Hyoscyamine
Hyoscyamine sulfate
Also known as: Anaspaz · ED-SPAZ · HyoMax · Cystospaz · Neosol · NuLev · Symax · Levbid · Levsinex · IB-Stat · Egazil Duretter · Neo-Allospasmin · daturin · duboisine · tropine-L-tropate
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Irritable bowel syndrome | 0.003-0.006 mg/kg PO two to three times a day | PO | q8-12h | — | 🌎 NA |
| Long-term management of symptomatic patients with sinus node disease | 0.003-0.006 mg/kg PO q8h | PO | q8h | Long-term | 🌎 NA |
- Irritable bowel syndrome: Assumed to be for immediate release oral dosage forms.
- Long-term management of symptomatic patients with sinus node disease: Assumed to be for immediate release oral dosage forms.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Hyoscyamine is a belladonna alkaloid and the highly active levo-isomer of racemic atropine. Because it is the active enantiomer, it is significantly more potent than atropine in both its central and peripheral anticholinergic effects.
In veterinary medicine, its use is relatively limited but it serves as a valuable alternative to other anticholinergics (like glycopyrrolate or atropine). It is primarily utilized for:
- Gastrointestinal hypermotility: Managing conditions like irritable bowel syndrome (IBS) by reducing spasms.
- Cardiovascular support: Treating symptomatic bradycardia or sinus node disease.
- Perioperative use: Potentially reducing hypersalivation, urinary spasms, and respiratory secretions, though clinical data in animals is sparse.
Clinical Pearl: Because hyoscyamine crosses the blood-brain barrier more readily than quaternary anticholinergics like glycopyrrolate, patients should be monitored closely for central nervous system (CNS) side effects, such as paradoxical excitement or sedation.
Mechanism of action
Hyoscyamine acts as a competitive, reversible antagonist of acetylcholine at muscarinic receptors (M1-M5) located on smooth muscle, cardiac muscle, and glandular cells.
- Gastrointestinal/Urinary: Blocks parasympathetic stimulation → relaxes smooth muscle → decreases GI and urinary tract motility and spasms.
- Secretions: Inhibits glandular muscarinic receptors → dose-dependent reduction in salivary, bronchial, pharyngeal, and gastric secretions.
- Cardiovascular: Blocks vagal (parasympathetic) tone at the sinoatrial (SA) node → increases heart rate.
- Ocular: Paralyzes the pupillary sphincter muscle → causes mydriasis (pupil dilation).
Safety & warnings
Contraindications
- Hypersensitivity to hyoscyamine or other belladonna alkaloids
- Glaucoma (narrow or wide angle)
- Intestinal obstruction or intestinal atony
- Toxic megacolon
- Severe ulcerative colitis
- Obstructive uropathy
- Acute hemorrhage with cardiovascular instability
Adverse effects
- Mydriasis (pupil dilation)
- Xerostomia (dry mouth)
- Constipation
- Urinary retention
- Xerophthalmia (dry eyes)
- Tachycardia
- CNS effects (somnolence, delirium, or paradoxical excitement at higher doses)
Precautions
Use with extreme caution in patients with renal dysfunction, as drug elimination may be reduced. Carefully consider the risks before using in patients with tachyarrhythmias, cardiac valve disease, or congestive heart failure due to its chronotropic effects. May aggravate myasthenia gravis. FDA Pregnancy Category C; use during pregnancy only if clearly needed. It is excreted in trace amounts in milk; while likely safe, risk to nursing offspring cannot be entirely ruled out.
Drug interactions
May interfere with the gastrointestinal absorption of hyoscyamine.
Additive anticholinergic actions and increased risk of adverse effects.
Additive anticholinergic actions and increased risk of adverse effects.
Hyoscyamine may antagonize and counteract their gastrointestinal prokinetic effects.
Monitoring
- Clinical efficacy (resolution of GI spasms or improvement in heart rate)
- Heart rate and rhythm
- Bowel elimination (monitor for constipation/ileus)
- Urinary elimination (monitor for urinary retention)
Pharmacokinetics
Absorption
Rapidly and nearly completely absorbed after oral or sublingual administration. Extended-release oral dosage forms may have somewhat reduced oral bioavailability.
Distribution
Distributed widely throughout the body. Readily enters the central nervous system (CNS) and crosses the placenta.
Metabolism
Partially hydrolyzed in the liver to tropic acid and tropine.
Elimination
The majority of the drug is excreted unchanged in the urine.
Overdose
Significant overdosage can be life-threatening; contacting an animal poison control center is strongly advised. The LD50 in rats is 375 mg/kg.
Clinical Signs of Toxicity:
- Intensified anticholinergic effects: severe tachycardia, mydriasis, urinary retention, and absent gut sounds (ileus).
- CNS signs: behavioral changes, severe depression, delirium, or seizures.
Treatment:
- Decontamination: Protocols to decrease oral absorption (e.g., emesis, activated charcoal) should be considered if the overdose was recent and the patient is asymptomatic.
- Antidote: Severe anticholinergic effects can be treated with physostigmine or neostigmine, but this should only be done under the strict guidance of a poison control center due to the risk of cholinergic crisis.
- Supportive Care: Delirium or excitement can be managed with small doses of short-acting barbiturates or benzodiazepines. Hyoscyamine can be removed by hemodialysis.
Available products
Formulations
- Oral tablets (immediate, chewable, orally disintegrating, sublingual)
- Extended-release tablets and capsules
- Oral solution and elixir
- Oral spray
- Injection (solution)
Human-labeled
- Hyoscyamine Oral Tablets: 0.125 mg (regular & chewable), 0.15 mg
- Hyoscyamine Orally Disintegrating Tablets: 0.125 mg, 0.25 mg
- Hyoscyamine Sublingual Oral Tablets: 0.125 mg
- Hyoscyamine Extended/Sustained-Release Oral Tablets: 0.25 mg, 0.375 mg
- Hyoscyamine Extended/Timed-Release Oral Capsules: 0.375 mg
- Hyoscyamine Oral Solution: 0.125 mg/mL
- Hyoscyamine Oral Elixir: 0.025 mg/mL
- Hyoscyamine Oral Spray: 0.125 mg/spray
- Hyoscyamine Injection: 0.05 mg/mL
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Unless otherwise advised by the manufacturer, oral products should be stored at room temperature in tight containers and protected from light. Injectable products should be stored at room temperature and protected from freezing.
Client information
Hyoscyamine is used to calm spasms in the gastrointestinal tract or to help increase a slow heart rate.
- Water Access: This medication frequently causes dry mouth. Ensure your pet always has access to plenty of fresh water.
- Monitor Elimination: Watch your pet's bathroom habits closely. Contact your veterinarian immediately if your pet is straining to urinate, unable to urinate, or becomes constipated.
- Other Side Effects: Contact your veterinarian if you notice dry eyes, difficulty swallowing, or unusual changes in behavior (such as extreme sleepiness, restlessness, or agitation).
- Missed Dose: If you miss a dose, give it when you remember. If it is close to the next dose, skip the missed dose. Do not double up.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.