Montelukast (Singulair) is an oral leukotriene receptor antagonist (LTRA) for the maintenance treatment of asthma and to relieve symptoms of seasonal allergies. It is not useful for the treatment of acute asthma attacks, so patients should also be supplied with rescue medication, such as an albuterol inhaler. Montelukast blocks the action of leukotriene D4 on the cysteinyl leukotriene receptor CysLT1, thus inhibiting bronchoconstriction.