Orlistat (marketed under the trade name Xenical by Roche; or over-the-counter as alliAlways rendered with a lowercase a, i.e. as alli. by GlaxoSmithKline (, like the English word "ally")?also known as tetrahydrolipstatin?is a drug designed to treat obesity. Its primary function is preventing the absorption of fats from the human diet, thereby reducing caloric intake. It is intended for use in conjunction with a physician-supervised reduced-calorie diet. Orlistat is the saturated derivative of lipstatin?a potent natural inhibitor of pancreatic lipases isolated from the bacterium Streptomyces toxytricini. However, due to simplicity and stability, orlistat rather than lipstatin was developed into an anti-obesity drug. Pharmacology Orlistat works by inhibiting