Mechanism of action
Atropine is a competitive antagonist at muscarinic acetylcholine receptors (M1-M5). By blocking the binding of acetylcholine to these receptors, atropine inhibits parasympathetic (cholinergic) neurotransmission throughout the body.
Receptor-Specific Effects
- M1 receptors (CNS, gastric parietal cells): Blockade reduces gastric acid secretion and can cause CNS effects at high doses
- M2 receptors (heart): Blockade removes vagal tone on the SA and AV nodes, increasing heart rate and AV conduction
- M3 receptors (smooth muscle, glands): Blockade causes bronchodilation, reduced salivary/sweat gland secretion, reduced GI motility, urinary retention, and mydriasis
- M4 and M5 receptors (CNS): Less clinically relevant for peripheral atropine effects
Atropine's tertiary amine structure allows it to cross the blood-brain barrier, unlike quaternary ammonium anticholinergics such as glycopyrrolate. At therapeutic doses, cardiovascular and peripheral effects predominate; at toxic doses, CNS effects including agitation, delirium, and hyperthermia occur.
- Atropine competitively blocks muscarinic (M1-M5) acetylcholine receptors, causing parasympathetic inhibition
- First-line treatment for symptomatic bradycardia and organophosphate poisoning (muscarinic symptoms only)
- Very low IV doses may cause paradoxical bradycardia via central vagal stimulation
- Anticholinergic toxidrome: tachycardia, mydriasis, dry flushed skin, urinary retention, hyperthermia, delirium
- Contraindicated in narrow-angle glaucoma (risk of acute angle-closure) and obstructive uropathy
- Crosses blood-brain barrier (tertiary amine); causes CNS effects at high doses unlike glycopyrrolate (quaternary)
- In organophosphate poisoning, titrate to dry secretions, not to pupil size—pralidoxime reverses nicotinic effects
- Ophthalmic atropine causes cycloplegia lasting 6-12 days and mydriasis lasting 7-14 days
Indications
Atropine has diverse clinical applications across multiple specialties:
Cardiovascular
- Symptomatic bradycardia: First-line agent for hemodynamically unstable bradycardia, particularly in the setting of high vagal tone or AV block
- Asystole/pulseless electrical activity (PEA): Used in advanced cardiac life support protocols, though evidence is limited
Toxicology
- Organophosphate and carbamate poisoning: Reverses muscarinic symptoms (bronchospasm, bronchorrhea, bradycardia, miosis, salivation). Pralidoxime is added to reverse nicotinic effects
- Nerve agent exposure: Antidote for chemical warfare agents
Anesthesia
- Preoperative medication: Reduces oral and respiratory secretions
- Reversal of neuromuscular blockade: Given with neostigmine to prevent muscarinic side effects
Ophthalmology
- Mydriasis and cycloplegia: For fundoscopic examination and refraction testing
- Uveitis treatment: Prevents posterior synechiae formation
Other Uses
- Reduction of secretions in palliative care
- Diagnostic testing in sick sinus syndrome
Pharmacokinetics
Absorption: Well absorbed after oral, intramuscular, subcutaneous, intravenous, and endotracheal administration. Ophthalmic preparations produce systemic absorption, especially in children.
Distribution: As a tertiary amine, atropine crosses the blood-brain barrier and placenta. Volume of distribution is approximately 2-4 L/kg. It is minimally protein-bound (approximately 50%).
Metabolism: Hepatic metabolism via enzymatic hydrolysis and oxidation. Approximately 50% is metabolized to tropine and tropic acid; the remainder is excreted unchanged.
Elimination: Renal excretion of unchanged drug and metabolites. Half-life is 2-3 hours in adults, slightly longer in children and elderly patients.
Onset and Duration:
- IV: Onset 1-2 minutes, peak effect 2-4 minutes
- IM/SC: Onset 10-15 minutes
- Oral: Onset 30-60 minutes
- Ophthalmic: Mydriasis peaks at 30-40 minutes, lasts 7-14 days; cycloplegia lasts 6-12 days
Dosing follows the product label and varies by indication.
Adverse effects
Adverse effects result from parasympathetic blockade and follow predictable patterns. The classic mnemonic describes anticholinergic toxicity but also applies to therapeutic use at higher doses.
Common Adverse Effects
- Cardiovascular: Tachycardia, palpitations; paradoxical initial bradycardia at very low doses due to central stimulation of the vagal nucleus
- Ocular: Mydriasis, cycloplegia, photophobia, blurred vision, precipitation of acute angle-closure glaucoma in susceptible individuals
- Gastrointestinal: Dry mouth, reduced GI motility, constipation
- Genitourinary: Urinary retention, especially in elderly men with prostatic hypertrophy
- Dermatologic: Decreased sweating (anhidrosis), flushed dry skin, hyperthermia
- CNS: Restlessness, confusion, agitation (at higher doses)
Serious Adverse Effects
- Anticholinergic toxidrome: Hyperthermia, delirium, hallucinations, seizures, coma at toxic doses
- Acute angle-closure glaucoma: Medical emergency in predisposed patients
- Severe tachyarrhythmias: Particularly in patients with cardiac disease
- Heat stroke: Due to impaired sweating
Pediatric and Geriatric Considerations
Children and elderly patients are more susceptible to CNS effects. Infants may develop atropine fever.
Contraindications and warnings
Absolute Contraindications
- Narrow-angle glaucoma: Risk of acute angle-closure crisis
- Obstructive uropathy: Urinary retention will worsen
- Obstructive GI disease: Including paralytic ileus, pyloric stenosis, toxic megacolon
- Myasthenia gravis: May precipitate cholinergic crisis in patients on anticholinesterases, though used cautiously with neostigmine reversal
- Hypersensitivity to atropine or belladonna alkaloids
Relative Contraindications and Warnings
- Prostatic hypertrophy: Increased risk of urinary retention
- Hyperthyroidism: Tachycardia may be poorly tolerated
- Coronary artery disease: Tachycardia increases myocardial oxygen demand
- Congestive heart failure: Tachycardia may worsen cardiac function
- Chronic lung disease: Reduced secretions may lead to mucus plugging
- Autonomic neuropathy: Unpredictable effects
- Down syndrome: Increased sensitivity to mydriatic effects
Pregnancy and Lactation
Pregnancy Category C (FDA classification, now replaced by narrative labeling). Atropine crosses the placenta. Tachycardia may occur in the fetus. Use when benefit outweighs risk. It is the antidote of choice for organophosphate poisoning in pregnancy.
Lactation: Excreted in breast milk in small amounts; may suppress lactation. Use with caution.
Drug interactions
Pharmacodynamic Interactions
Additive anticholinergic effects with:
- Antihistamines (first-generation: diphenhydramine, promethazine)
- Tricyclic antidepressants (amitriptyline, imipramine)
- Antipsychotics (especially low-potency phenothiazines like chlorpromazine)
- Antiparkinson drugs (benztropine, trihexyphenidyl)
- Antispasmodics (dicyclomine, oxybutynin)
- Over-the-counter sleep aids and cold preparations
Combination increases risk of anticholinergic toxicity: confusion, urinary retention, severe constipation, hyperthermia.
Antagonistic interactions:
- Cholinergic agents: Atropine antagonizes bethanechol, pilocarpine, and acetylcholinesterase inhibitors (donepezil, rivastigmine, neostigmine)
- Metoclopramide: Atropine may reduce prokinetic effects
Pharmacokinetic Interactions
Atropine may slow GI transit and alter absorption of other oral medications. Sustained-release formulations may be particularly affected.
Clinical Significance
Patients on multiple medications with anticholinergic properties are at high risk for delirium, falls, and cognitive impairment, especially the elderly. The Anticholinergic Cognitive Burden Scale is used to assess cumulative risk.
Clinical pearls
- Paradoxical bradycardia: Very low IV doses may cause transient bradycardia due to central vagal stimulation before peripheral blockade predominates. This is why minimum recommended IV doses in adults are used to avoid this effect.
- Organophosphate poisoning: Atropine treats muscarinic symptoms (SLUDGE syndrome: Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis) but does NOT reverse nicotinic effects (fasciculations, weakness, paralysis). Pralidoxime is required for nicotinic receptor reactivation.
- Atropinization endpoint: In toxicology, atropine is titrated to dry pulmonary secretions and reverse bronchospasm, NOT to normalize pupil size or heart rate. Mydriasis and tachycardia are expected.
- Ophthalmic examination: The prolonged cycloplegia from atropine (up to 2 weeks) makes it less practical for routine dilation compared to tropicamide or phenylephrine. Reserve atropine for pediatric refractions and uveitis.
- Nerve agent autoinjector: Military and first responder kits contain atropine autoinjectors (often combined with pralidoxime) for rapid treatment of chemical warfare exposure.
- Heat intolerance: Warn patients that anhidrosis from atropine increases risk of hyperthermia, especially in hot environments or during exercise.
- Red as a beet, dry as a bone, blind as a bat, mad as a hatter, hot as a hare: Classic mnemonic for anticholinergic toxicity. Physostigmine is the antidote for severe central toxicity but is rarely used due to risk of cholinergic crisis and seizures.
- Differentiate from sympathomimetic toxicity: Both cause tachycardia and mydriasis, but anticholinergic toxicity features dry skin and decreased bowel sounds, whereas sympathomimetic toxicity features diaphoresis and hyperactive bowel sounds.
Anticholinergic toxicity: Red as a beet (flushed), Dry as a bone (anhidrosis), Blind as a bat (mydriasis), Mad as a hatter (delirium), Hot as a hare (hyperthermia), Full as a flask (urinary retention)
Practice questions
1. A 67-year-old farmer is brought to the emergency department after collapsing in his field. He is diaphoretic with excessive salivation, lacrimation, and urination. On examination, he has miosis, bradycardia (48/min), wheezing, and diffuse muscle fasciculations. Which of the following is the most appropriate initial pharmacologic treatment?
2. A 72-year-old woman with a history of glaucoma develops severe bradycardia (heart rate 38/min) after a myocardial infarction. Her blood pressure is 82/50 mmHg and she appears confused. The resident prepares to administer atropine. Which of the following is the most important consideration before giving this medication?
3. A 28-year-old woman receives intravenous atropine at a low dose before surgery to reduce secretions. Two minutes later, her heart rate decreases from 72/min to 56/min. Which of the following best explains this response?
Frequently asked questions
How does atropine work?
Atropine works by competitively blocking muscarinic acetylcholine receptors throughout the body, preventing parasympathetic nervous system activation. This blockade increases heart rate by removing vagal tone on the SA and AV nodes, dilates pupils, reduces secretions from salivary and sweat glands, decreases gastrointestinal motility, and relaxes smooth muscle in the airways and bladder. Because atropine is a tertiary amine, it can cross the blood-brain barrier and cause central nervous system effects at higher doses.
What are the side effects of atropine?
Common side effects of atropine include dry mouth, blurred vision, dilated pupils, increased heart rate, urinary retention, constipation, and decreased sweating. More serious adverse effects include severe tachycardia, acute angle-closure glaucoma in susceptible individuals, hyperthermia due to impaired sweating, and central nervous system effects such as confusion, agitation, hallucinations, and delirium at higher doses. Elderly patients and children are particularly susceptible to CNS effects and anticholinergic toxicity.
Is atropine a controlled substance?
No, atropine is not a controlled substance under the United States Controlled Substances Act. It is available by prescription for most formulations, though some ophthalmic preparations may have varying regulations. While not controlled, atropine is a potent medication that requires careful dosing and monitoring, and misuse can lead to serious anticholinergic toxicity. It is stocked in emergency departments, ambulances, and autoinjectors for organophosphate exposure.
What is atropine used for?
Atropine is primarily used to treat symptomatic bradycardia, especially when caused by high vagal tone or in emergency cardiac situations. It is the antidote for organophosphate and carbamate insecticide poisoning and nerve agent exposure, where it reverses life-threatening muscarinic symptoms like bronchospasm and excessive secretions. In ophthalmology, atropine causes pupil dilation and paralysis of accommodation for eye examinations and to prevent complications in uveitis. It is also used preoperatively to reduce secretions and alongside neostigmine to reverse neuromuscular blockade.
Atropine vs glycopyrrolate—what is the difference?
The main difference is that atropine is a tertiary amine that crosses the blood-brain barrier and can cause CNS side effects, while glycopyrrolate is a quaternary ammonium compound that does not cross into the CNS, making it safer for patients at risk of confusion or delirium. Both are muscarinic antagonists, but glycopyrrolate has a longer duration of action and causes less tachycardia, making it preferred for reducing secretions perioperatively. Atropine is preferred when rapid IV treatment of bradycardia is needed or when CNS penetration is desired.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: A03BA01WHO ATC/DDD index: S01FA01
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.