Atropine
**Atropine** is a classic anticholinergic agent used primarily in veterinary medicine to treat bradycardia, dilate pupils for ophthalmic examinations, and manage specific toxicities (such as organophosphate and carbamate poisoning). It is also used in conjunction with anticholinesterase drugs during the antagonism of neuromuscular blockade. > **Clinical Warning:** Routine administration prior to anaesthesia as part of premedication is no longer recommended. It is better to monitor heart rate and administer atropine specifically to manage a low heart rate if necessary.
Mechanism: Atropine competitively **blocks the action of acetylcholine** at **muscarinic receptors** located at the terminal ends of the parasympathetic nervous system. This blockade โ reverses parasympathetic effects โ produces **mydriasis** (pupil dilation), **tachycardia**, **bronchodilation**, and general inhibition of gastrointestinal function.
Dosing by species
- Ophthalmic (mydriasis) ยท 1 drop in the affected eye ยท topical ยท q12-24h to cause mydriasis, then once q24-96h to maintain mydriasis
- Bradyarrhythmias ยท 0.01-0.03 mg/kg i.v. or 0.03-0.04 mg/kg i.m. ยท IV/IM ยท PRN ยท Low doses may exacerbate bradycardia; repetition of the dose will usually promote an increase in heart rate. IM dose can be given to prevent development of bradycardia during administration of potent opioids such as fentanyl.
- Organophosphate poisoning ยท 0.2-0.5 mg/kg (ยผ dose i.v., ยพ i.m., s.c.) to effect; repeat as necessary; or 0.1-0.2 mg/kg (ยฝ i.v., ยฝ i.m.) then i.m. q6h ยท IV/IM/SC ยท PRN or q6h
- Neuromuscular blockade antagonism ยท 0.04 mg/kg i.v. ยท IV ยท PRN ยท with edrophonium (0.5-1.0 mg/kg)
- Ophthalmic (mydriasis) ยท 1 drop in the affected eye ยท topical ยท q12-24h to cause mydriasis, then once q24-96h to maintain mydriasis
- Bradyarrhythmias ยท 0.01-0.03 mg/kg i.v. or 0.03-0.04 mg/kg i.m. ยท IV/IM ยท PRN ยท Low doses may exacerbate bradycardia; repetition of the dose will usually promote an increase in heart rate. IM dose can be given to prevent development of bradycardia during administration of potent opioids such as fentanyl.
- Organophosphate poisoning ยท 0.2-0.5 mg/kg (ยผ dose i.v., ยพ i.m., s.c.) to effect; repeat as necessary; or 0.1-0.2 mg/kg (ยฝ i.v., ยฝ i.m.) then i.m. q6h ยท IV/IM/SC ยท PRN or q6h
- Neuromuscular blockade antagonism ยท 0.04 mg/kg i.v. ยท IV ยท PRN ยท with edrophonium (0.5-1.0 mg/kg)
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Glaucoma
- Lens luxation
- Keratoconjunctivitis sicca (KCS / dry eye)
Adverse effects
- Sinus tachycardia
- Blurred vision (due to mydriasis)
- Drying of bronchial secretions
- Increased intraocular pressure (IOP)
- Reduced tear production
- Ventricular arrhythmias
- Hypersalivation (especially in cats, due to bitter taste of ophthalmic drops)
- Ileus (rare in small animals)
- Vomiting (rare in small animals)
Drug interactions
- Bromides, iodides, sodium bicarbonate, alkalis, noradrenaline ยท Incompatible when mixed together ยท major
- Antihistamines ยท May enhance the activity of atropine ยท moderate
- Pethidine ยท May enhance the activity of atropine ยท moderate
- Benzodiazepines ยท May enhance the activity of atropine ยท moderate
- Phenothiazines ยท May enhance the activity of atropine ยท moderate
- Thiazide diuretics ยท May enhance the activity of atropine ยท moderate
- Sympathomimetics ยท May enhance the activity of atropine ยท moderate
- Alpha-2 agonists ยท Combining is not recommended ยท major
- Amitraz ยท May aggravate toxicity signs, leading to hypertension and gut stasis ยท major
- Tricyclic Antidepressants ยท Additive anticholinergic effects ยท moderate
- Alpha-2 Agonists (e.g., Dexmedetomidine) ยท May cause paradoxical hypertension and significantly increased myocardial workload; concurrent use is generally discouraged ยท major
- Metoclopramide ยท Atropine antagonizes the prokinetic effects of metoclopramide on the GI tract ยท moderate
Monitoring
- Heart rate and rhythm (ECG)
- Pupillary light reflex and pupil size
- Intraocular pressure (IOP)
- Tear production (Schirmer Tear Test)
Overdose
Overdose may lead to severe sinus tachycardia, ventricular arrhythmias (which may be treated with lidocaine if severe), extreme mydriasis, severe drying of secretions, and gastrointestinal stasis.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.