July 13, 2010 -- An initial review has found the diet pill Qnexa to be safe and effective, according to a U.S. Food and Drug Administration report released this morning.

However, the FDA's look into two previous studies involving Qnexa found a high rate of participants who dropped out due to adverse effects. Some participants taking a high dose of the pill lost an average of 10 percent of their starting body weight.

According to the report, 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. And, around 40 percent of all participants either taking high or low doses of Qnexa did not complete the study for various reason.

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"Until you get into the nitty gritty of why did those people take the drug for an entire year, I worry that there was some side effects in this group that were missed," said ABC News' Senior Health and Medical Editor, Dr. Richard Besser on Good Morning America.

The FDA will more thoroughly review Thursday the efficacy and safety of Qnexa, one of three new weight-loss drugs.

Experts hope Qnexa, and the other drugs, Lorcaserin and Contrave, will 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 a thickening of the heart valve. Meridia was pulled from the European drug market in 2009, and the FDA recently warned consumers about Alli's link to severe liver damage.

"It's shown a 10 to 15 percent weight-loss in patients, which is very impressive," said Dr. Ken Fujioka, director of the Scripps Clinic Center for Weight Management in San Diego. He is also on the advisory committee for Vivus, the company that makes Qnexa, as well as on the advisory committees for the companies that make Lorcascerin and Contrave.

"This drug isn't something new," said Besser. "It's a combination of two drugs that have been on the market for many years."

Qnexa contains the amphetamine phentermine -- one half the ingredients of the popular combination diet pill Fen-phen, ultimately pulled by its manufacturer Wyeth because of its link to heart damage.

Fujioka said although it proved dangerous in the combination, there were no safety issues with phentermine by itself.

According to Vivus, Inc., the company that makes Qnexa, phentermine helps suppress a person's appetite, while the other ingredient, the anti-convulsant topiramate, makes a person feel more satiated.

Some experts though, questioned the safety of a combination pill that appears to show striking similarities to previously recalled drugs.

"Whether any drug will become another Fen-phen is hard to predict," said Dr. Gerard Mullin, associate professor of medicine at the Johns Hopkins School of Medicine. "Every time we see a drug recalled, none were predictable, but it seems that similar classes of drugs can have inherent problems."

It's also unclear to some experts on whether topiramate actually works.

"The anticonvulsant in both Contrave and Qnexa is a new variation on the theme of tweaking brain pathways to adjust appetite but does not appear to be more than a minor innovation to me, and one of questionable utility," said Dr. David L. Katz, associate professor adjunct in public health practice at the Yale School of Public Health.

Year-Long Studies on Qnexa, Lorcaserin, Contrave

"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.