Most proceedures they consider "safe" today involve stapling the stomach to less than 5% of it's original size.
Imagine your stomach the size of a cantelope. They reduce it to the size of a golfball. You cannot eat nearly as much, so you lose weight. There are many drawbacks to the surgery though. First you must be at least 150 lbs overweight. Minimum.
Second, you will feel very nauseated after you eat. Often times you will vomit numerous times when you do eat. But that will subside after a time. (Depending on the person, it may take 3 meals, others several years to get over the vomiting)
Third, all that excess "slack skin" will have to be removed at a later date. This will entail a surgery where the excess is cut away & your body is reshaped. It could include a liposuction to remove the excess fat cells remaining in the body.
My ex-wife & daughter had the procedure done. Ex lost about 160 lbs. Still is ugly, but now doesn't look like a beach ball anymore.
My daughter has lost 80 lbs, is looking much better, but still has a ways to go. It's been over a year for her, she will continue to loose for about another year.
Most authorities will say "you will lose a minimum of 140 lbs" Others will report the weight lost will range from 100-175 lbs. Just depends upon your body.
One last thing they won't tell you, or at least won't stress very hard...............you could die from this proceedure. Either immediately in surgery (common sense), or later from infections, loss of adequate nutrition (starvation without knowing it), or just plain depression. It is a very dangerous surgery, so if your considering it, make sure you go into it with your eyes wide open.
I would rather have open heart surgery, at least the techniques & outcomes are vastly improved.
And I know healthcare!