Alli is a low fat diet I believe. It was created to help our body by pass digesting fat. It has been around as a prescription drug for several years. The one side effect that I have heard the most about is problems with having episodes of incontinence because of the explosive bowel movements.
If you go to their website
http://www.myalli.com/ there is quite a bit of information to be found.
Here is a quote from the website:
The active ingredient in alli attaches to some of the natural enzymes in the digestive system, preventing them from breaking down about a quarter of the fat you eat. Undigested fat cannot be absorbed and passes through the body naturally. The excess fat is not harmful. In fact, you may recognize it in the toilet as something that looks like the oil on top of a pizza.
Because of the way in which alli works, eating meals with too much fat may cause side effects known as treatment effects
Hope this helps!