Unlike most prescription diet pills, Xenical (also known as Orlistat), is not an appetite suppressant. It is a Fat blocker. It helps to maintain weight loss when used in conjunction with a low calorie diet.

In general, it blocks fat that you eat from absorbing through the digestive tract and into the bloodstream.

However, it can also cause certain vitamins from being absorbed into the body - hence, people who take Xenical should also be taken with multivitamins.
Possible side effects include oily spotting, gas, oily, or fatty stools, an increased number of bowl movements, or an inability to control bowel movements.