Bronchodilator, brong koh dy LAY tuhr or brong koh duh LAY tuhr, is any of a group of drugs that open up the small breathing tubes in the lungs. These tubes are called bronchioles. Doctors prescribe bronchodilators to treat respiratory illnesses, including asthma, bronchitis, and emphysema. The drugs relax the muscles in the bronchioles, thereby expanding the tubes and making breathing easier. This can relieve such symptoms as coughing and wheezing.