Orlistat (Xenical, Roche) is one of a handful of antiobesity drugs that, when used appropriately, can cause significant weight loss with acceptable safety.1 It inhibits the gut lipases that hydrolyse ingested triglyceride (which constitutes almost all dietary fat) and decreases the absorption of lipid, which is the most energy dense nutrient. In clinical trials, such as those included in a systematic review by Rucker and colleagues published in this week’s BMJ, up to a third of obese people taking the standard therapeutic dosage (120 mg three times daily) lost at least 10% of their initial weight.1 This is the threshold value that is generally assumed to confer clinically important reductions in the metabolic and cardiovascular risks associated with obesity.2

The drug acts only in the gut lumen and—apart from potential deficiencies of fat soluble vitamins with chronic use—it seems to be safe. The main side effect is steatorrhoea (excess fat in the faeces), usually as a result of eating food high in fat, which obese people should avoid. Other side effects include faecal incontinence. Orlistat is widely prescribed under medical supervision to supplement—not replace—lifestyle modifications, primarily eating less and exercising more, which remain the key to success in managing obesity.

In 2006, the American Food and Drug Administration granted GlaxoSmithKline (GSK) approval to sell a 60 mg preparation of orlistat (Alli) “over the counter”—that is, directly from …