19.3 Toxicology: Poisoning Recognition & Antidotes
Key Takeaways
- Initial management of any poisoned patient is supportive and follows ABCs (airway, breathing, circulation) before decontamination or antidotes; an antidote is never a substitute for securing the airway and correcting hypoxia.
- Activated charcoal (1 g/kg, ideally within 1 hour of ingestion) adsorbs many drugs but is contraindicated for acids, alkalis, metals (including lithium and iron), and whenever the airway is unprotected or consciousness is depressed without prior intubation.
- Organophosphate poisoning produces a cholinergic crisis (SLUDGE plus miosis and muscle fasciculations); treat with atropine for muscarinic symptoms and pralidoxime chloride to reactivate acetylcholinesterase.
- Paracetamol (acetaminophen) hepatotoxicity is caused by NAPQI, which depletes glutathione; N-acetylcysteine replenishes glutathione and is indicated using the Rumack-Matthew nomogram (4-24 h post-ingestion, treat if level >150 µg/mL at 4 h) or if there is any evidence of hepatotoxicity.
- Tricyclic antidepressant cardiotoxicity (QRS widening >100 ms, ventricular arrhythmias, hypotension) is treated with intravenous sodium bicarbonate, which alkalinises plasma and blocks fast sodium channels; class Ia/Ic antiarrhythmics are contraindicated because they worsen Na channel blockade.
Last updated: August 2026
General Management of the Poisoned Patient
The first principle of toxicology is supportive care comes first. Before reaching for an antidote, secure the ABCs: protect the airway (intubate if GCS < 8 or the gag reflex is absent), ensure adequate breathing and oxygenation, and support circulation with IV fluids and vasopressors as needed. An antidote is an adjunct, never a substitute for resuscitation.
Decontamination options
| Method | When to use | When NOT to use |
|---|---|---|
| Activated charcoal 1 g/kg PO | Within 1 hour of ingestion of an adsorbable drug | Acids, alkalis (corrosives), metals (lithium, iron, heavy metals), hydrocarbons; unprotected airway or decreased consciousness without prior intubation; ingestion >1 h ago with delayed-release drugs only |
| Gastric lavage | Within 1 h of a life-threatening ingestion, after airway is secured | Corrosives, hydrocarbons (aspiration risk); largely superseded by charcoal for most drugs |
| Whole bowel irrigation (polyethylene glycol) | Iron, lithium, swallowed drug packets ('body packers') | Bowel obstruction, ileus, unstable patient |
| Skin decontamination | Cutaneous exposure (organophosphate pesticides, chemical burns) | — remove clothing, copious water irrigation |
Exam trap: Activated charcoal does not bind lithium, iron, strong acids, alkalis, or hydrocarbons. Giving charcoal for an iron overdose is both useless and may complicate endoscopy. For iron, the answer is whole bowel irrigation plus deferoxamine.
Exam trap: Do not give activated charcoal to a drowsy patient who has not been intubated — the risk is pulmonary aspiration of charcoal, which causes a severe pneumonitis. Secure the airway first, then consider charcoal.
High-Yield Antidote Table
| Toxin | Toxidrome / clue | Antidote | Mechanism / key detail |
|---|---|---|---|
| Organophosphates | Cholinergic crisis: SLUDGE (salivation, lacrimation, urination, defecation, GI cramps, emesis) + miosis, muscle fasciculations, bradycardia | Atropine + pralidoxime (2-PAM) chloride | Atropine blocks muscarinic effects; pralidoxime reactivates acetylcholinesterase (must be given early, before the enzyme ages) |
| Paracetamol (acetaminophen) | Initially asymptomatic; 24-72 h later AST/ALT >1000, RUQ pain, coagulopathy | N-acetylcysteine (NAC) | Replenishes glutathione, detoxifies NAPQI; use Rumack-Matthew nomogram from 4-24 h post-ingestion; treat if paracetamol level >150 µg/mL at 4 h (or any hepatotoxicity) |
| Opioids (morphine, heroin, fentanyl) | Respiratory depression, sedation, miosis ('pinpoint pupils'), hypotension | Naloxone 0.4-2 mg IV/IM, repeat as needed | Competitive μ-opioid antagonist; short half-life (30-90 min) so re-dosing or infusion may be needed for long-acting opioids |
| Tricyclic antidepressants | QRS widening >100 ms, ventricular arrhythmias, hypotension, anticholinergic signs (mydriasis, urinary retention) | IV sodium bicarbonate | Alkalinises plasma (pH 7.45-7.55) and blocks fast Na channels; avoid class Ia/Ic antiarrhythmics (procainamide, quinidine, flecainide) which worsen blockade |
| Benzodiazepines | Sedation, ataxia; rarely life-threatening alone | Flumazenil | Competitive GABA-A antagonist; caution: can precipitate seizures in chronic users or mixed overdoses with proconvulsants |
| Digoxin | Nausea, vomiting, visual disturbances (yellow-green halos), arrhythmias, hyperkalaemia | Digoxin-specific Fab antibody fragments (Digibind/DigiFab) | Bind digoxin in plasma; indicated for life-threatening arrhythmia, K > 5 mmol/L, or massive ingestion |
| Warfarin | Elevated INR, bleeding | Vitamin K (phytomenadione); PCC for major bleeding | Replenishes vitamin K-dependent clotting factors; PCC (prothrombin complex concentrate) acts fastest for life-threatening haemorrhage |
| Heparin (unfractionated) | Bleeding, prolonged aPTT | Protamine sulfate | Binds heparin; 1 mg neutralises ~100 units of UFH; partial effect against LMWH |
| Iron | Vomiting, GI bleeding, shock, metabolic acidosis | Deferoxamine (desferrioxamine) | Chelates iron; indicated for significant iron ingestion with symptoms or serum iron > expected toxic level |
| Methanol / ethylene glycol | Visual disturbance (methanol), renal failure (ethylene glycol), severe metabolic acidosis with anion gap | Fomepizole (or IV ethanol) ± haemodialysis | Inhibits alcohol dehydrogenase, preventing formation of toxic metabolites (formic acid, oxalic acid); dialysis removes parent alcohol and metabolites |
| Cyanide | Soot in mouth (fire victim), bitter almond smell, rapid collapse, metabolic acidosis with high lactate | Hydroxocobalamin (preferred) or sodium thiosulfate ± amyl nitrite | Hydroxocobalamin binds cyanide to form cyanocobalamin (vitamin B12); thiosulfate donates sulfur to convert cyanide to thiocyanate |
| Beta-blockers | Bradycardia, hypotension, hypoglycaemia, AV block | Calcium chloride (or gluconate), glucagon | Calcium overcomes channel blockade; glucagon increases cAMP independent of β-receptors |
| Calcium channel blockers | Bradycardia, hypotension, hyperglycaemia (paradoxical) | Calcium chloride, glucagon, high-dose insulin euglycaemic therapy (HIET) | HIET shifts myocardial metabolism to carbohydrates and improves contractility; often the most effective intervention for severe CCB poisoning |
Recognition Patterns and Exam Traps
- Pinpoint (constricted) pupils appear in both opioid and organophosphate poisoning. Distinguish by context: organophosphates add profuse secretions (SLUDGE) and muscle weakness; opioids add respiratory depression and a history of IV drug use or fentanyl exposure. The antidotes differ: naloxone for opioids, atropine plus pralidoxime for organophosphates.
- Dilated pupils (mydriasis) feature in TCA and anticholinergic (atropine, antihistamine) overdose. TCAs add cardiotoxicity (QRS widening) and require sodium bicarbonate, not physostigmine.
- Anion-gap metabolic acidosis in an overdose context points to methanol or ethylene glycol (treat with fomepizole), iron, paracetamol (late), salicylates, and ethylene glycol. The 'MUDPILES' mnemonic (Methanol, Uraemia, DKA, Paraldehyde/Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates) still circulates.
- Sodium bicarbonate appears in two overdose scenarios on the exam: TCA cardiotoxicity (QRS widening, arrhythmias) and salicylate toxicity (alkalinise urine to trap salicylate ions). Know which clinical setting the question describes.
- Flumazenil is contraindicated in patients with a history of seizures or in mixed overdoses involving proconvulsant drugs (e.g. TCAs) because it can precipitate refractory seizures.
- N-acetylcysteine is most effective when given within 8 hours of paracetamol ingestion but still benefits patients who present later, even after 24 h, if hepatotoxicity is present; do not withhold NAC because 'the window has passed'.
Test Your Knowledge
A farm worker presents with profuse salivation, lacrimation, urination, diarrhoea, GI cramps, emesis, miosis, and muscle fasciculations after accidental exposure to an organophosphate pesticide. Which combination of antidotes is most appropriate?
A
B
C
D
Test Your Knowledge
A 19-year-old presents 6 hours after ingesting 15 g of paracetamol (acetaminophen). The 4-hour serum paracetamol level is 180 µg/mL. What is the mechanism of toxicity and the correct antidote?
A
B
C
D
Test Your Knowledge
A patient with a history of depression presents after an intentional overdose of amitriptyline (a tricyclic antidepressant). ECG shows QRS duration of 140 ms and the blood pressure is 88/50 mmHg. Which intervention is most appropriate, and which class of drugs must be avoided?
A
B
C
D
Test Your Knowledge
Which statement about the use of activated charcoal in acute poisoning is correct?
A
B
C
D