Naloxone

Why this drug is interesting

Naloxone is close to the platonic ideal of an antidote: a competitive antagonist with high receptor affinity, negligible intrinsic activity, rapid onset, no meaningful toxicity of its own, and no ceiling problem. Give enough and the opioid comes off the receptor.

Almost all of its clinical difficulty comes from two mismatches. The first is temporal — naloxone's half-life of 1–1.5 hours is shorter than that of most opioids it is used against, so reversal is a window rather than a cure. The second is conceptual — the drug is frequently dosed to wake the patient up, when the physiological indication is only to make them breathe.

The UK position has changed — RCEM and NPIS, 2024

Route

"The preferred route of use for naloxone in the Emergency Department is intravenous," because it allows careful titration; intramuscular, intranasal and intraosseous routes "should generally only be considered in the emergency department for patients in whom intravenous access is difficult or not possible," and are associated with a longer time to peak concentration.¹

Dosing

First, decide which arm of the pathway the patient is in

There is a fourth route into treatment that is not an entry band. A patient who initially met the top row and needed no naloxone, but who then deteriorates to RR <10/min or SpO₂ <92% on air, is arrowed across into the same titrated-naloxone limb. This is why the monitoring interval in the top row matters: the pathway assumes someone is still looking.

Therapeutic targets

"Therapeutic targets should be a respiratory rate of over 10 breaths per minute and oxygen saturations of greater than 92% on room air" — in the absence of pre-existing respiratory disease such as COPD, where 88–92% may be the routine target. Level of consciousness (AVPU, GCS) is useful to monitor but is not the endpoint. Nasal end-tidal CO₂, where available and where clinicians are experienced with it, may be used as an adjunct.¹

After a response

• Monitor RR, SpO₂, ETCO₂ and AVPU every 15 minutes for the first hour, then every 30 minutes for the subsequent 3 hours
• If the patient deteriorates again (RR <10): repeat titrated boluses of 100–200 µg every 60 seconds, to a maximum of 2000 µg, then start an infusion

Children and neonates

The RCEM guideline covers adults only. Product labelling gives 10–20 µg/kg IV in children, repeated at 2–3 minute intervals until satisfactory respiration and consciousness. In neonates the ~3 hour half-life is longer than in adults, but so is the duration of most transplacental opioid, so monitoring requirements do not relax.

The naloxone infusion

"Following an initial response, if the patient subsequently deteriorates and requires further IV boluses of naloxone to maintain adequate ventilation, they will require a naloxone infusion."¹

Preparation

Mix 4 mg (ten 400 µg/1 mL vials) with 30 mL of 0.9% sodium chloride (or dextrose) to give a 40 mL solution at 100 µg/mL, for infusion by pump.

With high-dose infusions the syringe may need replacing quickly, and it may be appropriate to write a follow-on prescription at the same time as the first.

Monitoring and adjustment

• Observations every 15 minutes for the first hour, then every 30 minutes
• Breakthrough toxicity (RR <10, SpO₂ <92% on air, or a concerning ETCO₂ trace): further boluses of 100–200 µg every 60 seconds up to a maximum of 2 mg, then increase the hourly infusion rate by 60% of that total bolus dose
• Emerging withdrawal: reduce the infusion rate, generally by 50% in the first instance. If the patient is significantly agitated, stop the infusion temporarily and restart after 30–60 minutes at 50% of the previous rate once withdrawal settles
• Any change of rate resets the monitoring — back to every 15 minutes for an hour, then every 30 minutes

Stopping the infusion

• Unless toxicity recurs or withdrawal emerges, continue at the same rate for at least four hours before starting to titrate down
• Then down-titrate by 25% of the maximum infusion rate every two hours, with continued close observation
• Do not generally stop a naloxone infusion at night (midnight to 06:00) unless the patient is in withdrawal — recurrence of toxicity is harder to detect in a sleeping patient
• The guideline notes that down-titration will rarely happen in the ED unless admission is significantly delayed

Pharmacology

Mechanism

Naloxone is an N-allyl derivative of oxymorphone and a competitive antagonist at μ, κ and δ opioid receptors, with greatest affinity for μ. It has essentially no agonist activity, so in an opioid-naive person it produces almost no discernible effect — one of the reasons it is safe to give diagnostically. The RCEM guideline notes it "can be a useful diagnostic agent in a patient with drowsiness and significant respiratory depression."¹

Because antagonism is competitive, the dose required depends on the receptor occupancy it must overcome. High-affinity or high-potency opioids — buprenorphine especially, and the potent synthetics — may require substantially larger doses or an infusion, and buprenorphine's very high receptor affinity means reversal can be incomplete at conventional doses.

Kinetics

Onset

IV within 1–2 minutes; IM/SC within about 5 minutes

Duration of antagonist effect

1–4 hours, dose-dependent (both UK labels) — routinely shorter than the opioid

Plasma half-life

1–1.5 hours after parenteral administration; approximately 3 hours in neonates

Metabolism

Extensive hepatic glucuronidation; poor oral bioavailability due to first-pass metabolism

Excretion

Renal, as metabolites

Precipitated withdrawal and other complications

Features of acute opioid withdrawal

Yawning, coughing, sneezing, running nose, lacrimation, hypertension, tachycardia, dilated pupils, diarrhoea, cool clammy skin, fine muscle tremor, nausea, irritability, restlessness and anxiety.¹

Adverse effects — and an honest caveat about attribution

Reported adverse effects include nausea, vomiting, sweating, tachycardia, tremor, hyperventilation and hypertension — "these effects are more likely to occur when excessive doses of naloxone are used." Less commonly, particularly in patients taking opioids for pain who have pre-existing cardiac disease: hypo- or hypertension, pulmonary oedema, atrial and ventricular fibrillation, and cardiac arrest.¹

Contraindications

Hypersensitivity to naloxone or the excipients. There is otherwise no contraindication to giving naloxone to a patient who is not ventilating adequately — which is the correct framing: in true opioid-induced respiratory arrest, the risk calculus is not close.

Observation, discharge and secondary prevention

Always consider alternative diagnoses

The guideline's footnote A directs that investigation "should include BM, VBG, paracetamol concentration ± CT head." Failure to respond to an adequate dose of naloxone should prompt reconsideration: co-ingestion, hypoxic brain injury, head injury, intracranial event, hypoglycaemia or another sedative. Note that urine drug screening has no role in immediate management — the diagnosis is clinical.

Secondary prevention

Take-home naloxone provision varies by local commissioning, but both hospital and community drug services increasingly supply intramuscular or nasal naloxone with training — an intervention known to reduce drug-related deaths. An ED attendance after an overdose is an unusually good moment to arrange it.

Intranasal naloxone

Concentrated intranasal preparations are licensed in the UK for emergency use in suspected opioid overdose, including in the community. Onset is slower than IV and absorption less predictable, particularly with epistaxis or nasal pathology, but it removes the need for venous access or a sharp — which in the prehospital and peer-administration setting is the decisive advantage. In the ED it is a fallback for difficult access, not a first choice.

Special situations

• Buprenorphine — very high receptor affinity; reversal may be incomplete even at high doses, and prolonged infusion plus ventilatory support may be needed.
• Methadone and modified-release opioids — infusion and prolonged observation, not boluses and discharge. Extend observation to up to 12 hours.
• Potent synthetic opioids — fentanyl analogues and nitazenes — the guideline notes these are "often highly potent and/or longer acting than heroin," are frequently sold as heroin without the user's knowledge, and have driven death spikes in England in 2017, 2021 and 2023. Larger total doses and prolonged infusions should be anticipated.
• Co-ingestants — alcohol, benzodiazepines and gabapentinoids such as pregabalin exacerbate respiratory depression; stimulants may mask the toxidrome and will be unmasked by over-reversal.
• Body packers and stuffers — assume ongoing absorption; these patients need critical care input, not a single dose.
• Chronic pain patients on high-dose opioids — full reversal produces both withdrawal and uncontrolled pain; titrate very cautiously. Note that patients on prescribed opioids for cancer pain or palliative care are outside the RCEM guideline's scope — follow TOXBASE.
• Neonates of opioid-dependent mothers — naloxone may precipitate severe withdrawal including seizures; specialist neonatal advice.

Critical appraisal

1. Naloxone's effectiveness has never been established by randomised controlled trial, and never will be. Withholding an antidote from an apnoeic patient is not a randomisable question. The evidence is mechanistic, observational and overwhelming — a reminder that RCT evidence is not the only legitimate warrant for practice, and that demanding it everywhere is itself a form of poor reasoning.
2. The starting dose was genuinely contested, and the UK has now taken a position. Licensed ranges and prehospital protocols differ largely because they weight rapid reversal against precipitated withdrawal differently. The RCEM/NPIS solution — band by severity, titrate small in the stable patient, escalate fast in the peri-arrest one — resolves the argument sensibly, but it is a consensus resolution rather than a trial result.
3. Observation periods remain the weakest part of the guidance, and the guideline admits it. Its own research recommendation is that "research to establish safe and acceptable criteria for discharge after naloxone administration would be beneficial." The four-and-six-hour rule is consensus-derived. Clinical judgement about the specific opioid involved beats any fixed interval.
4. The adverse effect profile may be overstated. By naming the attribution problem explicitly — that pulmonary oedema and convulsions may belong to the opioid rather than the antidote — the guideline is more honest than most teaching on this drug, and it slightly weakens the case for the extreme caution that some clinicians apply.
5. The ground is still shifting under the dosing. Higher and repeated doses are more often required with high-potency synthetics, and the guideline's own escalation already tops out at a figure it says may not be enough. Anything written about adequate naloxone dosing in the nitazene era — including this page — should be checked against current national guidance rather than relied upon.

References

1. 1
   Gill R, Wood DM, Blundell MS, Thanacoody RH, Humphries C, Dargan PI. Guideline for the Assessment and Management of Acute Opioid Toxicity in Adults in the Emergency Department. Royal College of Emergency Medicine and National Poisons Information Service Best Practice Guideline, April 2024. rcem.ac.uk. Source of all dosing, infusion, monitoring, observation and secondary prevention content above, including Figure 1 and Table 2. Verified 23 Aug 2026. Also published as: Emerg Med J 2024 — PubMed.
2. 2
   Naloxone 400 micrograms/ml solution for injection/infusion — Summary of Product Characteristics, hameln (product 6344). electronic Medicines Compendium. Compare ADVANZ (product 6589), emc. Both labels give 0.1–0.2 mg for partial reversal preserving analgesia and 0.4–2 mg for suspected overdose, and both state in §5.1 that the duration of antagonist effect is 1–4 hours. Sections 4.2, 4.4, 5.1. Verified 24 Aug 2026.
3. 3
   TOXBASE — opioids and naloxone. National Poisons Information Service. toxbase.org (NHS login required. NPIS 0344 892 0111. The guideline directs discussion with a clinical toxicologist where the presentation is uncertain, and TOXBASE governs opioid toxicity in palliative care and cancer pain, which the guideline excludes.)
4. 4
   NHS England. Support to minimise the risk of distress and death from inappropriate doses of naloxone — patient safety alert. england.nhs.uk
5. 5
   Olfson M, Crystal S, Wall M, Wang S, Liu SM, Blanco C. Causes of death after nonfatal opioid overdose. JAMA Psychiatry 2018;75(8):820–7. PubMed Source of the 24-fold mortality figure at 12 months.
6. 6
   Office for Health Improvement and Disparities. Widening the availability of naloxone — UK government guidance on take-home naloxone supply. gov.uk