Health officials say that the first over-the-counter slimming pill to go on sale in Britain could be available in pharmacies as early as next year. The drug, which has been marketed in the US under the name Alli since June, is a half-strength version of a pill called Xenical that is currently only available in Britain under prescription for treating obesity.

The active ingredient of the drug, orlistat, does not affect diet but works by reducing the body's ability to absorb fats from food, and is taken as 60mg capsules three times a day with meals.

GlaxoSmithKline (GSK), which manufactures the pills, claims the treatment can help people lose 50% more weight than by dieting alone. However, an independent study into the drug by the University of Kentucky prior to its launch in the US earlier this year found a more modest weight loss among people who were only moderately overweight rather than obese.

During the four-month trial those taking the drug lost 3-7kg (7-15lb), provided they also followed a strict low-fat diet, which helps avoid the side-effect of excessive diarrhoea.

Dr James Anderson, who led the Kentucky study, stressed that despite the greater weight loss achieved by taking the pills, a healthy diet and exercise were still essential to lose weight and keep it off.

The pharmaceutical giant applied for a licence to sell the anti-obesity drug over the counter in October, and confirmed it had been accepted for review by the EU regulator, the European Agency for the Evaluation of Medicinal Products (EMEA). If the EMEA approves the application, GSK will be granted a licence to sell Alli without prescription in all 27 European Union member states. It has not confirmed where it would launch the drug first.

"We want to see people achieving gradual sustained weight loss by using Alli in tandem with healthy eating, a low-fat diet and increased exercise. We've said all along that this is no magic pill. If people are looking for a quick fix this is not it, but it is a powerful motivator," said John Clarke, president of GSK consumer healthcare.

The drug sold £77m in the first five weeks of going on sale in the US, despite headlines focusing on the pills' unpleasant side-effects. The drug can lead to "oily stools" because it blocks a compound called lipase, which normally breaks down fat in the intestine. Undigested fats cannot be absorbed and pass straight through.

The drug reduces fat absorption in the body by about one third, and this can also lower the amount of vitamins absorbed by the body, because many, including A, D, E and K, are fat-soluble.

Effective treatments for obesity could have a dramatic effect on the nation's health. Government statistics suggest that nearly 25% of the British population is obese, a rise of almost 10 percentage points in 15 years. Obesity has been linked to an increase in the risk of type 2 diabetes, cardiovascular disease and cancers.