Terbutaline Sulfate — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	1.25-5 mg/dog (total dose) or 0.1-0.2 mg/kg	q8-12h	Duration: As needed for clinical effect Notes: Start at low end and titrate to effect; monitor heart rate.
Dog	SC	0.01-0.02 mg/kg	q6-8h	Duration: For acute bronchospasm Notes: Use with caution in cardiac disease.
Cat	PO	0.625-1.25 mg/cat (total dose) or 0.1-0.2 mg/kg	q8-12h	Duration: As needed for clinical effect Notes: May cause agitation; monitor for tachycardia.
Horse	PO	0.05-0.1 mg/kg	q12h	Duration: For heaves; may be used long-term Notes: Oral bioavailability is low; may need higher doses.
Horse	IV	0.01-0.02 mg/kg	q6-8h	Duration: For acute bronchospasm Notes: Administer slowly; monitor cardiovascular effects.
Cattle	IM or SC	0.01-0.02 mg/kg	q8-12h	Duration: Short-term for bronchospasm Notes: Not approved; use with veterinary supervision.
Small Ruminants	IM or SC	0.01-0.02 mg/kg	q8-12h	Duration: Short-term for bronchospasm Notes: Not approved; use with veterinary supervision.
Rabbit	PO	0.1-0.2 mg/kg	q8-12h	Duration: As needed Notes: Limited data; use cautiously.

Clinical Indications & Species Uses

General Indications

Bronchodilation in obstructive airway diseases
Tocolytic agent for uterine relaxation

Dog (Canine)

Bronchodilation in chronic bronchitis
Tracheal collapse
Asthma (feline-like) in dogs
Uterine relaxation (tocolytic) for obstetric procedures

Cat (Feline)

Bronchodilation in feline asthma
Chronic bronchitis

Horse (Equine)

Bronchodilation in recurrent airway obstruction (heaves)
COPD
Tocolytic for management of premature labor (rarely used)

Pharmacology & Mechanism of Action

Drug Class: Beta-2 Adrenergic Agonist | Pharmacological Group: Sympathomimetic

Mechanism of Action: Terbutaline is a selective beta-2 adrenergic receptor agonist that stimulates adenylate cyclase, increasing intracellular cyclic AMP (cAMP) in smooth muscle cells. Elevated cAMP activates protein kinase A, which phosphorylates myosin light chain kinase, reducing its activity and leading to smooth muscle relaxation. In the respiratory tract, this causes bronchodilation. In the uterus, it reduces uterine contractions. It also inhibits mast cell degranulation, reducing release of histamine and other mediators.

Pharmacodynamics: Terbutaline produces relaxation of bronchial smooth muscle, resulting in bronchodilation and decreased airway resistance. It also relaxes uterine smooth muscle, inhibits premature labor, and may cause peripheral vasodilation with reflex tachycardia. It has minimal effects on beta-1 receptors at therapeutic doses, but at high doses may cause cardiac stimulation. It also increases mucociliary clearance and may reduce microvascular permeability.

⚡ Pharmacokinetics Summary

Absorption: Terbutaline is well absorbed after oral administration, but undergoes significant first-pass metabolism, reducing bioavailability. After subcutaneous or intravenous administration, absorption is rapid. In horses, oral bioavailability is approximately 20-30%.

Distribution: Terbutaline is widely distributed throughout the body, crosses the placenta, and is distributed into milk. It is not highly protein-bound (approximately 20%).

Metabolism: Terbutaline is metabolized in the liver primarily by sulfation (in humans and some animals) and to a lesser extent by glucuronidation. In horses, metabolism may involve conjugation.

Excretion: Terbutaline and its metabolites are excreted primarily in the urine, with a small amount in feces. Renal excretion involves both glomerular filtration and active tubular secretion.

Half-Life: Dog: 1-2 hours; Cat: 1-2 hours; Horse: 1-2 hours; Cattle: 1-2 hours (estimated); Rabbit: 1-2 hours (estimated).

Bioavailability: Oral: 20-30% in horses; 50-70% in dogs (estimated); subcutaneous: near 100%.

Protein Binding: Approximately 20%

Available Formulations & Strengths

Oral Tablet 2.5 mg, 5 mg (PO)

Injectable Solution 1 mg/mL (1 mL ampule) (SC, IM, IV)

Oral Syrup 0.3 mg/mL (human formulation) (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to terbutaline or other sympathomimetics
Cardiac arrhythmias, especially tachyarrhythmias
Hypertension
Hyperthyroidism
Concurrent use with monoamine oxidase inhibitors (MAOIs)
Use as a tocolytic in cases of fetal distress or uterine hemorrhage

Warnings & Clinical Precautions:

Use with caution in animals with cardiovascular disease, diabetes mellitus, or seizure disorders.
May cause hypokalemia; monitor serum potassium with prolonged use.
In food animals, observe withdrawal times; not approved for use in food animals in many countries.
In horses, may cause sweating, muscle tremors, and restlessness.
In small animals, may cause hyperactivity or anxiety.
Use with caution in pregnant animals; may delay labor.

Adverse Effects & Reactions

Common:

Tachycardia
Tremors
Nervousness
Hyperactivity
Vomiting (in cats)
Sweating (in horses)

Serious / Severe:

Cardiac arrhythmias
Hypotension
Hypokalemia
Pulmonary edema (rare)
Hyperglycemia

Rare:

Paradoxical bronchospasm
Allergic reactions
Seizures

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Beta-blockers (e.g., propranolol)	Antagonizes bronchodilatory effects; may cause severe bronchoconstriction	High
Corticosteroids	Increased risk of hypokalemia	Moderate
Diuretics (e.g., furosemide)	Increased risk of hypokalemia	Moderate
MAOIs	Increased risk of hypertensive crisis	High
Tricyclic antidepressants	Enhanced cardiovascular effects	Moderate
Xanthines (e.g., theophylline)	Additive bronchodilation but increased risk of cardiac and CNS stimulation	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Severe tachycardia
Hypertension
Cardiac arrhythmias
Tremors
Seizures
Hypokalemia
Hyperglycemia
Lactic acidosis

Emergency Treatment Protocol: Discontinue drug. Provide supportive care: monitor ECG, blood pressure, and electrolytes. Administer fluids for hypotension. Use beta-blockers (e.g., propranolol) cautiously to control tachycardia, but avoid in animals with bronchospasm. Treat seizures with diazepam. Correct hypokalemia with potassium supplementation. In severe cases, consider activated charcoal if recent oral ingestion.

Food Animal Withdrawal Times

🥩 Meat: 30 days🥛 Milk: 7 days

Not approved for food animals in the US; withdrawal times are extrapolated from human data and may not be reliable. Consult regulatory authorities.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F), excursions permitted between 15-30°C (59-86°F).

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

Handling & Special Conditions: Protect from light and moisture. Keep in tightly closed container.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for human use; not approved for veterinary use in most countries, but may be used off-label.

Extra-Label (Off-Label) Use: In the US, extra-label use in food animals is prohibited unless under a valid VCPR and with extended withdrawal times. In non-food animals, extra-label use is permitted under AMDUCA.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Terbutaline is primarily used as a bronchodilator in veterinary medicine, especially for conditions like feline asthma, canine chronic bronchitis, and equine heaves. It is also used as a tocolytic to relax the uterus. Due to its beta-2 selectivity, it has fewer cardiac side effects than non-selective beta-agonists, but tachycardia can still occur. It is important to monitor heart rate and serum potassium during therapy. In horses, oral bioavailability is low, so higher doses may be needed. In food animals, use is limited due to lack of approval and withdrawal concerns. Always use the lowest effective dose and consider alternative therapies if adverse effects are problematic.

References & Literature

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