Singulair is one type of a class of drug called anti-leukotrienes or leukotriene inhibitors. It blocks the activity of undesirable leukotrienes , produced by the body, that cause the symptoms you describe. It is often helpful for the treatment of nasal allergies, its biologic action different than that of antihistamines and inhaled nasal steroids but its actions often complementary to these drugs.
It is often effective for the symptoms you describe.
Good luck