Prescription diet drug Xenical (orlistat) has come under renewed attack by a consumer-advocacy group even as the Food and Drug Administration weighs approval of an over-the-counter version of the diet pill.

Public Citizen, in a letter to the FDA, charged that undisclosed data linking Xenical to a higher risk of colon cancer was not provided to FDA advisory panels that earlier this year recommended permitting a half-strength version of the diet drug to be sold without a prescription as Alli (pronounced Al-EYE).

Two FDA advisory committees, following a Jan. 23rd meeting at which they heard arguments both for and against approving the low-dose version of Xenical, voted 11-to-3 in favor of non-prescription sale of the drug.

The newsletter Cancer Letter, in its June 2nd edition, reported that FDA reviewers were aware of studies linking Xenical with precancerous lesions in the colon of animals, but did not provide the data to the advisory committees before their January meeting.

"The advisory committee wasn't alerted to the information," said Sidney Wolfe of the advocacy group Public Citizen. "What's the point of having FDA advisory committees if you're not going to give them all the information to made a decision?

"There is more than enough evidence to take orlistat off the market as a prescription drug and thereby end the process of switching it to over-the-counter status," Wolfe added.

Dr. Neal L. Benowitz of the University of California at San Francisco, a member of the Nonprescription Drugs Advisory Committee, said he was troubled by the idea that information had not been shared with the panel, but didn't "think this would have changed anything.

"I think the FDA should definitely consider this, make sure there's no evidence of cancer risk, and make sure there's long-term surveillance," Benowitz told the Newark Star Ledger.

GlaxoSmithKline, which hopes to market the over-the-counter version of Xenical, contended in a statement that the pill is safe and that the issue raised by the Cancer Letter was reviewed before Xenical was approved as a prescription diet drug in 1999.

Glaxo said human and animal data has confirmed there is "no association" between colon cancer and orlistat.

"The concerns raised by Public Citizen have been addressed and dismissed by leading scientists," said Glaxo.