Vetdrugs 2.0Hyoscyamine is a belladonna alkaloid with atropine-like antimuscarinic activity. It is used infrequently in veterinary practice, principally when reduction of parasympathetic effects is desired. Proposed clinical applications include selected gastrointestinal spasm or hypermotility disorders, excessive secretions and vagally mediated bradycardia. Most veterinary experience is empirical rather than trial-based.
Group
Antimuscarinic
Mechanism of Action
Hyoscyamine competitively blocks acetylcholine at muscarinic receptors. Reduced signaling in gastrointestinal smooth muscle and glands decreases motility, spasm and secretions. Cardiac muscarinic blockade reduces vagal slowing of the sinoatrial and atrioventricular nodes, potentially increasing heart rate. These effects reflect broad parasympathetic antagonism rather than a selective action on the underlying gastrointestinal or cardiac disease.
Dose Rates
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Gastrointestinal antispasmodic | 0.003–0.006 mg/kg | PO | Every 8 h | |
| Irritable bowel syndrome | 0.003–0.006 mg/kg | PO | Every 8–12 h | Immediate-release formulation only. |
| Symptomatic sinus-node disease | 0.003–0.006 mg/kg | PO | Every 8 h | Immediate-release oral formulation. |
Indications
- Empirical treatment of selected gastrointestinal spasm or hypermotility disorders in dogs.
- Reduction of excessive salivation or other secretions in selected veterinary patients.
- Treatment of selected clinically significant vagally mediated bradycardia when an antimuscarinic is appropriate.
Contraindications
- Glaucoma
- Ileus or gastroparesis
- Tachycardia
- Use cautiously with renal impairment, myasthenia gravis, urinary obstruction, intestinal obstruction or toxic megacolon.
Interactions
- Antagonizes cholinergic and prokinetic drugs such as metoclopramide.
- Avoid mixing with alkaline solutions.
- Antacids containing aluminium, magnesium or calcium can reduce absorption; first-generation antihistamines and other antimuscarinics have additive effects.
Adverse effects
- Dry mouth, constipation, ileus, tachycardia and urinary retention
Toxicity
Excess antimuscarinic exposure may cause marked tachycardia and ileus; stop treatment and provide supportive care.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Hyoscyamine 0.125 mg tablet | Tablet | 0.125 mg / tablet | PO |
| Hyoscyamine 0.025 mg/mL oral solution | Oral Liquid | 0.025 mg / mL | PO |
| Hyoscyamine sulfate 0.15 mg tablet | Tablet | 0.15 mg / tablet | PO |
| Hyoscyamine sulfate 0.125 mg/mL oral solution | Oral Liquid | 0.125 mg / mL | PO |
Monitoring and Administration
Monitor heart rate and gastrointestinal motility.
Extended-release and immediate-release products have different delivery characteristics. Store tightly closed at room temperature.
Hyoscyamine 0.375 mg extended-release tablet: 0.375 mg per 1 tablet. Extended-release presentation; not interchangeable with immediate-release dosing.
Sulfate product strengths and the free-base parent identity must not be exchanged without an explicit equivalent basis. Oral solution 0.125 mg/mL is five times the 0.025 mg/mL elixir concentration.