The UK's first over-the-counter weight loss pill is now available at pharmacies around the country, but its makers warn that it may not be the 'magic bullet' for obesity that the public is hoping for.

Alli has been marketed as a weight loss capsule which, when added to a reduced fat diet, can help people lose 50% more weight than by dieting alone. It contains Orlistat, a lipase inhibitor which until now was available by prescription only. The drug acts on the gastrointestinal tract to reduce absorption of dietary fat and so can cause unpleasant side effects such as flatulence, oily leakage from the rectum and liquid stools after a high-fat meal.

However, nutritional expert Dr Donald Hensrud from the Mayo Clinic warns that exciting results from tightly-controlled clinical trials on obese patients motivated to lose weight may not be predictive of the drug's efficacy in the real world. The pill will not work without accompanying lifestyle changes he says, and patients who are looking for a pill that will make them slimmer with minimal effort on their part will be sorely disappointed.

Alli is intended for over-18s with a BMI of 28 or more, requiring the individual to first consult a pharmacist who will confirm if the buyer is a suitable candidate for treatment; a two-week supply costs £32 (at Boots). (bbc.co.uk 2009; 21 April, timesonline.co.uk 2009; 20 April)