The whole point of taking a weight loss product is that it is SUPPLEMENTING a change in your diet and exercise. If you have taken the necessary strides to change what you eat and you are trying to get the right amount of exercise, you shouldn't gain any back. The product isn't the "answer" to permanent weight loss. Diet and exercise are the fundamental keys to maintianing your weight loss.

Depends on the kind of product and if you are going to still use diet and exercise. You have to give more details to better answer your question, like what kind of diet pill, what else you are doing, etc.