Early evidence shows labeling proposed to sell GlaxoSmithKline’s weight-loss drug Xenical over-the-counter may not lead to safe use, U.S. regulatory staff said in documents released on Friday.

One study showed that some diabetics and other patients who are not supposed to take the drug, a low-dose alternative to Roche Holding AG’s prescription version of Xenical, did not understand the risk after reading the label, U.S. Food and Drug Administration staff reviewers said.

FDA staff said nonprescription Xenical helped more patients taking the drug for six months lose weight compared to those taking a placebo. It was not as effective after four months, they said, citing company data.

The documents were released ahead of an FDA advisory panel meeting scheduled for Monday to discuss whether or not the agency should approve the nonprescription drug.

Shares of Glaxo, which acquired the U.S. rights to market the both versions of Xenical from Roche last year, were down 19 cents to $50.56 a share in early morning trading on the New York Stock Exchange. In London, its shares were barely changed.

Xenical, also known as orlistat, helps prevent fat from being absorbed by the body, but can cause excess gas and oily discharge.

It can also lead to hepatitis, gallstones and kidney stones. In their review, FDA staff noted these potential safety concerns, adding that it was still unclear how Xenical causes such problems.