Montelukast sodium is a prescription medicine that blocks substances in the body called leukotrienes. This may help to improve symptoms of asthma and allergic rhinitis. Montelukast sodium does not contain a steroid. Montelukast sodium is used to: 1. Prevent asthma attacks and for the long-term treatment of asthma in adults and children ages 12 months and older. Do not take montelukast sodium if you need relief right away for a sudden asthma attack. If you get an asthma attack, you should follow the instructions your healthcare provider gave you for treating asthma attacks. 2. Prevent exercise-induced asthma in people 6 years of age and older. 3. Help control the symptoms of allergic rhinitis (sneezing, stuffy nose, runny nose, itching of the nose). Montelukast sodium is used to treat outdoor allergies that happen part of the year(seasonal allergic rhinitis) in adults and children ages 2 years and older, and indoor allergies that happen all year (perennial allergic rhinitis) in adults and children ages 6 months and older.