Toxidromes
The five classic patterns of poisoning (cholinergic, anticholinergic, sympathomimetic, opioid, sedative) and the clues that tell them apart.
A toxidrome is a cluster of signs that points to a group of toxins, even before the exact substance is known. Pupils, skin, temperature, heart rate, bowel sounds and mental state are enough to recognise the main patterns at the bedside and start the right treatment.
Cholinergic
Acetylcholine builds up when acetylcholinesterase is blocked, mainly by organophosphate and carbamate insecticides, which are important causes of poisoning where pesticides are widely used. Muscarinic signs are salivation, tearing, bronchorrhoea, bronchospasm, sweating, vomiting, diarrhoea, urination, miosis and bradycardia; nicotinic signs are fasciculations and weakness that can stop breathing. Atropine dries the airways and supports the heart rate; pralidoxime can reactivate the enzyme.
Anticholinergic (antimuscarinic)
Blocking muscarinic receptors gives the opposite picture: dilated pupils, blurred vision, dry mouth, hot, dry, flushed skin, fever, tachycardia, urinary retention, reduced bowel sounds and agitated delirium with hallucinations. Many medicines cause it: first-generation antihistamines, tricyclic antidepressants, antispasmodics, some antipsychotics and atropine itself. Care is mainly supportive; physostigmine reverses central and peripheral effects in selected cases, with atropine kept at hand.
Sympathomimetic
Cocaine and amphetamines release or block reuptake of noradrenaline, dopamine and serotonin. Patients are agitated, hypertensive and tachycardic, with dilated pupils and sweating, and can develop hyperthermia, seizures, arrhythmias, stroke, myocardial infarction and rhabdomyolysis. Sweating is the bedside clue that separates this pattern from anticholinergic poisoning. Benzodiazepines are the first step for agitation.
Opioid and sedative-hypnotic
Opioids cause the triad of pinpoint pupils, unconsciousness and respiratory depression; naloxone reverses it, but its effect can wear off before a long-acting opioid, so the patient is watched. Sedative-hypnotics depress consciousness with fewer specific signs; combined with opioids or alcohol, the risk of fatal overdose rises sharply.
Look-alikes to remember
- Serotonin syndrome (serotonergic drugs together): agitation, tremor, clonus, hyperreflexia, sweating, fever.
- Neuroleptic malignant syndrome (antipsychotics): lead-pipe rigidity, high fever, autonomic instability.
- Hypoglycaemia, head injury, infection and withdrawal states can mimic any toxidrome.
Test yourself
Think of the answer, then tap the question to see it.
A patient has dilated pupils, tachycardia and fever. What single sign helps separate sympathomimetic from anticholinergic poisoning?
The skin: sweaty in sympathomimetic poisoning, hot and dry in anticholinergic poisoning.
Which toxidrome causes miosis, heavy secretions, bradycardia and fasciculations, and what is the antidote pair?
The cholinergic toxidrome (e.g. organophosphates); atropine plus pralidoxime.
What is the opioid overdose triad?
Pinpoint pupils, unconsciousness and respiratory depression.
Why is a patient watched after naloxone even if they wake up?
Naloxone can wear off before the opioid, so breathing can slow again.
Example drugs
Connections
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - ATROPINE SULFATE injection, solution (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - PHYSOSTIGMINE SALICYLATE injection (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - AMPHETAMINE SULFATE tablet (DailyMed (U.S. National Library of Medicine), 2026); Opioid overdose (World Health Organization, 2026).
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Representative model, for educational purposes only.