With alli (pronounced Al-eye) now approved for over-the-counter sale, it is expected to be marketed at less than half the cost of the only two brand-name prescription diet drugs currently on the market.

A Glaxo spokesperson said the drug would cost about 60 cents a pill. People using the drug would take three pills a day, one before each meal containing fat.

Full-strength prescription Xenical costs about $2 a pill and Meridia, the only other FDA approved prescription weight-loss drug, goes for about $3 a pill.

The biggest concern for regulators concerning alli is whether young people concerned about their weight are likely to abuse it the same way they do laxatives and diet pills.

''There is a potential for abuse," said Dr. David L. Katz, director of the Prevention Research Center at the Yale University School of Medicine.

Katz said young women with eating disorders, high-school wrestlers trying to make weight, and gymnasts who fear gaining weight are among the groups of teen-agers likely to attempt to acquire alli, even if sales are limited to teens over 18.,

''People will be tempted to try it," Katz said.

According to the Centers for Disease Control and Prevention, 7.3 percent of Massachusetts high school students took diet remedies without physician oversight in 2003, the most current figures available.

More than 6 percent of high school students also said they vomited or used laxatives to lose weight or to avoid gaining weight.

The FDA staff also has expressed concern that diabetics taking medication, transplant patients on the immunosuppressive drug cyclosporine, and people taking blood- thinners may not understand guidance on a proposed label advising them that they should not use the drug.

The FDA staff also express concern that the drug can reduce the absorption of some fat-soluble vitamins, and is recommending that those taking alli also take a multivitamin at bedtime.