"We did very detailed studies and found it wasn't the phentermine. It was phenfluramine that was linked to the heart issues," Katz said.
Fujioka said the trials for Qnexa, Lorcaserin and Contrave lasted about a year. The drugs were tested on about 4,500 patients each.
However, others believe there have not been enough studies done to show that these drugs are safe for most patients.
"Just because these compounds don't have the same chemical mix as Fen-phen doesn't mean they don't have a chemical mix that can cause long-term harm," said Joanne P. Ikeda, nutritionist emeritus and former co-director of the Center on Weight and Health, University of California, Berkeley. "I think we should demand long-term (three- to five-year) safety studies for these drugs."
Keith Ayoob, director of the nutrition clinic at the Rose R. Kennedy Center at the Albert Einstein College of Medicine, agreed.
"The new drugs seem safe, but so did the Fen-phen combo," he said.
Many experts, including Ayoob, said it is safe to assume that there will be side effects with these drugs.
"It is difficult to believe the risk of memory loss and other neurological effects found with topimarate will provide adequate safe use," said Dr. George L. Blackburn, associate director of nutrition at Harvard Medical School.
Blackburn said if the drugs do end up on the market, they will target those who are obese and possibly morbidly obese.
"Studies have looked at the morbidly obese -- about 7 percent of our population falls into this category -- that's a group that has a huge need," said Fujioka.
Though risky, invasive procedures such as gastric bypass surgery clearly offer better results than taking diet pills. But many obese Americans may not qualify for surgery. And given the grim history of many diet pills that have come and gone, many experts said they don't think the answer to the obesity epidemic can be found in a pill.
"The more effective drugs are less safe, and the safer drugs are less effective," said Katz. "[More than half] of overweight adults [are] metabolically healthy. Why would we put metabolically healthy people on a drug when they don't need it?"
Regardless of whether a person undergoes surgery or takes diet pills, both options must be accompanied by an overall healthier lifestyle that includes diet and exercise, according to Besser.
"You can't just use the drug and say that's the secret to weight loss," said Besser.