Singulair (Montelukast) is used to control long-term asthma. It is also used to treat allergic rhinitis or hay fever. There are two types of allergic rhinitis. Outdoor allergies that happen during part of the year are also known as seasonal allergic rhinitis. Indoor allergies that happen all year are also known as perennial allergic rhinitis. This drug works by blocking substances in the body called leukotrienes that make asthma worse. Blocking leukotrienes improves asthma control. It also improves allergic rhinitis symptoms such as sneezing and stuffy, runny, itchy nose.