Guaifenesin — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	2-5 mg/kg	q8h or q12h	Duration: 5-7 days or as needed Notes: Commonly used in combination with other antitussives or bronchodilators. Not FDA-approved for veterinary use, but commonly used off-label.
Cat	PO	2-5 mg/kg	q8h or q12h	Duration: 5-7 days or as needed Notes: Use with caution in cats with renal impairment. Palatability may be an issue.
Horse	IV	50-100 mg/kg (as slow IV infusion)	Single dose for anesthesia induction	Duration: Given to effect for muscle relaxation during anesthesia Notes: Administered as a 5-10% solution in sterile water or saline. Often combined with ketamine or thiopental for induction. For expectorant use: 50-100 mg/kg PO q12h (but oral absorption is variable).
Horse	PO	50-100 mg/kg	q12h	Duration: 5-7 days Notes: For expectorant use; may be less effective due to variable absorption.
Cattle	PO	2-5 mg/kg	q12h	Duration: 5-7 days Notes: For expectorant use; not commonly used in food animals due to withdrawal times.
Small Ruminants (Sheep/Goats)	PO	2-5 mg/kg	q12h	Duration: 5-7 days Notes: For expectorant use; use with caution in animals with renal disease.
Rabbit	PO	2-5 mg/kg	q12h	Duration: 5-7 days Notes: Limited data; use with caution.

Clinical Indications & Species Uses

General Indications

As an expectorant to relieve cough and promote clearance of respiratory secretions
As a centrally acting muscle relaxant in large animal anesthesia (especially horses)

Dog (Canine)

As an expectorant for cough associated with respiratory tract infections, bronchitis, and tracheitis
As an adjunct in the management of kennel cough (infectious tracheobronchitis)

Cat (Feline)

As an expectorant for cough associated with respiratory tract infections, bronchitis, and asthma (though less commonly used)

Horse (Equine)

As a muscle relaxant during anesthesia (as an adjunct to induction and maintenance)
As an expectorant for respiratory conditions such as chronic obstructive pulmonary disease (COPD) and pneumonia

Cattle (Bovine)

As an expectorant for respiratory disease complex (shipping fever) and pneumonia
As a muscle relaxant in some anesthetic protocols (rarely used)

Small Ruminants (Sheep / Goat)

As an expectorant for respiratory infections (e.g., pneumonia) in sheep and goats

Pharmacology & Mechanism of Action

Drug Class: Expectorant | Pharmacological Group: Mucolytic/Expectorant

Mechanism of Action: Guaifenesin is thought to act by increasing the volume and reducing the viscosity of respiratory tract secretions. It stimulates the gastric mucosal reflex (via vagal afferents) which increases respiratory tract fluid, thereby enhancing the removal of mucus. It also has a mild central depressant effect on the cough center, but its primary use in veterinary medicine is as a muscle relaxant (centrally acting) when used at high doses, particularly in horses. The muscle relaxant effect is due to depression of polysynaptic reflexes in the spinal cord and brainstem, leading to general muscle relaxation without loss of consciousness.

Pharmacodynamics: At therapeutic doses for expectoration, guaifenesin increases respiratory tract fluid and reduces mucus viscosity, facilitating its expulsion. At higher doses (as used in equine anesthesia), it produces dose-dependent muscle relaxation, sedation, and ataxia. It has minimal effect on cardiac and respiratory function at typical doses, but high doses can cause respiratory depression. The onset of muscle relaxation after IV administration is rapid (within minutes) and lasts for 10-30 minutes.

⚡ Pharmacokinetics Summary

Absorption: Rapidly absorbed from the gastrointestinal tract after oral administration. Peak plasma concentrations occur within 0.5-1 hour. In horses, oral bioavailability is approximately 50-60% due to first-pass metabolism.

Distribution: Widely distributed throughout the body, including the central nervous system. It crosses the blood-brain barrier. Volume of distribution is approximately 0.7-1 L/kg. Protein binding is low (approximately 10-20%).

Metabolism: Extensively metabolized in the liver, primarily via O-demethylation and oxidation. The major metabolite is beta-(2-methoxyphenoxy)-lactic acid, which is inactive.

Excretion: Excreted primarily in the urine as metabolites and unchanged drug (less than 5% unchanged). Renal excretion is via glomerular filtration and tubular secretion. In horses, the elimination half-life is approximately 1-2 hours.

Half-Life: Horse: 1-2 hours; Dog: 1-3 hours; Cat: 1-2 hours (estimated)

Bioavailability: Oral: ~50-60% in horses; not well established in other species

Protein Binding: Approximately 10-20%

Available Formulations & Strengths

Oral Tablet 100 mg, 200 mg, 400 mg (PO)

Oral Liquid/Syrup 100 mg/5 mL, 200 mg/5 mL (PO)

Injectable Solution 50 mg/mL, 100 mg/mL (for IV use in horses) (IV)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to guaifenesin or any component of the formulation
Use in animals with known renal impairment (due to potential accumulation of metabolites)
Use in animals with myasthenia gravis (due to muscle relaxant effects at high doses)
Concurrent use with other CNS depressants (e.g., barbiturates, opioids) may cause excessive sedation

Warnings & Clinical Precautions:

For IV use in horses, administer slowly to avoid respiratory depression and hypotension.
Oral formulations may contain alcohol or sugar; use with caution in diabetic animals.
Use with caution in animals with hepatic impairment.
In food animals, observe withdrawal times.
Not approved for veterinary use in many countries; use should be based on clinical judgment and legal regulations.
In horses, high doses may cause ataxia and recumbency; ensure proper padding and monitoring during anesthesia.

Adverse Effects & Reactions

Common:

Nausea
Vomiting
Diarrhea
Drowsiness
Ataxia (at high doses)

Serious / Severe:

Respiratory depression (at high IV doses)
Hypotension (at high IV doses)
Allergic reactions (rare)

Rare:

Hemolytic anemia (reported in humans, not in animals)
Methemoglobinemia (reported in humans, not in animals)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
CNS depressants (barbiturates, opioids, phenothiazines)	Additive CNS depression and respiratory depression	High
Neuromuscular blocking agents	Additive muscle relaxation	Moderate
Anticholinergics	May reduce the expectorant effect by drying secretions	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Ataxia
Recumbency
Respiratory depression
Hypotension
Coma
Muscle weakness

Emergency Treatment Protocol: Discontinue drug immediately. Provide supportive care: maintain airway, assist ventilation if needed, administer IV fluids for hypotension, and monitor vital signs. In severe cases, use CNS stimulants (e.g., doxapram) if respiratory depression is significant. There is no specific antidote.

Food Animal Withdrawal Times

🥩 Meat: 7 days🥛 Milk: 3 days

Withdrawal times are not established for guaifenesin in food animals. The values provided are estimates based on pharmacokinetic data and should be used with caution. Consult regulatory guidelines and a veterinarian for specific recommendations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (20-25°C, 68-77°F). Protect from freezing.

Handling & Special Conditions: Keep container tightly closed. Protect from moisture. Injectable solutions should be used within 24 hours after dilution.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use as an OTC expectorant.

Extra-Label (Off-Label) Use: In the US, guaifenesin is not FDA-approved for veterinary use, but it can be used legally under the Animal Medicinal Drug Use Clarification Act (AMDUCA) in an extra-label manner by or on the order of a veterinarian. For food animals, a valid veterinarian-client-patient relationship (VCPR) is required, and withdrawal times must be observed.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Guaifenesin is a versatile drug in veterinary medicine. Its primary use is as an expectorant for cough associated with respiratory diseases, but it is also widely used as a muscle relaxant in equine anesthesia. In horses, it is often combined with ketamine and/or thiopental for induction of anesthesia, providing smooth muscle relaxation and recumbency. For expectorant use, it is often combined with other agents such as bronchodilators or antitussives. It is generally well tolerated, but high doses can cause CNS depression and ataxia. In food animals, withdrawal times must be considered. Always use according to legal regulations and clinical judgment.

References & Literature

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