alli works by attaching itself to the natural enzymes in the digestive system. This stops the body from breaking down the fat you eat. If fat is not digested, it passes through the body naturally. alli only attaches itself to enzymes that break down fat -- so it has no bearing on carbs or proteins. Those nutrients are absorbed by the body as they are supposed to be.

The main ingredient in alli is orlistat, which is the key ingredient used in Xenical, a drug prescribed to those individuals who suffer from morbid obesity. However, alli has half the strength of Xenical.

One of the greatest perks of alli is that there are none of the effects that are commonly associated with other diet pills -- racing heart, sleeplessness and jitteriness. alli proponents point out that it works on the digestive system, not the heart or brain. But that does not mean it's without consequence.

Among the side effects are gas with oily spotting, loose stools and more frequent stools that may be hard to control. The more fat you eat, the more likely you are to suffer the uncomfortable effects.