Pharmacies are reporting brisk sales of Alli (pronounced like the noun ''ally''), which is sold by drug maker GlaxoSmithKline and is the first over-the-counter diet drug to win FDA approval. Unlike other prescription weight-loss drugs such as Meridia and the generic phentermine, Alli doesn't make you feel full, reduce cravings or curb your appetite. Instead, it prevents the body from breaking down and absorbing fat.

The active ingredient in Alli is orlistat, which is found in a higher dose in the prescription diet drug Xenical. Alli blocks about 25 percent of the fat you eat; Xenical blocks one-third of the fat you ingest. If you consume about 2,000 calories a day and eat about 30 percent fat, the fat-blocking benefits of Alli would translate to about 150 calories a day. A pound of weight loss equals 3,500 calories.

The downside of Alli is the fat it blocks can come out of your body in embarrassing ways. The Glaxo Web site, myalli.com, warns the drug can cause gas with oily discharge as well as frequent or loose stools. The site suggests it's probably a ''smart idea'' to wear dark pants and bring a change of clothes to work if you use Alli.

To avoid the side effects, Glaxo suggests limiting fat intake to 15 grams a meal. Many Americans consume 80 to 100 grams of fat a day. Glaxo officials concede that many people would lose weight on their own with a diet that's moderate in fat, but that the pill helps them lose more weight.

''If you'd lose 10 pounds on a diet, you'll lose 15 pounds by adding Alli to your diet,'' says Vidhu Bansal, director of medical affairs for Glaxo's consumer-health division.

If someone is consuming a diet already low in fat and high in carbohydrates, they likely won't get much benefit from Alli. However, doctors say most people are eating far more fat than they realize.