is shared; almost nothing else is, and every statement about observation periods needs a specific drug attached to it before it means anything.

References

1
Liu S, Kim D-I, Oh TG, et al. Neural basis of opioid-induced respiratory depression and its rescue. Proc Natl Acad Sci U S A 2021;118(23):e2022134118. PMC8201770 Open access. Source of the lateral parabrachial µ-opioid-receptor neuron findings — correlation with respiratory rate, its abolition by morphine, and the chemogenetic silencing and rescue experiments — and of the three quoted limitations of naloxone: reappearance of respiratory depression from its short half-life, failure to reverse high-affinity agonists such as carfentanil and buprenorphine, and a catecholamine surge at high doses causing cardiopulmonary arrest. Mouse work; the human extrapolation is flagged in the appraisal. Verified 1 Sep 2026 from the full text.
2
Stangeland M, Dale O, Skulberg AK. Nitazenes: review of comparative pharmacology and antagonist action. Clin Toxicol (Phila) 2025;63(6):393–406. PubMed 40422647 Systematic review, 35 included articles. Source of the 1950s development and abandonment for high toxicity, the in vitro versus in vivo potency discrepancy, the median parenteral naloxone dose of 1.20 mg and pre-hospital median of 0.8 mg, the 6-of-30 naloxone infusion figure and its comparison with current guideline observation times, the post-mortem concentration overlap, and the polysubstance finding. Verified 1 Sep 2026 from the abstract.
3
Codeine Phosphate Tablets BP 30mg — Summary of Product Characteristics, Ranbaxy (UK) Limited. emc product 2616 — and Codeine phosphate 30 mg Tablets, Aurobindo/Milpharm. emc product 7031. §4.2–4.4 of both products were fetched and read separately, and they agree. Source of the CYP2D6 activation statement, the up-to-7% deficiency figure, the ultra-rapid-metaboliser prevalence table reproduced above, and the four contraindications quoted. Verified 1 Sep 2026.
4
EXTRIP workgroup — published recommendations index. extrip-workgroup.org/recommendations Cited for an absence: none of the workgroup's 22 published poison recommendations covers an opioid. Verified 1 Sep 2026.
5
Royal College of Emergency Medicine / National Poisons Information Service. Joint best practice guidance on the use of naloxone in opioid poisoning, April 2024. The current UK source for severity-banded naloxone dosing; summarised on the ResusDoc naloxone monograph. Doses are deliberately not reproduced on this page.
6
TOXBASE — opioids; heroin; methadone; nitazenes. National Poisons Information Service. toxbase.org (NHS login required. NPIS: 0344 892 0111. The authoritative UK source for naloxone regimens, observation periods and disposition — and the place where changes in the illicit supply are reflected first. Login-gated, so not quoted here.)

Opioids

At a glance