They were dubbed weight-loss wonder drugs, the prescription Phen-Fen arrived in the early 1990's - proving to be the most potent appetite suppressant ever to hit the market.

"It was so successful at inducing weight loss that clinics rose up around the country solely devoted to prescribing Phen-Fen," says Dr. Bryan Roth of the University of North Carolina.

Success that seemed too good to be true.

Heart valve problems appeared in some patients, leading the f-d-a to pull the diet drug cocktail. Drug maker Wyeth faced legal claims that would total billions, and said it had no plans to bring the drug back to market. It seemed the Phen-Fen run was over.

"They basically dropped that drug, and unless the mechanism is bloody obvious, they don't pursue it," says Roth.

But scientist Bryan Roth and his team at the University of North Carolina are. Roth has one of the few labs around the country that analyzes drug side effects, and was the first to tackle the mechanism behind Phen-Fen's dangerous heart effects.

"It was a serotonin receptor. It was found specifically in the heart," says Roth.

The discovery was confirmed, resurrecting the possibility of the return of Phen-Fen.

"You could make drug that targets basically the healthy, or magic receptor for weight gain and misses the receptor for side effects," says Roth.

A number of companies are racing to do just that. One drug is now being tested in people - so far, safety data looks good.

"If it is safe, then that would be a major advance," says Roth.

It’s an advance that would not only impact the treatment of obesity, but could prove the concept that a drug comeback is possible.