Gastric bypass surgery is any of a number of similar surgical procedures where the stomach is sub-divided and both parts of it are attached to the small intestine. It is used to treat morbid obesity. The procedures reduce the functional size of the stomach, changing the physiology of the patient's digestive tract. The surgery has both high risks and high rewards. Against control groups, the long-term mortality rate of patients who receive the surgery drops 40%. However, about 15% of patients have complications and about 1 in 200 die within six months of the procedure being performed.