Orlistat is a potent and selective inhibitor of various lipase enzymes responsible for the metabolism of fat. It acts in the gastrointestinal (GI) tract via covalent binding to the serine residues located on the active site of both gastric and pancreatic lipase. When orlistat is taken with food containing fat, it partially inhibits the hydrolysis of triglycerides. This decreases absorption of monoaclglycerides and free fatty acids, contributing to weight maintenance and weight loss.