Liraglutide, marketed as Victoza, outperformed the established anti-obesity drug orlistat, commercialised as Alli or Xenical, among 564 European volunteers, it suggests.
The 135 men and 429 women volunteers were divided into three groups.
One received daily liraglutide at four different dosages; the second received orlistat; and the third were given a harmless lookalike called a placebo.
In the fortnight before the trial, all the volunteers began a regime that combined daily exercise with a cut in calorie intake by 500 calories.
By comparison, 500 calories is roughly a quarter of the recommended daily energy intake for women, and about a fifth of the recommended intake for men.
Five months later, the liraglutide patients had each lost between 4.8 and 7.2 kilos (10.5 and 15.8 pounds) on average depending on the dosage; the orlistat volunteers had lost 4.1 kilos (9.0 pounds); and the placebo group 2.8 kilos (6.1 pounds).
Seventy-six percent of the high-dosage liraglutide takers lost more than five percent of their weight, compared with 44 percent in the orlistat group and 30 percent in the placebo group.
There was also a big reduction in "prediabetes" conditions among the liraglutide group.
Liraglutide was "well-tolerated," according to the study.
"Nausea and vomiting occurred more often in individuals on liraglutide than in those on placebo, but adverse events were mainly transient and rarely led to discontinuation of treatment."
Liraglutide, which is administered into the skin through an injector, was initially developed as a treatment to control blood glucose levels for patients with Type 2 diabetes.
But one of its impacts is to brake the emptying of the contents of the stomach into the intestine, thus prolonging a feeling of satiety.
The study was led by Arne Astrup, a professor of nutrition at the University of Copenhagen. He is also an advisor to liraglutide's manufacturers, the Danish company Novo Nordisk A/S. The study data was vetted by The Lancet, a peer-reviewed journal.
The authors say the results offer "a new mode of action" for treating obesity, for which only a few effective and safe drugs are available.
Obesity has reached epidemic-level proportions in many rich economies as well as in fast-growing emerging countries.
Over the past 20 years, the rate of obesity has tripled and is more than 30 percent in some European countries, according to an estimate published last year.
Volunteers in the study had a body mass index (BMI) of between 30 and 40. A BMI, one's weight in kilos divided by one's height in metres squared, that is above 30 is generally categorised as obese.