A prescription diet drug that blocks the absorption of fat is "no magic pill" but will nevertheless help control calorie intake, the drug manufacturer said Monday as the Food and Drug Administration considered whether to approve the pill for over-the-counter sales.

In 1999, the regulatory agency approved orlistat, marketed as Xenical, for sale as a prescription drug. The drug blocks the absorption of fat and GlaxoSmithKline Consumer Healthcare is seeking to sell an over-the-counter version of the pill.

If the company is successful, the pill would become the first weight-loss drug to be approved for nonprescription sales. A nonprescription version of the drug could cost consumers an estimated $12 to $25 a week, the company said.

"There is no magic pill for weight loss and orlistat is not a magic pill. Orlistat is a tool that will help people control their calorie intake and modify their diet," said John Dent, the pharmaceutical company's senior vice president of research and development.

The pill's effect ends once its use is stopped, said Dr. Julie Golden, a medical officer in the FDA's division of metabolism and endocrinology products. A previous study showed a progressive weight gain in patients once they discontinued use of orlistat, Golden said.

The proposed over-the-counter version, called Alli (pronounced "ally"), would contain half the dose of the prescription capsule and would be intended for use by overweight adults. Two FDA advisory committees were to vote on recommending approval late Monday.

The agency usually follows the nonbinding recommendations of its outside panels of experts.

An earlier, internal FDA review found the drug is a "safe and effective weight loss agent," but held off on concluding whether it should be sold without a prescription. The review found that over-the-counter use of the drug could lead to vitamin deficiencies and encourage abuse.

On Monday, FDA panel members questioned whether consumers would be able to distinguish Alli from non-approved dietary supplements also sold as weight-loss aids. They also expressed concern about its effect on vitamin intake; half of patients enrolled in trials of the drug failed to understand the need to take supplemental vitamins at least two hours before or after using the pills.

When taken with meals, the drug blocks the absorption of about one-quarter of any fat consumed. That fat is passed out of the body in stools, which can be loose or oily as a result. Other side effects include gas, incontinence and oily spotting. About half of patients in trials experienced such side effects, the company said.

In six-month clinical trials, obese people who took orlistat lost on average 5.3 pounds to 6.2 pounds more than did those who were given dummy pills, according to FDA documents. The drug's effect on weight loss is "gradual and modest," said Steve Burton, Glaxo's vice president of weight control.

The primary concern of FDA reviewers was the drug's potential to create vitamin deficiencies, since its use also blocks absorption of fat-soluble vitamins like D, E, K and beta-carotene. The company has recommended patients take multivitamins when using the drug.

However, at least 47 percent of people ignored that advice in drug trials. Furthermore, just 35 percent of diabetes patients in a study correctly stated the drug was inappropriate for them, according to FDA documents.

Those sort of results worried FDA reviewers, who questioned the risk of selling the drug directly to consumers "without the principal involvement of a learned intermediary," or prescribing doctor.

The reviewers also showed concern about the increased potential for abuse or misuse of a prescription-free version of the drug, especially among bulimics or binge-eaters who could develop vitamin deficiencies due to chronic use. The company said there was a "very low" potential for abuse.

British-owned GlaxoSmithKline's U.S. operations are based in Philadelphia and Research Triangle Park, N.C.