Also Known As: Orlistat, Xenical, Alli, tetrahydrolipstatin

Orlistat (marketed as a prescription under the trade name Xenical by Roche in most countries, or over-the-counter as Alli by GlaxoSmithKline in the United Kingdom and the United States), 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.

The effectiveness of orlistat in promoting weight loss is definite, though modest. Pooled data from clinical trials suggest that people given orlistat in addition to lifestyle modifications, such as diet and exercise, lose about 2–3 kilograms (4.4–6.6 lb) more than those not taking the drug over the course of a year. Orlistat also modestly reduces blood pressure, and appears to prevent the onset of type 2 diabetes, whether due to weight loss itself or to other effects; in a large randomized controlled trial, orlistat was found to reduce the incidence of diabetes by nearly 40% in obese people.

Orlistat is notorious for its gastrointestinal side effects (sometimes referred to as treatment effects), which can include steatorrhea (oily, loose stools). These decrease with time, however, and are the most frequently reported adverse effects of the drug. In the United States, the European Union, and Australia, orlistat is available for sale without a prescription. Over-the-counter approval was controversial in the United States, with consumer advocacy group Public Citizen repeatedly opposing it on safety and efficacy grounds.Generics of orlistat are available in India and Russia.

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