"The FDA is very sensitive to issues about drugs that target systems that control mood, addiction and reward," said Niswender.
Lorcaserin is another combination drug that works by increasing the amount of serotonin, leading to increased satiety.
"Weight loss is not stellar - it's at the lower end of the weight-loss range," said Fujioka, an adviser to Arena Pharmaceuticals, who makes Lorcaserin. "Its safety looks good."
Experts believe combination drugs are key to non-surgical weight loss, and while they say drugs should only be one part of a multifaceted approach to weight loss that includes lifestyle changes, they believe the development of a diet drug that's both safe and effective is long overdue.
"Out of the past 50 years, there have only been three medications approved, and one is already off the market," said Kushner. "A third of the population is obese, and we only have a few medications."