Serious side effects of weight loss surgery include:
Bleeding in stool. Your stool can be reddish or black, which can be serious. Tell your doctor about this right away, or go to an emergency room.
Blood clots. Blood clots in the lungs are rare and happen less than 1% of the time. They can be life-threatening. But blood clots can usually be prevented with blood thinning drugs and frequent activity.
Leaks. Leaks in the new connections made by weight loss surgery are rare, but serious. They usually happen within 5 days of the surgery. Belly pain and feeling ill are common symptoms. Call your doctor.