There are many weight-loss pills helping overweight people to lose excess weight. Alli is a less-potent version of the prescription diet pill, Xenical (orlistat). It is a fat blocker; the main ingredient in the pill binds with the digestive enzymes that would normally break down fat from the meal that you eat. Because Alli attaches to these enzymes, it prevents them from digesting about a quarter of the fat you just ate, allowing it to pass through the digestive system and out of the body, undigested and unabsorbed. Instead of affecting your brain or nervous system, Alli does all of its work in your digestive system. If you experience unpleasant side effects such as loose or more frequent stools that are hard to control or gas with oily discharge, it is an indication that you are eating too much fat. Meal planning is an important aspect of a successful diet. You should eat a calorie controlled diet. It is also recommended to eat three large meals throughout the course of the day, with healthy snacks in between meals.