Washington ? A pharmaceutical company that seeks federal approval for over-the-counter sales of a prescription diet pill that blocks the absorption of fat faces questions on the pill’s potential for creating vitamin deficiencies and encouraging abuse.

Two Food and Drug Administration advisory committees are to discuss on Monday allowing over-the-counter sales of orlistat, which GlaxoSmithKline Consumer Healthcare proposes to market as Alli (pronounced “ally”). That version would be half the dose of the prescription form of the drug, known as Xenical, which won FDA approval in 1999.

An FDA reviewer found the drug is a “safe and effective weight loss agent,” but held off on recommending whether the Nonprescription Drugs and the Endocrinologic and Metabolic Drugs advisory committees should in turn recommend to the FDA that it approve the drug for sale without a prescription.

The drug acts by blocking the absorption of about one-quarter the fat a patient consumes. That fat is then passed out of the body in stools, which can be loose or oily as a result.

In six-month clinical trials, obese subjects 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 released Friday.

FDA reviewer Dr. Karen Feibus said the primary concern with the drug is its potential to create vitamin deficiencies, because its use also would block absorption of fat-soluble vitamins like D, E, K and beta-carotene.

The company recommends patients take multivitamins when using the drug, said Steve Burton, GlaxoSmithKline’s vice president of weight control. Whether that would happen remains unclear; at least 47 percent of the people involved in trials of the drug did not take multivitamins as recommended, the FDA said.

Orlistat may also block some drugs, including cyclosporine, used to prevent rejection of kidney, liver and heart transplants. Orlistat shouldn’t be taken with that drug, the FDA said.

Furthermore, just 35 percent of diabetes patients in a study correctly stated the drug was not appropriate for them to use, according to agency documents.

The regulatory agency also is concerned about increased potential for abuse or misuse of the prescription-free version of the drug, especially among bulimics or binge-eaters who could develop vitamin deficiencies from chronic use of the drug. There are at least four published reports of women suffering from bulimia using the prescription form of the drug as a purgative. An estimated 22 million people worldwide have used the drug.

“We believe there is a very low potential for misuse and abuse. But we need to be vigilant,” Burton said.

The unpleasantness of the drug’s side effects, including “involuntary leakage” of undigested fat, should limit its abuse, said Dr. Howard Eisenson, director of the Duke Diet and Fitness Center in Durham, N.C.