Once the Drug Enforcement Administration clears it, the drug will be marketed in the U.S. under the name Belviq. But the FDA’s approval comes with a warning.

The Food and Drug Administration has approved marketing of the weight-loss drug lorcaserin, the first prescription anti-obesity medication to win the FDA’s blessing since the agency approved orlistat in 1999.

Once it is cleared by the Drug Enforcement Administration, the drug will be marketed in the United States under the commercial name Belviq.

The medication won marketing approval as a drug for “chronic weight management in adult patients” with a body mass index greater than 30, or for those with a BMI of 27 or above with a weight-related condition such as high blood pressure, elevated cholesterol or Type 2 diabetes.

The FDA’s approval came with a significant warning to consumers likely to flock to a medication that could help boost their weight loss: The safety and effectiveness of Belviq when used alongside other diet medications has not been established.

Nor, the FDA said, has the effect of Belviq on the long-term risk of having, or dying from, a heart attack or stroke.

The approval comes after a lengthy review by an agency under pressure to help Americans lose weight but also to screen out drugs that could prove dangerous once embraced by a large number of patients.

Just over 1 in 3 adult Americans is considered obese, and an additional 36% are overweight, prompting widespread concern that Type 2 diabetes, heart disease and certain cancers linked to obesity will overwhelm the U.S. healthcare system. Already, obesity-related disease is estimated to account for $147 billion in annual healthcare costs.

Belviq, made by San Diego-based Arena Pharmaceuticals, is one of four weight-loss drugs the FDA has anguished over in recent years. Safety concerns have prompted the agency to order the makers of two other candidate drugs to conduct and comb through additional research. A third drug was withdrawn after it was linked to an elevated suicide risk.

In the 28 months since the makers of Belviq submitted the drug for FDA consideration, the agency also demanded the withdrawal of Meridia after studies showed it increased the risk of heart attack and stroke. That left only two FDA-approved weight-loss drugs on the market: orlistat and phentermine.

The result has been — to the growing frustration of many obesity experts — almost no pharmaceutical aid for a steadily growing population of Americans desperate for help in shedding pounds.

Belviq offers would-be dieters modest benefits. One clinical trial found that two-thirds of patients on the drug lost 5% of their body weight, while one-third lost at least 10%, after one year of taking the drug as a supplement to diet and exercise. Subjects’ average weight loss was 17 to 18 pounds.

Still, even this kind of modest weight loss can help lower a patient’s blood pressure, improve blood cholesterol measures and hold off or delay the onset of Type 2 diabetes, experts said.

As a means of boosting weight loss results without surgery, lorcaserin “addresses one of the greatest therapeutic gaps we have in medicine,” said Dr. Arya M. Sharma, an obesity expert and professor of medicine at the University of Alberta in Canada.

The FDA’s decision came after the makers of lorcaserin allayed fears, raised in clinical trials, that the drug might damage heart valves. Experts were also worried that animals given a higher dose than the amount intended for patients had an increased incidence of brain and breast tumors.

By last May, Arena and its partner, Tokyo-based Eisai Pharmaceuticals, brought the FDA data showing that such worrisome side effects were highly unlikely to emerge at the doses it expected patients would be prescribed.

On Wednesday, Arena said it would conduct further studies to assess the safety and effectiveness of Belviq’s long-term use and its safety for weight management in children. The drug is not FDA-approved for children, but doctors may legally prescribe it to them off-label.

Still to come are FDA decisions on two other weight-loss drug candidates. Qnexa, a combination of the diet drug phentermine and the anticonvulsant topiramate, sponsored byVivus Inc. of Mountain View, is now under review by the FDA.

Another medication — Contrave, a combination of the anti-alcoholism drug naltrexone and the antidepressant bupropion — also awaits an FDA decision.

Dr. Lee Kaplan, head of Massachusetts General Hospital’s weight management program, said that while lorcaserin’s effects appeared modest, there were some patients who could be expected to respond to the drug with major weight loss.

Lorcaserin “is not going to be the magic bullet. But it offers a very important tool…. It’s a tool I know I’ll use,” he said.

Kaiser Permanente weight management specialist Dr. Jack Der-Sarkissian said the new pill “has a lot of promise” but expressed concerns about the effect it would have upon patients’ expectations.

“I anticipate a lot of patients will want to try these,” he said. “My worry is that patients would jump on a pill more readily than the other means of weight loss - behavioral modification - that have a long history of safety.”

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