Alli is claimed to be a "fat inhibitor," which basically means that it won't burn fat that already exists, but it aims to block fat from being absorbed.  Alli is a product that has been relatively popular over the past couple of years because it produces moderate results. Alli has a reputation for causing some unwanted side effects.  Users have had some unpleasant experiences using Alli including some of the following: Gas with oily spotting, loose stools, more frequent uncontrollable stools.