The drug inhibits digestion and its manufacturers hope it will provide an effective new way to combat obesity.

Clinical trials of Xenical, also known as Orlistat, produced an average weight loss of 10% in a group of obese patients over a two year period.

Such a weight reduction should make the drug acceptable for licensing by European authorities.

The drug works by blocking fat absorption in the intestines. However, it requires careful monitoring as it can upset the gastro-intestinal tract, and the long-term impact of taking the drug is unknown.

Significant weight loss

Researchers led by Professor Lars Sjostrom from Sahlgrenska University Hospital, Goteburg, Sweden, studied 688 obese patients recruited at 15 European centres.

The patients were either given Xenical three times daily or a dummy placebo for one year in combination with a low calorie diet.

In the second year the allocation of Xenical and the placebo was reversed for a number of the volunteers.

During year one the Xenical group lost an average 10.2% of bodyweight compared to 6.1% in the placebo group.

In the second year, patients who continued on Xenical regained on average half as much weight as those who were switched to a placebo.

Cholesterol down

Obesity is defined as weighing 20% more than ideal

Total cholesterol and concentrations of glucose and insulin decreased more in the Xenical group than in the placebo.

The scientists, who published their results in the Lancet, concluded: "Orlistat taken with an appropriate diet promotes clinically significant weight loss and reduces weight regain in obese patients over a two-year period."

But they noted that adverse gastro-intestinal effects were more common in the Xenical group, showing that treatment with the drug needed careful monitoring.

A 'milestone'

Writing in the Lancet, an American expert, George Bray from Louisiana State University, called the findings "another milestone in the effort to treat obesity effectively".

In his commentary, Dr Bray said advances in the understanding of control of energy balance and the genetic basis for obesity were providing a new framework for drug discovery.

Future treatments for obesity were likely to ameliorate associated diseases, such as high blood pressure, at the same time.

"Orlistat is the first step in this process, which may help defuse the time bomb of obesity," said Dr Bray.

Breast cancer fear

Xenical is marketed by Roche and is set for launch in the US next year, and a European Union advisory panel recently recommended approval.

The drug was backed for approval in the US in May 1997, but Roche withdrew its application to study concerns that the drug was linked to breast cancer - a link which the US Food and Drug Administration has since accepted does not exist.

Obesity, defined as weighing 20% more than ideal bodyweight, is linked to a range of clinical conditions, including heart disease, diabetes and cancer.