An accidental discovery has led to the development of a treatment for obesity that is claimed to be twice as effective as the best existing drugs at helping the overweight shed excess pounds.

The drug, tesofensine, has such a powerful effect in reducing hunger that scientists say it could be regarded as the chemical equivalent of stomach stapling or gastric-bypass surgery. When combined with an appropriate diet, they say patients using it could lose 20 per cent of their weight in six months.

Effective treatments for obesity have a huge potential market but research is still at an early stage and bigger trials are awaited. Two-thirds of adults are overweight in Britain and the Department of Health estimated last week that the cost to the NHS could rise to £6.3bn by 2015 if no action was taken to curb its growth.

Tesofensine, developed by the Danish biotech company, Neurosearch, was being tested as a treatment for Alzheimer's and Parkinson's diseases when researchers noticed the mainly elderly patients were losing weight. The company asked Professor Arne Astrup, of the department of human nutrition at the University of Copenhagen, to test it as a weight-loss treatment. The trial among 161 obese patients, whose average weight was 100kg (16st), showed those on the highest dose of the drug lost 12.8ks (2st) over six months, when combined with "mild" diet. This was 10kg more than those given a placebo.

"To my great surprise the trial showed weight loss far and away greater than I had expected. We tested it with a mild diet, but if it were given with a tough diet we could expect weight loss of 20kg [more than 3st] that could be maintained. That is comparable with gastric surgery."

Existing drugs – orlistat, which its makers are applying to be made available over the counter in the UK next year, sibutramine and rimonabant – achieve about half the level of weight loss, the researchers said.

British specialists were cautious. Professor Iain Broom, of Robert Gordon University, said: "This is not a drug with a new mechanism of action. It has the same mechanism as sibutramine... We should be a little circumspect about accepting these claims and await the results of the more relevant Phase 3 studies."

Professor Steve O'Rahilly, of the MRC Centre for Obesity at the University of Cambridge, said: "If we could treat obesity, like we treat high blood pressure, with safe, effective and affordable drugs this would be an enormous boon to health care. However, to date obesity drugs that have been effective have not been safe, and conversely those that are safer, are relatively ineffective."

Tesofensine: How it works

Tesofensine suppresses hunger and prevents over-eating by targeting brain chemicals linked to appetite. This leads to an energy deficit so the body burns off excess fat. It works in a similar way to sibutramine and rimonabant, which are marketed in the UK as Reductil and Accomplia, but targets an additional brain chemical, dopamine. It has a different mechanism from orlistat, marketed as Xenical, which inhibits the absorption of fat from the gut. The makers of Xenical, GlaxoSmithKline, are seeking a licence to sell it over the counter in the UK.