Orlistat, the generic form of Xenical, works by reducing the amount of fat absorbed from your diet; it is not an appetite suppressant.
When fat enters the body, it is broken down by enzymes called lipases, so that the body can digest it. Orlistat attaches itself to the enzymes in the body so that they cannot break down the fat, which stops about one third of the fat from your meal from being absorbed, so that your body passes the fat rather than storing it.
The undigested fat is then passed through your digestive system with no unwanted effects on your heart rate or sleeping pattern. Orlistat has been tested in hundreds of clinical studies and has proven to be very safe, yet very efficient.
Orlistat should only be used by those who are medically obese; those with a BMI of 30 or more or a BMI of 28 or more in those with other risk factors such as diabetes or hypertension.