Los Angeles: The first over-the-counter diet drug approved by the US Food & Drug Administration has gone on sale. In some cities there were stampedes as the drug called Alli sold out.

Alli is a lower dose version of the prescription-only drug called Xenical that blocks absorption of fat. It works by disabling some of the natural enzymes in the digestive system that break down fat for absorption. When those enzymes can’t do their job, excess fat passes through the body.

Those who use alli “may recognize it in the toilet as something that looks like the oil on top of pizza,” according to the product Web site – www.myalli.com Nutrients absorbed from carbohydrates and proteins are not affected

The drug blocks about one-quarter of fat consumed. When used along with a healthy diet plan and regular exercise, about half of people taking Alli in clinical studies lost 5 percent of their body weight in six months.

But the drug has some unpleasant side effects. Digestive side effects include gas with oily spotting, loose stool, and hard-to-control bowel movements, reports its manufacturer GlaxoSmithKline.These side effects are more likely when a person consumes more than 30 percent of fat in a meal.

The drug is relatively expensive at $40 to $50 for 20- and 30-day starter kits may have put customers off.