Atropine Sulfate — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	SC, IM, IV	0.02-0.04 mg/kg (preanesthetic); 0.04 mg/kg for bradycardia; 0.1-0.2 mg/kg for organophosphate toxicity (repeat as needed)	Preanesthetic: once; Bradycardia: as needed; Toxicity: every 10-15 minutes until signs of atropinization (mydriasis, tachycardia, dry mouth)	Duration: As needed Notes: Titrate to effect; monitor heart rate and mucous membranes.
Cat	SC, IM, IV	0.02-0.04 mg/kg (preanesthetic); 0.04 mg/kg for bradycardia; 0.1-0.2 mg/kg for organophosphate toxicity	Preanesthetic: once; Bradycardia: as needed; Toxicity: every 10-15 minutes until atropinization	Duration: As needed Notes: Cats may be more sensitive to CNS effects; use with caution.
Horse	IV, IM	0.01-0.02 mg/kg for bradycardia; 0.05-0.1 mg/kg for organophosphate toxicity	Bradycardia: as needed; Toxicity: every 10-15 minutes until atropinization	Duration: As needed Notes: Monitor for ileus; atropine can cause colic.
Cattle	IV, IM, SC	0.02-0.04 mg/kg for bradycardia; 0.1-0.2 mg/kg for organophosphate toxicity	Bradycardia: as needed; Toxicity: every 10-15 minutes until atropinization	Duration: As needed Notes: Monitor for bloat; atropine reduces rumen motility.
Small Ruminants (Sheep, Goats)	IV, IM, SC	0.02-0.04 mg/kg for bradycardia; 0.1-0.2 mg/kg for organophosphate toxicity	Bradycardia: as needed; Toxicity: every 10-15 minutes until atropinization	Duration: As needed Notes: Similar to cattle; monitor for rumen stasis.
Rabbit	SC, IM, IV	0.02-0.05 mg/kg (preanesthetic); 0.1-0.2 mg/kg for organophosphate toxicity	Preanesthetic: once; Toxicity: every 10-15 minutes until atropinization	Duration: As needed Notes: Rabbits have high atropinase activity; may require higher doses or more frequent administration.
Birds/Poultry	IM, IV	0.01-0.02 mg/kg (preanesthetic); 0.1-0.2 mg/kg for organophosphate toxicity	Preanesthetic: once; Toxicity: every 10-15 minutes until atropinization	Duration: As needed Notes: Use with caution; birds may be sensitive to anticholinergics.

Clinical Indications & Species Uses

General Indications

Reduction of salivary, bronchial, and gastrointestinal secretions
Treatment of bradycardia
Antidote for cholinergic toxicity (organophosphates, carbamates)
Ophthalmic mydriasis and cycloplegia

Dog (Canine)

Preanesthetic to reduce secretions and prevent bradycardia
Treatment of sinus bradycardia and atrioventricular block
Antidote for organophosphate and carbamate toxicity
Treatment of cholinergic drug overdose
Ophthalmic use for uveitis and cycloplegia

Cat (Feline)

Preanesthetic to reduce secretions and prevent bradycardia
Treatment of sinus bradycardia and atrioventricular block
Antidote for organophosphate and carbamate toxicity
Ophthalmic use for uveitis and cycloplegia

Horse (Equine)

Treatment of bradyarrhythmias
Antidote for organophosphate toxicity
Preanesthetic to reduce salivation and bronchial secretions
Treatment of colic due to spasmodic conditions (as an antispasmodic)

Cattle (Bovine)

Antidote for organophosphate and carbamate toxicity
Treatment of bradyarrhythmias
Preanesthetic to reduce secretions

Small Ruminants (Sheep / Goat)

Antidote for organophosphate and carbamate toxicity
Treatment of bradyarrhythmias

Rabbit & Small Mammals

Antidote for organophosphate toxicity
Preanesthetic to reduce secretions (though rabbits are relatively resistant, higher doses may be needed)

Avian & Poultry

Antidote for organophosphate toxicity
Preanesthetic to reduce secretions

Exotic & Other Species

Antidote for organophosphate toxicity in various exotic species
Ophthalmic use for uveitis in some species

Pharmacology & Mechanism of Action

Drug Class: Anticholinergic / Antimuscarinic | Pharmacological Group: Parasympatholytic

Mechanism of Action: Atropine is a competitive antagonist of acetylcholine at muscarinic receptors. It blocks the actions of acetylcholine at postganglionic parasympathetic neuroeffector junctions, thereby inhibiting parasympathetic nerve impulses. This leads to a decrease in secretions (salivary, bronchial, gastric), relaxation of smooth muscle in the gastrointestinal and urinary tracts, and an increase in heart rate by blocking vagal tone. Atropine also blocks the effects of cholinergic agonists such as pilocarpine and organophosphates.

Pharmacodynamics: Atropine produces dose-dependent effects: low doses decrease salivary and bronchial secretions, moderate doses increase heart rate and cause pupillary dilation, and high doses inhibit gastrointestinal motility and bladder tone. It also has central nervous system effects at higher doses, including excitation and delirium. The duration of action is dose-dependent and species-specific.

⚡ Pharmacokinetics Summary

Absorption: Atropine is well absorbed from the gastrointestinal tract and from parenteral sites. Oral absorption is rapid but may be variable due to decreased gastric emptying. Onset of action after IV administration is immediate; after IM or SC, it is within minutes.

Distribution: Atropine is widely distributed throughout the body, including the central nervous system. It crosses the blood-brain barrier and placenta. It is distributed into milk and can be found in ocular tissues after topical application.

Metabolism: Atropine is metabolized in the liver primarily by hydrolysis to tropine and tropic acid. Some metabolism occurs in the tissues. The metabolic rate varies among species; rabbits have high atropinase activity, making them relatively resistant.

Excretion: Atropine is excreted primarily in the urine, both as unchanged drug and metabolites. A small amount is excreted in feces. Excretion is pH-dependent; acidic urine increases elimination.

Half-Life: Dog: ~1.5-2 hours; Cat: ~2-3 hours; Horse: ~1-2 hours; Cattle: ~1-2 hours; Rabbit: ~0.5-1 hour (due to high atropinase activity).

Bioavailability: Oral bioavailability is variable, estimated at 20-50% due to first-pass metabolism and variable absorption. Parenteral administration provides complete bioavailability.

Protein Binding: Approximately 18-20% bound to plasma proteins.

Available Formulations & Strengths

Injectable Solution 0.1 mg/mL, 0.4 mg/mL, 0.5 mg/mL, 1 mg/mL (IV, IM, SC)

Ophthalmic Solution 1% (10 mg/mL) (Ophthalmic)

Ophthalmic Ointment 1% (Ophthalmic)

Oral Tablet 0.3 mg, 0.4 mg, 0.6 mg (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to atropine or other belladonna alkaloids
Glaucoma or narrow-angle glaucoma
Tachyarrhythmias
Gastrointestinal obstruction or ileus
Urinary bladder obstruction
Myasthenia gravis (unless used for anticholinesterase overdose)
Severe renal or hepatic impairment (use with caution)

Warnings & Clinical Precautions:

Use with caution in animals with cardiovascular disease, especially those with tachycardia or hypertension.
May cause constipation and urinary retention.
In horses, atropine can cause ileus and colic; use with caution.
In ruminants, atropine can cause rumen stasis and bloat.
Use with caution in animals with fever, as atropine impairs sweating and thermoregulation.
Ophthalmic use may cause local irritation and systemic absorption; use with caution in animals with cardiac disease.
In organophosphate toxicity, atropine should be used in conjunction with pralidoxime (2-PAM) for best outcomes.
Monitor for signs of atropinization (dry mouth, mydriasis, tachycardia, ileus) to avoid overdose.

Adverse Effects & Reactions

Common:

Dry mouth
Mydriasis
Tachycardia
Constipation
Urinary retention
Decreased gastrointestinal motility
Blurred vision (if ophthalmic)

Serious / Severe:

Severe tachycardia or arrhythmias
Central nervous system excitation (agitation, seizures, hallucinations)
Paralytic ileus
Hyperthermia (due to impaired sweating)
Respiratory depression (at high doses)

Rare:

Hypersensitivity reactions
Acute angle-closure glaucoma
Psychosis or delirium
Coma (in severe overdose)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Other anticholinergics (e.g., glycopyrrolate, scopolamine)	Additive anticholinergic effects, increased risk of toxicity.	High
Cholinergic agents (e.g., bethanechol, pilocarpine)	Antagonistic effects; atropine blocks the action of cholinergic drugs.	Moderate
Cholinesterase inhibitors (e.g., neostigmine, physostigmine)	Antagonistic effects; atropine can reverse the effects of these drugs.	Moderate
Digoxin	Atropine may increase digoxin absorption due to decreased GI motility, potentially increasing digoxin levels.	Moderate
Potassium chloride	Atropine may increase the risk of gastrointestinal ulceration when given with oral potassium.	Mild
Antacids or adsorbents	May reduce absorption of oral atropine; separate administration by at least 1-2 hours.	Mild
Halogenated anesthetics (e.g., halothane)	Atropine may increase the risk of arrhythmias when used with halogenated anesthetics.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Severe tachycardia
Hypertension
Hyperthermia
Mydriasis
Dry, flushed skin
Delirium, agitation, hallucinations
Seizures
Respiratory depression
Paralytic ileus
Urinary retention

Emergency Treatment Protocol: Treatment is primarily symptomatic and supportive. Discontinue atropine immediately. For severe CNS excitation or seizures, administer diazepam or barbiturates. For hyperthermia, use cooling measures. Physostigmine (0.02-0.04 mg/kg IV) can be used as an antidote to reverse central and peripheral anticholinergic effects, but use with caution as it may cause bradycardia and seizures. Supportive care includes IV fluids, cardiac monitoring, and management of arrhythmias. In cases of oral overdose, consider gastric lavage and activated charcoal if recent ingestion.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days🥚 Eggs: 0 days

Atropine is not approved for use in food animals in many countries; however, when used extra-label, withdrawal times may be extended. Consult regulatory guidelines. In the US, under AMDUCA, a withdrawal time of at least 7 days for meat and 3 days for milk is often recommended for cattle and sheep, but this is not an official withdrawal period. For poultry, a withdrawal time of at least 7 days for meat and eggs is recommended. Always check local regulations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 15-30°C (59-86°F). Protect from light.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Keep tightly closed. Do not freeze. Store ophthalmic preparations according to manufacturer's instructions.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in dogs, cats, horses, and cattle in certain formulations. Not approved for all species or all routes; many uses are extra-label.

Extra-Label (Off-Label) Use: Atropine is not approved for all species and indications; extra-label use is permitted under AMDUCA in the US for food animals, provided there is a valid veterinarian-client-patient relationship and appropriate withdrawal times are observed. For non-food animals, extra-label use is common.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Atropine is a fundamental drug in veterinary emergency medicine, particularly for the treatment of bradyarrhythmias and as an antidote for organophosphate and carbamate toxicity. It is also used as a preanesthetic to reduce secretions. However, its use requires careful monitoring due to potential adverse effects, especially in species like horses and ruminants where gastrointestinal motility is critical. In organophosphate toxicity, atropine should be administered aggressively, titrating to effect (mydriasis, dry mucous membranes, normal heart rate), and often combined with pralidoxime. For preanesthetic use, glycopyrrolate may be preferred in some cases due to fewer cardiac effects. Always consider species-specific differences in metabolism and response. In rabbits, higher doses may be needed due to atropinase activity. For ophthalmic use, atropine provides prolonged mydriasis and cycloplegia, useful for uveitis, but can cause systemic effects if absorbed. Withdrawal times for food animals should be determined based on regulatory guidance and the specific formulation used.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)