Singulair is a leukotriene (loo-koe-TRY-een) inhibitor. Leukotrienes are chemicals your body releases when you breathe in an allergen (such as pollen). These chemicals cause swelling in your lungs and tightening of the muscles around your airways, which can result in asthma symptoms. Singulair is used to prevent asthma attacks in adults and children as young as 12 months old. It is ... used to relieve runny nose and sneezing caused by allergies in adults and children as young as 6 months old.Do not give Singulair to a child without a doctor's advice. Singulair is also used to prevent exercise-induced bronchoconstriction (narrowing of the air passages in the lungs)..