From
WebMD: (this is specifically about bypass surgery)...
Recovery and Potential Complications
After gastric bypass surgery, people typically stay in the hospital for 2 to 3 days and return to normal activity within 2 to 3 weeks. About 10% of people have complications, which are usually minor and include:
Wound infections
Digestive problems
Ulcers
Bleeding
Nearly 1% to 5% of people have serious or life-threatening complications, such as:
Blood clot (pulmonary embolism)
Heart attack
Leak in the surgical connections with the intestines
Serious infection or bleeding
The risk of complications is lower at centers that do more than 100 weight loss surgeries per year. Deaths in the month following gastric bypass surgery are very rare (about 0.2% to 0.5%, or less than one in 200 people) when the procedure is done by a highly experienced surgeon.
Other health problems can also happen as a result of the surgery. For example, not absorbing as much of nutrients like iron and calcium can cause anemia and osteoporosis. But taking nutritional supplements and getting blood tests can make that less likely.