Pralidoxime

Why this drug is interesting

Organophosphate poisoning is one of the few toxicological emergencies with a genuinely two-drug answer. Atropine blocks the muscarinic effects — bronchorrhoea, bronchospasm, bradycardia, the wet, drowning presentation — and it is unequivocally life-saving. It does nothing at the nicotinic receptor, so it does not touch fasciculation, weakness or the respiratory muscle paralysis that kills the patients atropine has dried out.

Pralidoxime is supposed to fill that gap by reactivating the enzyme itself. The mechanism is elegant, the pharmacology is real, and the clinical evidence is the weakest of any antidote on this site.

Pharmacology

Mechanism

Organophosphates phosphorylate the serine residue in the active site of acetylcholinesterase, inactivating it. Acetylcholine accumulates at muscarinic and nicotinic synapses and at the neuromuscular junction, producing the cholinergic crisis. Pralidoxime's oxime group attacks the phosphorus atom, pulls the phosphoryl group off the enzyme, and frees the active site — the enzyme is reactivated rather than replaced.

Because it acts at the enzyme, pralidoxime is theoretically effective at every cholinergic site, including the nicotinic ones atropine cannot reach. That is the entire rationale for using it.

Ageing — the reason timing dominates

Salt confusion

Pralidoxime exists as several salts and they are not interchangeable by weight of salt: mesilate (the UK stockpile form, historically labelled P2S), chloride (US Protopam, and the salt used in the Eddleston trial), and iodide (used in the Pawar trial). Doses in the literature are usually quoted in mg/kg without stating the salt. When a dose is being given, the salt on the vial and the dose in the protocol must match.

Dosing

Adults — the WHO regimen

• At least 30 mg/kg pralidoxime salt as a loading dose, conventionally given over about 30 minutes, then
• 8 mg/kg/hour by continuous infusion.¹

Note the WHO figure is expressed in milligrams of salt, not of pralidoxime cation — see the salt warning above. Eddleston 2009 states the recommendation as "at least 30 mg/kg pralidoxime salt loading dose followed by 8 mg/kg infusion".¹

Slow administration matters: rapid injection of pralidoxime has been associated with hypertension, tachycardia, laryngospasm and muscle rigidity.

Children

No paediatric regimen is given on this page. There is no UK label, no controlled paediatric evidence, and the paediatric doses in circulation could not be traced to a primary source during preparation of this monograph. Paediatric dosing must come from TOXBASE and a direct conversation with NPIS and paediatric intensive care.

Duration

Continuation is guided by clinical response — resolution of fasciculation and weakness, reduction in atropine requirement — and where available by red cell (or plasma) cholinesterase activity. Lipophilic organophosphates redistribute from fat over days, so relapse after apparent recovery is well described and infusions are often continued well beyond the first 24 hours.

Evidence

Two randomised trials frame the debate, and they answer different questions.

Eddleston 2009 — does pralidoxime work at all?

A double-blind, randomised, placebo-controlled trial in Sri Lanka: 235 patients with organophosphorus insecticide self-poisoning randomised to pralidoxime chloride 2 g over 20 minutes then 0.5 g/hour for up to 7 days (n=121) or saline (n=114). The primary outcome was mortality; secondary outcomes included intubation and duration of intubation. This is the WHO-recommended regimen, tested deliberately — the trial was designed to answer the objection that earlier negative studies had used doses that were too low.¹

Despite clear reactivation of red cell acetylcholinesterase in diethyl organophosphorus pesticide poisoned patients, we found no evidence that this regimen improves survival or reduces need for intubation in patients with organophosphorus insecticide poisoning. The reason for this failure to benefit patients was not apparent.

Eddleston et al., PLoS Medicine 2009 — conclusions¹

Pawar 2006 — if you use it, how much?

A randomised trial in India of 200 patients with moderately severe organophosphorus self-poisoning. After a 2 g loading dose over 30 minutes, half received 1 g/hour of pralidoxime iodide for 48 hours and half received 1 g every 4 hours. The high-dose continuous regimen produced better outcomes — less atropine, less intubation and fewer ventilator days.²

Read together: within a pralidoxime-treated population there is a signal that more may be better than less, on a dose difference much smaller than the salt figures suggest; against placebo, the WHO regimen did not demonstrate benefit and trended towards harm. Those findings are not contradictory, but they do not add up to a recommendation.

Why the trials may not settle it

• Both trials studied pesticide self-poisoning in South Asia, typically with large ingestions and long pre-hospital times. Enzyme ageing may already have occurred in many patients.
• Organophosphates are not one drug. Dimethyl compounds age far faster than diethyl compounds; the responsiveness to oximes differs accordingly, and trials pooling them may dilute a real effect in a subgroup. The same authors' review puts it as: “the role of oximes is not completely clear; they might benefit only patients poisoned by specific pesticides or patients with moderate poisoning”.³
• Neither trial addresses nerve agents, where exposure is recognised within minutes and treatment is given by autoinjector at the scene — the setting in which the mechanism has the best chance of mattering.
• A Cochrane review of oximes for acute organophosphate pesticide poisoning (Buckley, Eddleston and colleagues) concluded the evidence was insufficient to determine whether they are harmful or beneficial.⁵

Practical use in the ED

1. Protect yourself. Organophosphate on clothing and skin contaminates staff. Decontaminate before or during resuscitation; use PPE; this is a scene and departmental safety issue, not only a patient one.
2. Recognise the syndrome. Muscarinic — salivation, lacrimation, urination, defecation, gastrointestinal upset, emesis, plus bronchorrhoea, bronchospasm, bradycardia and miosis. Nicotinic — fasciculation, weakness, tachycardia, hypertension, and ultimately respiratory muscle paralysis. Central — agitation, seizures, coma.
3. Give oxygen and atropine, and keep giving atropine. Titrate to clear chest and adequate oxygenation. Bronchorrhoea, not heart rate, is the endpoint.
4. Secure the airway early if weakness is progressing. Note that suxamethonium is metabolised by plasma cholinesterase, which is inhibited — expect markedly prolonged block; rocuronium is the more predictable choice.
5. Give a benzodiazepine for seizures — midazolam — and consider it early in severe exposure.
6. Contact NPIS on 0344 892 0111 and consult TOXBASE for the pralidoxime decision, the dose and the source of supply.
7. Consider activated charcoal where the route was oral — the 2026 CTRC guideline recommends charcoal for organophosphorus insecticides at any ingested dose — one of very few poisons with no dose threshold at all.⁴
8. Plan for relapse. Lipophilic agents redistribute; the intermediate syndrome of proximal and respiratory weakness appears 24–96 hours after the acute crisis. These patients belong in critical care.

Critical appraisal

Pralidoxime is the clearest example on this site of a drug whose mechanism is better than its evidence. Enzyme reactivation is demonstrable in vitro, the biochemistry of ageing is well described, and the nicotinic gap that atropine leaves is real. And yet the one placebo-controlled trial in humans found no benefit on mortality or intubation.

The honest position is that it remains standard in nerve agent protocols, is variably used in pesticide poisoning, and is given in the UK on expert advice rather than on a licence. Presenting it as an established antidote overstates the case; presenting it as discredited overstates a single negative trial conducted in a setting where ageing may already have foreclosed the mechanism.

This page is also the weakest-sourced on the site, and deliberately says so: no UK SmPC, doses from the literature, and a supply chain that runs through national emergency stock. That is an accurate description of the drug's position in UK practice, not a gap in the research.

References

1. 1
   Eddleston M, Eyer P, Worek F, et al. Pralidoxime in acute organophosphorus insecticide poisoning — a randomised controlled trial. PLoS Medicine 2009;6(6):e1000104. Full text · PMC2696321. 235 patients; pralidoxime chloride 2 g over 20 min then 0.5 g/h for up to 7 days versus saline — the WHO-recommended regimen, tested deliberately. Mortality 30/121 (24.8%) versus 18/114 (15.8%), adjusted HR 1.69 (95% CI 0.88–3.26, p=0.12). Source of the verbatim conclusion, the discussion's "most likely to be ineffective, and may be harmful", the WHO regimen wording, and the iodide-to-cation conversion (1 g iodide = 0.52 g cation). The largest and best-characterised trial — but not the only placebo-controlled one: the paper itself describes three earlier medium-sized RCTs, one of which used a placebo arm. Full text verified 4 Sep 2026.
2. 2
   Pawar KS, Bhoite RR, Pillay CP, et al. Continuous pralidoxime infusion versus repeated bolus injection to treat organophosphorus pesticide poisoning: a randomised controlled trial. Lancet 2006;368:2136–2141. PubMed 17174705. 200 patients; 2 g load then 1 g/h pralidoxime iodide for 48 h versus 1 g every 4 h. Favoured the higher-dose continuous regimen.
3. 3
   Eddleston M, Buckley NA, Eyer P, Dawson AH. Management of acute organophosphorus pesticide poisoning. Lancet 2008;371:597–607. PubMed 17706760. Narrative review from the authors of the trial above. Source for the pesticide-to-pesticide variability and the position that "the role of oximes is not completely clear; they might benefit only patients poisoned by specific pesticides or patients with moderate poisoning". Only the abstract was accessible (thelancet.com returns 403); nothing on this page is sourced to its full text.
4. 4
   Hoegberg LCG, Gosselin S, Buckley NA, Wood DM, et al. Recommendations from the Clinical Toxicology Recommendations Collaborative on the administration of activated charcoal in acute oral overdose. Clinical Toxicology 2026;64(6):419–475. PubMed 41906697. Table 4: organophosphorus insecticides carry a single-dose recommendation at any dose, GRADE D. Full text read 4 Sep 2026.
5. 5
   Buckley NA, Eddleston M, Li Y, Bevan M, Robertson J. Oximes for acute organophosphate pesticide poisoning. Cochrane Database of Systematic Reviews 2011;(2):CD005085. Cochrane Library. Concluded the evidence was insufficient to determine whether oximes are harmful or beneficial. Cited from the review record; full text not retrieved for this page.
6. 6
   National Poisons Information Service. TOXBASE · NPIS 0344 892 0111. The required UK source: there is no pralidoxime product listed on the electronic Medicines Compendium (searched 31 Aug 2026), so there is no UK SmPC, licensed indication or licensed dose.