Arena's studies showed that patients taking Belviq, known generically as lorcaserin, had modest weight loss. On average patients lost just 3 to 3.7 percent of their starting body weight over a year. About 47 percent of patients without diabetes lost at least 5 percent of their weight or more, which was enough to meet FDA standards for effectiveness. By comparison, average weight loss with Qnexa is 11 percent, with more than 83 percent of patients losing 5 percent of their weight or more.
The FDA said patients should stop taking Belviq after three months if they fail to lose 5 percent of their body weight. Patients are unlikely to see any significant weight loss by staying with the drug.
Side effects with the drug include depression, migraine and memory lapses.
In May a panel of expert advisers to the FDA voted 18-4 to recommend approval of Arena's drug, concluding that its benefits "outweigh the potential risks when used long term" in overweight and obese people.
Experts say the challenge of weight loss drug development lies in safely turning off one of the body's fundamental directives: to eat enough food to maintain its current weight.
While several drugs are available for short-term weight loss, until Wednesday there was only one FDA-approved prescription drug for long-term weight loss: Xenical from Roche, which is seldom prescribed because of unpleasant digestive side effects and modest weight loss. Belviq is the first new prescription drug approved to treat obesity since Xenical's approval 13 years ago.
Other safety failures for diet pills have continued to pile up in recent years.
Four years ago Sanofi-Aventis SA discontinued studies of its highly anticipated pill Acomplia due to psychiatric side effects, including depression and suicidal thoughts. In 2010, Abbott Laboratories withdrew its drug Meridia after a study showed it increased heart attack and stroke.