Orlistat (the active ingredient in Alli) promotes weight loss by decreasing the amount of dietary fat absorbed in your intestines. Lipase, an enzyme found in the digestive tract, helps break down dietary fat into smaller components, so it can be used or stored for energy. Orlistat inhibits the work of lipase

Orlistat (brand name: alli) is an FDA-approved OTC medication that can help overweight adults lose weight. Orlistat works by blocking fat from being absorbed into the body after it is eaten during a meal. Approximately 5 to 10 pounds may be lost during the first 6 months of orlistat use