A diet pill that makes you feel full as soon as you start eating

It sounds like the answer to a dieter's prayers - a once-a-day pill which can help a woman drop two dress sizes in six months.

Scientists say they are astonished by the success of trials on tesofensine, a drug which could be on the market in three years.

By targeting part of the brain which controls appetite, it makes the user feel full soon after starting a meal and cuts the urge to snack.

Taken for six months, it is said to help achieve an average weight loss of a stone and a half - twice the amount achieved by any obesity drug already on the market.

Such a change in weight could have a dramatic effect on quality of life, as well slashing the risk of heart disease, diabetes and cancer.

Tesofensine pills could also cut the need for gastric banding, stomach stapling and other expensive and potentially dangerous operations carried out thousands of times a year in the UK as waistlines expand.

There are, however, likely to be concerns about the drug being used as a 'quick fix' to a problem which can in many cases be tackled with diet and exercise.

And there are side- effects, including mild nausea, diarrhoea, constipation, insomnia, mood changes, and, perhaps most importantly, increased heart rate.

Future trials will determine whether the side-effects outweigh the benefits.

Professor Arne Astrup, president of the International Association for the Study of Obesity, looked at how tesofensine affected the weight of more than 200 obese men and women.

All took the drug, or a dummy drug, for six months, while eating slightly less food than usual.

Even those taking very low-strength tablets lost an average of 10lb more than those on dummy pills - a figure comparable to obesity drugs already on the market.

Raising the dose doubled the weight loss, the European Congress on Obesity heard. Medium-strength pills, the variety likely to be marketed, produced an average weight loss of almost ten per cent.

This equates to a stone and a half - the equivalent of two dress sizes.

Professor Astrup, of the University of Copenhagen, said that encouraging users to lose weight before taking the tablets and to eat sensibly and exercise while taking them could double the weight loss.

'You could easily come up to 20 per cent weight loss which could offer an alternative to the surgical treatment of obesity which has become the only real cure or effective treatment that can provide a weight loss of that size.'

The researcher, who holds shares in NeuroSearch, the Danish drug company behind the pill, said: 'The patients say, "For the first time in my life, my appetite control is normal".'

Originally developed to treat Alzheimer's and Parkinson's diseases, tesofensine is a man-made compound which alters levels of three brain chemicals involved in appetite control - serotonin, dopamine and noradrenaline.

While it is too early to say how much it would cost, the three obesity drugs already on the market - Acomplia, Reductil and Xenical - cost no more than £2 a day.

Dr Nick Finer, an obesity expert at Addenbrooke's Hospital in Cambridge, described the weight loss achieved by tesofensine as 'striking' but expressed caution about sideeffects.

He said: 'In general a rise in heart rate is a bad thing, especially in people at risk of heart disease.'