Overweight volunteers who took Orexigen's experimental drug Contrave, designed to reduce cravings, lost about 13 pounds (6 kg) over a year, U.S. researchers reported on Thursday.
Those who got the drug said they felt less desire for sweet or starchy foods; felt fuller and had fewer cravings, the researchers said.
Their study, published in the Lancet medical journal, also suggested that feared side-effects from the drug were mild and went away over time.
But one expert said more studies are needed before regulators can assume the drug is safe.
Data from the company-sponsored study has already been presented to the U.S. Food and Drug Administration, which has a panel of advisers scheduled to consider approval in December.
The Lancet submitted it for review by experts before publishing it, said Frank Greenway of the Pennington Biomedical Research Center at Louisiana State University, who led the trial.
Contrave is a combination of the antidepressant bupropion, a generic version of GlaxoSmithKline's Wellbutrin, and naltrexone, used to treat alcoholism and other addictions.
Greenway and colleagues tested two formulations of Contrave against a placebo in 1,700 overweight or obese volunteers.
Only half the volunteers finished the trial, dropping out for various reasons.
"It is something that seems to be characteristic of obesity studies," Greenway said in a telephone interview.
"The people in the placebo group seemed to drop out more frequently because they were dissatisfied with the lack of weight loss. People in the drug arm seemed to drop out more because of side-effects. Overall, the groups seemed to be equal."
But of those who finished the trial and also got the highest dose of the drug, mean weight loss was 13 pounds (6 kg) compared to 3 pounds (1.4 kg) for those who got the placebo.
The drug did raise blood pressure slightly at first, a known side-effect of bupropion and a big worry for a drug being taken by overweight people. But this increase leveled off after a short time, Greenway reported.
"The safety is really nothing different from what we would expect from the components," Greenway said.
One expert said more study is needed to show if the drug is safe. The drugs have been linked to anxiety, insomnia and to raised blood pressure, as well as to thoughts of suicide.
But the trial showed no increase in any of these, said Arne Astrup of the University of Copenhagen in Denmark.
"These findings suggest that the combination might have fewer adverse psychiatric effects than its components given separately, but a meta-analysis of all continuing or completed phase 3 studies of this combination treatment is needed to assess this effect more accurately," Astrup wrote in a commentary in the Lancet.
Three U.S. companies are competing to get new weight loss drugs approved by the FDA.
FDA advisers voted against Vivus Inc's Qnexa earlier this month, expressing concerns that it could cause depression, memory loss and potential birth defects.
Arena Pharmaceuticals Inc has a drug called lorcaserin up for consideration in September.