Montelukast sodium is a selective cysteinyl leukotriene receptor (CysLT1) antagonist used for the chronic management of asthma and to relieve seasonal allergic rhinitis symptoms. By blocking leukotriene D4 binding in the airway, it reduces bronchoconstriction, mucus secretion, and inflammatory cell infiltration, and is typically dosed once daily as an oral tablet or granule.