For the first time, a drug has been shown to be so effective against obesity that patients can avoid many of its worst effects, including diabetes, researchers reported Wednesday.

Novo Nordisk’s drug Semaglutide is already marketed for the treatment of type 2 diabetes. In a clinical study published in the New England Journal of Medicine, researchers at Northwestern University in Chicago tested semaglutide at a much higher dose as an anti-obesity drug.

Nearly 2,000 participants in 129 centers in 16 countries injected semaglutide or a placebo weekly for 68 weeks. The ones who got the drug lost almost 15 percent of their body weightcompared with 2.4 percent on average among those who received the placebo.

More than a third of the participants who received the drug lost more than 20 percent of their weight. Symptoms of diabetes and pre-diabetes improved in many patients.

These results far exceed the weight loss seen in clinical trials with other anti-obesity drugs, experts say. The drug is a “game changer,” said Dr. Robert F. Kushner, an obesity researcher at Northwestern University’s Feinberg School of Medicine, who led the study. “This marks the beginning of a new era of effective obesity treatments.”

Dr. Clifford Rosen of the Maine Medical Center Research Institute, who was not involved in the study, said, “I think it has great potential for weight loss.” The gastrointestinal symptoms among the participants were “really marginal – nothing like weight loss drugs in the past,” added Dr. Rosen, editor of the New England Journal of Medicine and co-author of an editorial to the study.

For decades, scientists have been looking for ways to help a growing number of people with obesity. Five obesity drugs currently available have side effects that limit their use. The most effective phentermine causes an average weight loss of 7.5 percent and can only be taken for a short time. After stopping, this amount of weight is also regained.

The most effective treatment to date is bariatric surgery, which allows people to lose an average of 25 to 30 percent of their body weight, noted Dr. Louis Aronne, an obesity researcher at Weill Cornell Medicine in New York, who advises Novo Nordisk and studies semaglutide.

However, surgery is an invasive solution that permanently changes the digestive system. Only 1 percent of those who qualify do the procedure. Instead, most obese people try diet after diet with disappointing results.

The semaglutide study confirms what scientists already know, said Dr. Kushner: Willpower is not enough. In the new study, the participants who received the placebo and the diet and exercise counseling could not find any significant difference in weight.

In general, insurers have refused to pay for the weight loss drugs on the market. Semaglutide is likely to be expensive. The lower dose used to treat diabetes has an average retail price of nearly $ 1,000 per month. (Insurers usually pay for diabetes medication, Dr. Kushner noted.)

Dr. Caroline Apovian, co-director of the Center for Weight Management and Wellness at Brigham and Women’s Hospital and a member of the Novo Nordisk advisory board, said semaglutide’s effectiveness is “phenomenal” and the trial results may lead insurers to cover it.

Semaglutide is a synthetic version of a naturally occurring hormone that acts on appetite centers in the brain and intestines, causing feelings of satiety. A high-dose regimen of the drug hasn’t been studied long enough to see if it has serious long-term effects.

And it is expected that patients will have to take it for a lifetime to prevent weight loss from returning.

Qiana Mosely, who lives in Chicago, has tried dieting and drugs for years to lose weight, but to no avail. Then Ms. Mosely took the semaglutide study and lost 40 pounds, about 15 percent of her weight.

Ms. Mosely did not know until recently whether she was receiving the drug or the placebo. Although she tried to eat well and exercise, her weight “dropped too quickly,” she said. “It had to be the drugs.”

She said she didn’t experience any side effects. But when the study ended and she stopped receiving the drug, the weight came back. “I was so sad,” she said. She will endeavor to resume taking the medicine as soon as it becomes available.