The first new prescription diet pill in 13 years won approval from the Food and Drug Administration on Wednesday, providing a new option for the roughly one-third of American adults considered obese.

The drug, developed by Arena Pharmaceuticals of San Diego, has been known as lorcaserin and will be sold under the name Belviq by the American branch of the Japanese pharmaceutical company Eisai.
Doctors who treat obesity, and some patient advocates, have argued that there is a need for medicines to help people lose weight, something to plug a “treatment gap” between diet and exercise, which do not work for many people, and the more radical option of bariatric surgery.
Before Belviq’s approval, there was only one prescription medicine — Roche’s Xenical — approved for long-term use in weight loss.
The history of diet pills has been marked by many safety problems, which has made the F.D.A. reluctant to approve new drugs. Belviq itself was turned down by the agency in 2010, but Arena came back with new data that assuaged some of the agency’s safety concerns.
“Obesity threatens the overall well-being of patients and is a major public health concern,” Dr. Janet Woodcock, director of the drug evaluation center at the F.D.A., said in a statement. “The approval of this drug, used responsibly in combination with a healthy diet and lifestyle, provides a treatment option for Americans.”
Belviq is the first drug to reach the market for Arena. The company’s stock has more than quintupled in the last year, with much of the gain coming since an advisory committee to the F.D.A. recommended approval of Belviq on May 10.
The share price had fallen in recent days and on Wednesday was down slightly before trading was halted pending release of the news. When trading resumed, the price jumped 42 percent.
Arena said it was not clear yet when the drug would be available to patients and what it would cost. The Drug Enforcement Administration must first decide what level of controls to place on prescribing the drug since there is considered to be at least some potential for abuse.
Belviq provides only modest weight loss, leading to some debate among investors and analysts as to how well the drug will sell.
In clinical trials, those who took the drug lost an average of about 6 percent of their weight after a year, meaning a person weighing 220 pounds — the average weight of those in the trials — would weigh 207 pounds after a year. The patients in the trial who got a placebo lost about 2 percent to 3 percent of their weight.
Still, a smaller group of those treated, about 22 percent, lost at least 10 percent of their body weight on the drug.
Belviq works by activating a receptor in the brain, called serotonin 2C, in a way that helps suppress appetite.
The main safety concern about Belviq is that it works somewhat similarly to fenfluramine, a drug that was part of the popular fen-phen diet pill combination. It and a similar drug called dexfenfluramine were withdrawn from the market in 1997 after they were found to damage heart valves.
The F.D.A. said in a news release Wednesday that it was satisfied that Belviq was different enough from fenfluramine that it would not cause such problems. However, Arena committed to conducting six studies after the drug reaches the market, including one to see whether the drug increases the risk of heart attacks and strokes.