However, questions remain about the long term safety of the drug, sibutramine.
Researchers tested the impact of the drug - originally developed as an anti-depressant - on volunteers over a two year period.
The researchers recruited 605 obese patients from eight European centres.
The patients took a 10mg daily dose of sibutramine for six months.
They were then put on a diet tailored to their individual needs.
Almost eight out of 10 of the patients were able to lose more than 5% of their body weight.
These patients then took part in the second phase of the trial.
For the next 18 months, they were given either a 10mg daily dose of sibutramine or a placebo.
The dose of sibutramine was increased up to 20 mg per day if weight regain occurred.
At the end of the trial, 43% of the patients given sibultramine had maintained at least 80% of the weight loss achieved in the first phase of the trial.
This compared to just 16% of those given the placebo.
Tell-tale compounds
The researchers also tested for bloodstream levels of compounds which in higher concentrations are linked to disease, such as VLDL cholesterol, insulin, C peptide and uric acid.
They found that sibultramine patients were more likely to maintain lower levels of these compounds than those patients given a placebo.
In addition, the sibultramine patients registered higher levels of HDL cholesterol which is regarded as beneficial in protecting against heart disease.
However, the researchers also found that some of the patients on sibultramine had to stop taking the drug because their blood pressure began to rise.
They warn that patients given sibutramine on a long-term basis need to have their blood pressure carefully monitored.
Lead researcher Professor Philip James, a member of the International Obesity Task Force from Rowett Research Institute, Aberdeen, said that sibultramine was only the second effective drug for obesity after Xenical.
He told BBC News Online: "This is the first trial specifically designed to see if an obesity drug can help people to maintain weight loss and the evidence is that there is a hugely significant effect, with a tripling of the number of people who can keep their weight down once they have lost 10% of their original body weight."
Professor James said the problems of obesity had been "grossly underestimated" by the medical profession, and the world at large.
The World Health Organisation has recently stated that obesity represents the biggest unrecognised health problem in the world.
Professor James said: "This is the opening of a new era in the management of obesity.
"Research has shown that a 10% reduction in weight reduces the risk of mortality by 25% over the following ten years."
At present, it appears that neither Xenical or sibultramine work for all patients. Researchers do not know why.
The research is published in The Lancet medical journal.