Gastric bypass surgery is major surgery with both short and long-term risks. Short-term risks include excessive bleeding, infection, blood clots, respiratory complications, leaks in the gastrointestinal system, and an adverse reaction to anesthesia. Long-term complications are associated with the changes in your digestive system from the surgery and include bowel obstruction, dumping syndrome, gallstones, hernias, hypoglycemia, malnutrition, stomach perforation, ulcers, and vomiting. Many of the complications from gastric bypass procedures can be avoided by closely following your doctor's instructions before and after surgery.