Montelukast Sodium
**Montelukast sodium** is a leukotriene receptor antagonist. In veterinary medicine, it is primarily used off-label in **cats** for inflammatory conditions such as: * **Feline asthma** * **Inflammatory bowel disease (IBD)** * Chronic upper respiratory infections ("snufflers") * **Heartworm-associated respiratory disease (HARD)** syndrome > **Clinical Pearl:** While theoretically promising for feline asthma by blocking inflammatory mediators, clinical efficacy has been largely disappointing. It is generally considered a second- or third-line adjunctive therapy rather than a primary treatment. A small trial in horses with Recurrent Airway Obstruction (RAO) also failed to show efficacy. In humans, it is FDA-approved for allergic rhinitis and asthma.
Mechanism: Montelukast acts as a highly selective and competitive **cysteinyl leukotriene receptor (CysLT1) antagonist**. * Arachidonic acid → 5-lipoxygenase pathway → **Cysteinyl leukotrienes (LTC4, LTD4, LTE4)**. * These leukotrienes are potent inflammatory mediators released by **mast cells** and **eosinophils**. * By blocking the **CysLT1 receptor** in the airways and gastrointestinal tract, montelukast prevents leukotriene-mediated bronchoconstriction, mucosal edema, vascular permeability, and eosinophil recruitment.
Dosing by species
- Feline asthma · 0.5-1 mg/kg PO once daily (q24h) · PO · q24h · Anecdotal reports of efficacy; more conclusive evidence is needed before regular use.
- Adjunctive treatment of the chronic 'snuffler' · 0.25-0.5 mg/kg PO q24h · PO · q24h · i.e., 1/8th of a 10 mg Singulair tablet.
- Inflammatory Bowel Disease (IBD) · 0.5-1 mg/kg PO once daily · PO · q24h · For mild cases involving eosinophils and lymphocytes. May be used in combination with other drugs.
- Adjunctive treatment of feline heartworm · 2 mg total dose (PO) once daily · PO · q24h · Anecdotal evidence suggests it may help thwart acute, fatal lung injury when an adult worm dies.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Hypersensitivity to montelukast or any component of the product
- Not for use in reversing acute bronchospasm (asthma attacks)
Adverse effects
- No significant adverse effects reported in cats to date
- Humans (rare): behavioral effects (aggression, suicidal thoughts), palpitations, cholestatic hepatitis, allergic granulomatous angiitis
Drug interactions
- Phenobarbital · CYP-450 enzyme inducer; may reduce montelukast plasma concentrations and efficacy.
- Rifampin · CYP-450 enzyme inducer; may reduce montelukast plasma concentrations and efficacy.
- Prednisone / Prednisolone · Severe peripheral edema has been reported in a human receiving both drugs, presumably due to increased renal tubular sodium and fluid retention. Clinical significance in veterinary patients is unclear.
Monitoring
- Clinical efficacy (improvement in respiratory or GI signs)
Overdose
**Overdose Toxicity:** Montelukast is relatively safe in overdose situations. * **Animal Data:** Rats and mice survived oral doses of approximately 230X to 335X the usual human adult dose. * **Human Data:** Doses as high as 1,000 mg have been reported with the majority having no adverse effects. * **Clinical Signs (if present):** Headache, vomiting, psychomotor hyperactivity, thirst, somnolence, mydriasis, hyperkinesia, and abdominal pain. * **Treatment:** Basically supportive. No specific antidote is known.
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.