Generic name of Brethine is Terbutaline and it is used to treat or obstruct from symptoms caused by spasms or contractions of the muscles around the bronchioles (air tubes) associated with asthma, bronchitis, and emphysema. Brethine is a bronchodilator and it works by dilating (opening) the bronchioles of the lungs by relaxing the muscles around them. This helps for easier airflow into and out of the lungs