After food is eaten, it is digested in the stomach. The digested nutrients from food are absorbed, mainly in the intestines, into the bloodstream and are used as fuel for the body.When people eat more food than they need, the body stores the extra nutrients as fat, which accounts for weight gain.

Some diet drugs are able to prevent the body from absorbing some of the calories contained in food. People who take these drugs can eat the same amount of food and take in the same number of calories and still lose weight, because the drug causes their bodies to absorb and use fewer calories. The excess calories are eliminated in the stool. Drugs that prevent absorption of nutrients in the intestine may also prevent other drugs a person may be taking from being absorbed. For this reason, it is important not to take these diet pills at the same time as other medications, and to separate them by a few hours. Since these drugs affect the digestive system, including the stomach and intestines, they may also lead to side effects like diarrhea and flatulence (passing gas).

Currently, the only drug on the market that works by blocking the body’s absorption of calories is orlistat, which is sold under the brand name Xenical®. Orlistat specifically decreases the absorption of fats from food consumed.

Reference:

1. Diet Pills (Drugs: the Straight Facts) by Debra Henn and Deborah DeEugenio (Hardcover – Oct 2004)