Montelukast Sodium — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	0.5-1 mg/kg	q12h	Duration: Variable; often long-term for chronic conditions Notes: Administer on an empty stomach for optimal absorption. Use with caution in patients with hepatic impairment.
Cat	PO	2-5 mg per cat (total dose)	q24h	Duration: Variable; often long-term for asthma management Notes: Commonly used as adjunctive therapy to corticosteroids. May take 2-4 weeks for full effect.
Horse	PO	0.1-0.2 mg/kg	q12h	Duration: Variable; often used during flare-ups or as maintenance Notes: Limited evidence; may be used as adjunct to environmental management and corticosteroids.
Rabbit	PO	1-2 mg/kg	q24h	Duration: Variable; based on clinical response Notes: Limited evidence; use with caution.

Clinical Indications & Species Uses

General Indications

Allergic respiratory conditions
Leukotriene-mediated inflammation

Dog (Canine)

Allergic bronchitis
Chronic cough
Eosinophilic bronchopneumopathy
Feline asthma (off-label use in dogs)

Cat (Feline)

Feline asthma
Chronic bronchitis
Allergic airway disease

Horse (Equine)

Recurrent airway obstruction (RAO) - adjunctive therapy
Inflammatory airway disease (IAD) - adjunctive therapy

Rabbit & Small Mammals

Respiratory disease with allergic component (limited evidence)

Exotic & Other Species

Guinea pigs: respiratory disease (limited evidence)

Pharmacology & Mechanism of Action

Drug Class: Leukotriene receptor antagonist | Pharmacological Group: Anti-inflammatory / Anti-asthmatic

Mechanism of Action: Montelukast is a selective and orally active leukotriene receptor antagonist that specifically inhibits the cysteinyl leukotriene receptor (CysLT1). Cysteinyl leukotrienes (LTC4, LTD4, LTE4) are pro-inflammatory mediators released from mast cells, eosinophils, and other inflammatory cells. By blocking the CysLT1 receptor, montelukast prevents leukotriene-mediated bronchoconstriction, increased vascular permeability, mucus secretion, and eosinophil recruitment, thereby reducing airway inflammation and bronchospasm.

Pharmacodynamics: Montelukast antagonizes the effects of cysteinyl leukotrienes at the CysLT1 receptor, leading to bronchodilation, reduced airway edema, and decreased mucus production. It also reduces eosinophilic inflammation in the airways. In veterinary species, its efficacy is variable; it is used primarily for its anti-inflammatory effects in allergic and respiratory conditions, though evidence is limited in many species.

⚡ Pharmacokinetics Summary

Absorption: Rapidly absorbed after oral administration. In humans, peak plasma concentrations occur within 3-4 hours. In dogs, absorption is rapid with peak levels at approximately 1-2 hours. Food may affect absorption; administration on an empty stomach is often recommended for consistent absorption.

Distribution: Highly protein-bound (approximately 99% in humans). Distributed widely, with low penetration into the central nervous system. In animals, distribution is not well characterized but is expected to be similar.

Metabolism: Extensively metabolized in the liver via cytochrome P450 enzymes (CYP3A4, CYP2C9, and CYP2C8). Metabolites are inactive.

Excretion: Primarily eliminated via bile in feces. In humans, less than 1% is excreted unchanged in urine. In animals, similar biliary excretion is expected.

Half-Life: Dogs: approximately 3-4 hours; Cats: approximately 2-3 hours (estimated); Humans: 2.7-5.5 hours.

Bioavailability: Oral bioavailability is approximately 64% in humans; in dogs, it is approximately 73%.

Protein Binding: Approximately 99% in humans; similar in animals.

Available Formulations & Strengths

Oral Tablet 4 mg, 5 mg, 10 mg (PO)

Chewable Tablet 4 mg, 5 mg (PO)

Oral Granules 4 mg/packet (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to montelukast or any component of the formulation
Patients with phenylketonuria (chewable tablets contain phenylalanine)
Severe hepatic impairment (use with caution)

Warnings & Clinical Precautions:

Use with caution in patients with hepatic disease.
Not recommended for acute bronchospasm; should not replace rescue bronchodilators.
In cats, may take several weeks to achieve full therapeutic effect; do not discontinue abruptly.
Safety in pregnant or lactating animals has not been established; use only when clearly needed.
In horses, efficacy is variable; monitor clinical response.
Store at room temperature away from moisture and heat.

Adverse Effects & Reactions

Common:

Gastrointestinal upset (vomiting, diarrhea)
Lethargy
Increased thirst

Serious / Severe:

Hepatotoxicity (rare)
Hypersensitivity reactions (angioedema, urticaria)
Behavioral changes (agitation, restlessness)

Rare:

Eosinophilia
Churg-Strauss syndrome (in humans; not reported in animals)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Phenobarbital	May decrease montelukast plasma concentrations due to CYP3A4 induction.	Moderate
Rifampin	May decrease montelukast plasma concentrations due to CYP3A4 induction.	Moderate
Gemfibrozil	May increase montelukast plasma concentrations due to CYP2C8 inhibition.	Moderate
Corticosteroids	Additive anti-inflammatory effects; may allow for reduced corticosteroid dosage.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Vomiting
Diarrhea
Lethargy
Ataxia
Tremors (in severe cases)

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce vomiting if ingestion is recent and the animal is conscious. Administer activated charcoal to reduce absorption. Monitor for respiratory depression and provide supportive care as needed. There is no specific antidote.

Food Animal Withdrawal Times

Not approved for use in food animals; no withdrawal times established. Use in food animals is off-label and should be avoided.

Storage, Handling & Regulatory Information

Storage Temperature: Store at 20°C to 25°C (68°F to 77°F); excursions permitted between 15°C and 30°C (59°F and 86°F).

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

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

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use.

Extra-Label (Off-Label) Use: In the US, montelukast is not FDA-approved for veterinary use. Use in animals is extra-label and must comply with AMDUCA regulations. For food animals, extra-label use is prohibited if the drug is not approved for that species and if it may cause residues.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Montelukast is used off-label in veterinary medicine primarily for allergic respiratory conditions such as feline asthma and canine chronic bronchitis. It is not a bronchodilator and should not be used for acute bronchospasm. Clinical response may take several weeks; therefore, it is often used as an adjunct to corticosteroids. In horses, it may be considered for RAO but evidence is limited. Always monitor liver function during prolonged therapy. Due to lack of safety data in pregnant and lactating animals, use with caution. The drug is generally well tolerated, but gastrointestinal upset is the most common adverse effect.

References & Literature

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