(07-10) 22:09 PDT LEXINGTON, Ky. (AP) --

A drug used to quit smoking and treat depression helps obese people lose weight and keep it off, researchers report.

The drug, bupropion SR, is sold as Wellbutrin for depression and Zyban for nicotine addiction. In a yearlong study, it was shown to help reduce weight and keep it off when combined with diet and exercise, said Dr. James Anderson, professor of medicine and clinical nutrition at the University of Kentucky's College of Medicine.

About 51 million Americans are obese and another 61 million are overweight, according to the American Obesity Association.

"Obesity is a huge problem, especially among children, and the treatments we offer aren't very effective," Anderson said Tuesday.

"There are no magic bullets for treating obesity. Diet and exercise are still the keys to long-term weight loss. But this does give us another arrow in the quiver in helping to treat it."

The study, reported in July's Obesity Research, was paid for by the drug's manufacturer, GlaxoSmithKline.

During the first half of the 48-week study, 227 obese people were randomly assigned to take one of two doses of bupropion or a dummy pill each day. Participants also were on a restricted diet and asked to increase their exercise by about 50 percent.

Those on the higher dose of bupropion lost 10 percent of their body weight compared with 7 percent for those on the lower dosage. Those who took the dummy pill lost about 5 percent of their body weight.

In the second 24 weeks, 192 participants who completed the first part of the study were all given bupropion and were able to sustain their weight loss.

Those on the higher dose maintained an 8.6 percent loss and those on the lower dose maintained a 7.5 percent loss.

"This is a drug that people have taken for many years to treat depression with few problems or side effects," said Dr. Richard Atkinson, president of the American Obesity Association. "If it can be just as effective in treating obesity, it has the potential to help a lot of people."