If you eat an excessive amount of fat or calories, the excess is stored as fat by the body, resulting in weight gain. When you eat fat, your body breaks it down into its simplest components so that it can be digested. Unlike other weight-loss drugs you may have heard about that act in the brain or central nervous system to suppress appetite or to speed up metabolism, XENICAL works in your digestive system to block about one-third of the fat in the food you eat from being digested. Enzymes in your digestive system, called lipases, help digest (or break down) fat. When taken with meals, XENICAL attaches to the lipases and blocks them from breaking down some of the fat you have eaten. The undigested fat cannot be absorbed and is eliminated in your bowel movements. By working this way, XENICAL helps block about one-third of the fat in the foods you eat from being absorbed by your body.