Pharmacology · Antimuscarinics (anticholinergics)

Atropine !

The classic muscarinic blocker: it speeds a slow heart, dries secretions and is the key antidote for organophosphate insecticide poisoning.

Updated 2026-11-01

Class: Antimuscarinics (anticholinergics)
Receptor: Muscarinic acetylcholine receptors (M1 to M5)
ATC code: A03BA01
WHO Model List of Essential Medicines

Muscarinic acetylcholine receptors (M1 to M5)Atropine

The figure shows atropine (its active isomer, hyoscyamine) in the acetylcholine pocket of the human M1 muscarinic receptor. Atropine blocks all five subtypes about equally; its effect on the heart comes from M2.

Target in 3D: Muscarinic acetylcholine receptors (M1 to M5). PDB structure 9JEA (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Atropine, an alkaloid of belladonna (Atropa belladonna) and jimson weed, competitively blocks acetylcholine at muscarinic receptors on the heart, smooth muscle and glands. At the heart it removes vagal braking of the sinus and AV nodes, so the rate rises and AV conduction improves; it dries saliva and bronchial secretions and relaxes gut and bladder. Small doses can briefly slow the heart before the rate rises. It does not block nicotinic receptors, so it cannot reverse muscle weakness in organophosphate poisoning.

Occupies the acetylcholine pocket of muscarinic receptors
Vagal braking of the sinus and AV nodes is removed
Heart rate rises; AV conduction improves
Secretions dry; gut, bladder and bronchi relax

Pharmacokinetics

Distribution: Spreads throughout the body, crosses the blood-brain barrier and the placenta; about 44% bound to plasma proteins
Metabolism: Largely by enzymatic hydrolysis in the liver; organophosphate pesticides slow its metabolism
Elimination: Between 13 and 50% unchanged in the urine; not removed by dialysis
Half-life: More than doubled in children under two and in adults over 65
Onset and duration: Heart rate and saliva effects peak several minutes after intravenous injection

Indications

Organophosphate insecticide and nerve agent poisoning (muscarinic effects) Organophosphate and carbamate poisoning →
Poisoning by muscarine-containing mushrooms Mushroom poisoning (Amanita) →
Symptomatic bradycardia and vagal AV block, including bradyasystolic arrest Atrioventricular (heart) block →
Reduction of saliva and airway secretions and blockade of vagal reflexes in anaesthesia
Eye drops: dilating the pupil and relaxing focus (cycloplegia) in anterior uveitis Uveitis →

Adverse effects

Common

Dry mouth and thirst
Blurred vision and sensitivity to light
Fast heart rate and palpitations
Less sweating, heat intolerance and flushing
Constipation and difficulty passing urine, mainly in older adults

Contraindications

No absolute contraindication when used for life-threatening muscarinic effects
Otherwise caution or avoidance in angle-closure glaucoma, pyloric obstruction, prostatic enlargement with retention and coronary disease

Key interactions

Other anticholinergic drugs (tricyclics, antihistamines, phenothiazines): Additive toxicity: delirium, retention, hyperthermia
Cholinesterase inhibitors (neostigmine, pyridostigmine, physostigmine): Opposite effects; atropine blocks their muscarinic side effects
Mexiletine: Absorption of mexiletine is slowed

What is monitored

Heart rate, rhythm and blood pressure
In organophosphate poisoning: chest sounds and bronchial secretions, oxygen, pupils and heart rate after each dose
Temperature, bowel sounds, bladder (retention) and mental status

Administration and safety

Given intravenously for bradycardia and poisoning, titrated to heart rate, PR interval, blood pressure and symptoms
In organophosphate poisoning, repeated doses are often needed until secretions dry; large total amounts may be required
Single-dose vials: solution checked for clarity and the rest discarded

Nursing care

In poisoning, listen to the chest and check secretions after each dose: drying of secretions is the target, not the pupil size
Watch for fever, agitation and delirium of atropine excess; cool the patient if hot
Check the bladder for retention and the abdomen for silent bowel
Remove contaminated clothes and wash the skin in organophosphate exposure, with staff protection

Pregnancy and breastfeeding

Atropine crosses the placenta; life-saving treatment of poisoning or severe bradycardia is not withheld in pregnancy.

Test yourself

Think of the answer, then tap the question to see it.

What is the clinical endpoint for repeating atropine in organophosphate poisoning?

Drying of bronchial secretions with clear breathing (with adequate heart rate and blood pressure), not pupil size.

Why does atropine not reverse the paralysis of organophosphate poisoning?

The paralysis is nicotinic (neuromuscular junction) and atropine blocks only muscarinic receptors; an oxime such as pralidoxime is needed.

Which antidote reverses severe atropine delirium?

Physostigmine, a cholinesterase inhibitor that enters the brain.

Why can a very small dose of atropine briefly slow the heart?

Low doses block presynaptic muscarinic receptors on vagal nerve endings, briefly increasing acetylcholine release, before the M2 block of the pacemaker dominates.

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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); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 11 (McGraw Hill, 2023); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 12 (McGraw Hill, 2023); eEML - Electronic Essential Medicines List (World Health Organization, 2025).

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Representative model, for educational purposes only.