The drug helped people lose significant amounts of weight
An anti-obesity drug vastly outperforms currently available rivals, early trials suggest.
Danish tests of tesofensine, reported in The Lancet, found dieting patients on the highest doses lost up to 12.8kg (28.2lbs) in six months.
This is twice the level achieved by drugs such as sibutramine and rimonabant.
But UK experts warned that more trials were needed, and expressed concerned the results may have been hyped.
Tesofensine first came to the attention of obesity researchers when it caused unintended weight loss when given to overweight patients with Parkinson's or Alzheimer's disease.
It works by changing the way that three nerve signalling chemicals, noradrenaline, dopamine, and serotonin have their effects on the brain.
This in turn reduces appetite, so that the person will eat smaller meals and have a reduced urge to snack.
The Danish study, led by Professor Arne Astrup, from the University of Copenhagen, split a group of 203 obese patients into two groups.
Both groups were given a once-daily pill to take, and told to go on a moderate diet, but half the pills were tesofensine, in varying doses, and the other half were "dummy" placebo pills.
After six months, all were re-measured, and the researchers found that while the placebo group had lost an average 2.2kg (4.85lbs) those taking tesofensine had lost much more.
On the lowest dose, the average weight loss was 6.7kg (14.8lbs), the medium dose produced 11.3kg weight loss (24.9lbs) and the highest dose 12.8kg (28.2lbs).
This performance is roughly twice that achieved by the best weight-loss drugs already approved for use in Europe.
Blood pressure warning
The drugs did produce side-effects, ranging from dry mouth and insomnia to nausea and diarrhoea, with the highest dose increasing patients' blood pressure, a concern given that many obese patients may have heart problems or diabetes.
The researchers said that the middle dose was more promising because it produced almost as great a weight loss as the highest dose, without the worst side-effect.
They called for bigger trials to confirm their result, and the drug is unlikely to become available across Europe until these are completed over the next couple of years.
Professor Steve O'Rahilly, an obesity expert 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.
"The results with this new drug demonstrate that, over a six-month period, it is quite effective.
"However, as the drug is likely to have actions on parts of the brain not involved in weight control, the risk of serious side-effects on longer term administration will need to be watched very carefully."
Professor Iain Broom, of Robert Gordon University, said it was premature to claim that tesofensine significantly out-performed other anti-obesity drugs, as it had not been widely tested, unlike its rivals.
And Professor Mike Lean, a human nutrition expert from the University of Glasgow, said the drug seemed similar to sibutramine - which is licenced and has a very good safety record.
"The results are generally interesting but a lot more research is needed before anyone should be given it in routine practice."