The history of diet pills has been marked by many safety problems and product withdrawals, 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 the agency’s safety concerns.

Some patient advocates and doctors who treat obesity say there is a need for new medicines to help to plug a “treatment gap” between diet and exercise, which do not work for many people, and the more radical option of bariatric surgery. They say obesity itself is a serious disease that causes other health problems like diabetes and heart disease.

In announcing the approval of Belviq, the F.D.A. suggested that it agreed with that point of view. “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.

Belviq is the first drug to reach the market for Arena, which was founded in 1997. Its stock price has more than quadrupled in the last two months, with much of the gain coming after an advisory committee to the F.D.A. recommended approval of Belviq by a vote of 18 to 4 on May 10. On Wednesday, the stock rose 29 percent to $11.39.

Arena said it was not clear yet when the drug would be available to patients and what it would cost. Because the F.D.A. deemed that there was some potential for the drug to be abused, the Drug Enforcement Administration must now decide what controls to place on prescribers, a process that Arena said could take four to six months.