The Food and Drug Administration effectively approved the use of an obesity drug, Xenical, for adolescents on Monday. This is the first time a weight-loss drug has been permitted to treat overweight children.

A spokeswoman for the drug agency said it had decided to allow the manufacturer to add to the Xenical labels two studies that involved children to help doctors prescribe the correct doses for 12- to 16-year-olds.

Terence Hurley, a spokesman for Hoffman-La Roche, the manufacturer, said it had no plans to market the drug for teenagers, but added that it was good news for children who battle obesity.

"We're thrilled with the approval," Hurley said.

Xenical, one of three prescription weight-loss drugs on the market, works by blocking the body's ability to digest fat. Like the other two, Meridia and Phentermine, Xenical is described by most obesity experts as just moderately useful.

It has several unpleasant side effects, including problems with bowel control, and it works best when combined with a low-calorie diet that limits foods high in fat.

The FDA acted as studies show that 15 percent of American children are overweight, and doctors and parents are struggling to deal with increasing childhood obesity. The problem has become so widespread that experts are increasingly calling for preventive efforts against obesity and the serious health complications that often accompany it like heart disease and diabetes.

The decision to approve — at least to some extent — the expansion of Xenical to children was based on two studies. The first, conducted over 54 weeks, found that 357 teenagers who took the drug three times a day and followed low-calorie diets had a greater reduction in body mass index than 182 subjects given a placebo.

Dr. Marc S. Jacobson, director of the Center for Atherosclerosis Prevention at Schneider Children's Hospital in New Hyde Park, N.Y., said that 16 patients at his center were in the study over 12 months and that they all showed at least small improvements. That was an encouraging sign, Jacobson said, because children tend to add weight in adolescence.

Some studies show that Xenical can help adults lose 13 pounds a year.

"It's hard for people to lose weight under any circumstances," Jacobson said. "This gives a little bit of help for adolescents when they're also following a healthy diet with regular physical activity."

Though all 16 children finished the study, Jacobson said, one patient, unaware that he had to watch his diet while on the drug, came close to dropping out after he ate junk food one weekend and had some incontinence.

Because the drug generally blocks about one-third of the fat in any meal, patients have to limit foods like cheese and butter to avoid side effects.

Each patient in the study, Jacobson said, had regular visits with a nutritionist.

"It has side effects, like all drugs, but they seem to be manageable in clinical practice," the doctor said. "It's important for parents to get involved and understand what the treatment is. It's a lifestyle change for the family. It's not something you go on a few weeks and then go off."

The results of another study that were used to update the Xenical label showed a decrease in iron in patients taking it. That study also found that patients taking a placebo had similar iron deficiencies.

Xenical, also called orlistat, was first approved in 1999. Because it had not been studied in children at that time, its labeling said there was no information on its safety or efficacy for them.

After a drug is approved, doctors can prescribe it for "off-label" uses, meaning for patients or conditions not on the label. Some doctors have already been prescribing Xenical off label for obese teenagers, Jacobson said, adding that he had done so and considered the drug a "useful tool" when combined with diet and exercise.

But it brings about just modest weight loss. In studies, people who took it for a year lost 5 percent to 10 percent of their body weight.

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Xenical works in the gastrointestinal tract, blocking an enzyme that is needed to digest fat. Instead of being digested, one-third of the fat a person eats will accumulate in the intestines and be excreted in the stool. People who take the drug are warned to limit their fat intake. Jacobson said he advised children taking the drug to avoid some staples in the average teenager's diet like pizza and french fries.

"We try to make substitutions compatible with a patient's diet but also with their lifestyle," he said. "It's getting easier as McDonald's comes up with salads and grilled chicken. So kids can still eat out with their friends. Most places allow salad dressing on the side, so kids can have some control."

Cheaters can suffer an array of side effects like bloating, flatulence, oily stool, diarrhea and fecal incontinence, which act as a sort of built-in enforcement.

But by blocking fat absorption, the drug also blocks absorption of the fat-soluble vitamins A, D, E and K, as well as beta-carotene, meaning that patients have to take daily vitamin supplements.

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