Orlistat is a reversible inhibitor of lipases. It exerts its therapeutic activity in the lumen of the stomach and small intestine by forming a covalent bond with the active serine residue site of gastric and pancreatic lipases. The inactivated enzymes are thus unavailable to hydrolyze dietary fat in the form of triglycerides into absorbable free fatty acids and monoglycerides. As undigested triglycerides are not absorbed, the resulting caloric deficit may have a positive effect on weight control.
Orlistat is a lipase inhibitor for obesity management that acts by inhibiting the absorption of dietary fats. At the recommended therapeutic dose of 120 mg three times a day, orlistat inhibits dietary fat absorption by approximately 30%. It works by inhibiting pancreatic lipase, an enzyme that breaks down fat in the intestine. Without this enzyme, fat from the diet is excreted undigested and not absorbed by the body. Because some vitamins are fat soluble, the effect of orlistat is to reduce their body absorption. Therefore the drug should only be taken in conjuction with fatty meals, and a multivitamin tablet containing these vitamins (D E K and beta-carotene) should be taken once a day, at least 2 hours before or after taking the drug. In the March 15, 2004 issue of Cancer Research,  Steven J. Kridel et al. state that orlistat may also inhibit growth of prostate cancer, and in theory may be useful in treating other cancers, by interfering with the metabolism of fats.
Metabolized primarily within the gastrointestinal wall forming relatively inactive metabolites. Metabolites M1 (4-member lactone ring hydrolyzed) and M3 (M1 with N-formyl leucine moiety cleaved) accounted for approximately 42% of total radioactivity in plasma. M1 and M3 have an open beta-lactone ring and extremely weak lipase inhibitory activity (1000- and 2500-fold less than orlistat, respectively).
The results of a massive overdose of Xenical are unknown, although the drug seems relatively harmless.