Montelukast sodium is a medication used to treat asthma and seasonal allergic rhinitis. It is a leukotriene receptor antagonist that works by blocking the action of leukotrienes, which are chemicals released by the body that cause inflammation and narrowing of the airways in the lungs. By blocking leukotrienes, montelukast sodium helps to reduce inflammation and improve breathing in people with asthma and other respiratory conditions. Montelukast sodium is available in tablet and granule form and is usually taken once daily in the evening. It is a prescription-only medication and should be taken under the guidance of a healthcare professional.