As with any major surgery, gastric bypass carries risks such as bleeding, infection and an adverse reaction to the anesthesia. Possible risks specific to this surgery include:

Death. A risk of death has been associated with gastric bypass surgery. It has generally been reported to be one death per 200 to 300 surgeries. Higher risks have been reported amongst Medicare recipients. The risk varies depending on age, general health and other medical conditions. Talk to your doctor about the exact level of risk gastric bypass surgery may pose for you.

Blood clots in the legs. Blood clots in the legs are more likely to occur in very overweight people. Blood clots can be dangerous. In some cases, they travel to the lungs and lodge in the lungs' arteries causing a pulmonary embolism — a serious condition that damages lung tissue and can lead to death. Walking and using leg wraps that apply intermittent pressure to the leg can help reduce this risk of blood clots in the legs. Smoking has been shown to increase the risk of clotting in people undergoing gastric bypass surgery. Quitting smoking is strongly recommended.

Leaking at one of the staple lines in the stomach. This severe postoperative problem is treated with antibiotics. Many cases heal with time. Often, however, the leak can be serious enough to require emergency surgery.

Incision hernia. An incision hernia is a weakness in the incision. This is more likely to occur if you have an open procedure and a large abdominal incision. This usually requires surgical repair, depending on the symptoms and the extent of the hernia.

Narrowing of the opening between the stomach and small intestine. This rare complication may require either an outpatient procedure — in which a tube is passed through your mouth to widen (dilate) the narrowed opening — or a corrective surgery.
Dumping syndrome. This is a condition where stomach contents move too quickly through the small intestine, causing nausea, vomiting, diarrhea, dizziness and sweating. It's frequently experienced after eating sweets or high-fat foods.
Other potential complications of gastric bypass surgery include:

Vitamin and mineral deficiency (iron deficiency anemia, vitamin B-12 deficiency and vitamin D deficiency)
Dehydration
Gallstones
Bleeding stomach ulcer
Intolerance to certain foods
Kidney stones
Low blood sugar (hypoglycemia) related to excessive insulin production