Despite side-effects you wouldn't want to discuss at mealtime, a U.S. advisory panel has recommended that a fat-blocking pill be approved for over-the-counter sale, news services reported on Monday.

If the U.S. Food and Drug Administration agrees, orlistat would be the first weight-loss drug legally available without a prescription in the United States.

It is already available by prescription under the trade name Xenical.

Pittsburgh-based GlaxoSmithKline sees a big market for the drug on store shelves, costing $12 US a week. The company proposes to sell the drug under the trade name Alli at half the prescription dose.

In approving the prescription version in 1999, the FDA said it prevents enzymes in the gut from breaking fat down into digestible forms.

"Absorption of fat is decreased by about 30 per cent. Since undigested triglycerides are not absorbed, the reduced caloric intake may have a positive effect on weight control," the agency said in a statement at the time.

People who use it should take vitamin supplements because it also reduces absorption of fat-soluble vitamins, the FDA said.

It added that the most common side-effects are oily spotting, gas with discharge, fecal urgency, fatty/oily stools, and frequent bowel movements.