The FDA's Endocrinologic and Metabolic Drugs Advisory Committee voted 10-6 against recommending approval for the combination drug. The pill contains the amphetamine phentermine -- supposedly the safer of two active ingredients in the infamous diet pill Fen-phen, which was pulled from the market in 1997 because of its link to heart damage. Qnexa also contains the anticonvulsant topiramate.
An initial review found Qnexa to be safe and effective, with some participants in early clinical trials losing an average of 10 percent of their starting body weight.
"In terms of efficacy, it's far superior to anything on the market," said panel member Dr. Abraham Thomas, an endocrinologist at Henry Ford Hospital in Detroit.
The committee, however, was concerned about increased risk of psychiatric and cognitive issues uncovered in the company's trials, and said the there was a lack of data to rule out cardiovascular risks and the drug's potential to cause birth defects in women who become pregnant while taking it.
There were also concerns because 18 percent of participants taking a high dose withdrew from trials after experiencing mild side effects, such as tingling of the hands and feet, headache and constipation. About 40 percent of all participants either taking high or low doses of Qnexa did not complete the study for various reasons.
Dr. Sidney Wolfe, director of Public Citizen's Health Research Group, said he was pleased with the panel's decision.
"The two drugs that make up the combination of Qnexa are dangerous on their own," Wolfe said. "Topiramate is a drug that was studied several years ago to see if it worked for diabetics who are obese. It worked well for weight loss, but the side effects were so significant that it should not be used for weight loss."
As for phentermine, Wolfe said there are a number of adverse side effects associated with it, including increased pulse.
But Dr. Ken Fujioka, director of the Scripps Clinic Center for Weight Management in San Diego, said before Thursday's hearing that while phentermine was dangerous as part of the fen-phen combination, it is safe and effective on its own. Fujioka is on the advisory committee for Vivus, the company that makes Qnexa, and the advisory committees for companies that make two other proposed weight-loss drugs, Lorcascerin and Contrave.
Lorcaserin and Contrave go before the FDA for approval later this year. Although the panel voted against recommending Qnexa, the agency doesn't have to follow the panel's advice.
Experts said they hoped that Qnexa and the other drugs would provide effective weight-loss without the dangerous side effects that doomed other drugs, such as Fen-phen, Meridia and Alli. Fen-phen was pulled from the market in 1997 after it was linked to heart valve thickening. Meridia was pulled from the European drug market in 2009, and the FDA recently warned consumers about Alli's link to severe liver damage.