The proposed switch of GlaxoSmithKline’s obesity drug orlistat from prescription-only to over-the-counter use goes in front of a US Food and Drug Administration advisory panel later today, though comments posted by the agency ahead of the meeting suggest its safety and efficacy may be questioned.

The FDA said that orlistat, a pancreatic lipase inhibitor sold as Xenical, could be unsafe in some patients, such as diabetics, who sometimes need to alter their dose of insulin or anti-diabetic drugs after treatment with the drug.

“Preliminary evidence suggests that non-prescription labelling may not adequately direct the safe use of orlistat,” according to the FDA reviewer in the case.

GSK, which acquired the US rights to OTC orlistat from orginator Roche last year, is hoping to sell a half-dose version of the drug under the trade name Alli for non-prescription use.

The FDA reviewer has also questioned whether this 60mg dose will be effective enough to have a real benefit in overweight people, given that weight tends to come back quickly once treatment with orlistat is stopped. Orlistat blocks the absorption of dietary fat.

As a prescription product, Xenical has failed to achieve its once prophesised blockbuster status because it has certain side effects such as gas or anal discharge that have held back use. Nevertheless, it has seen annual sales peak in the 600 million Swiss franc region and, while sales had started to flag, the drug managed a 4% gain in the first nine months of 2005, helped by new labelling showing its benefits on glucose intolerance, as well as an approval for adolescent obesity.

The OTC version could be a runaway commercial success, according to analysts, who suggest that many overweight people in the USA will be tempted to try the product. At present, two thirds of Americans are overweight or obese, according to the Centers for Disease Control.