(Corrects share price in last paragraph from $2.11 to $12.11)

WASHINGTON - The first potential U.S. prescription weight-loss pill in more than a decade could move closer to market on Thursday if it can overcome safety hurdles that have plagued diet drugs for years and led to tepid sales despite the growing number of obese Americans.

Vivus Inc is seeking Food and Drug Administration support to sell its drug, Qnexa, to adults to use once-a-day along with exercise and diet changes to slim down.

On Thursday, a FDA advisory panel will weigh Qnexa -- the first of three potential new diet drugs -- and decide whether the benefits of shedding pounds outweigh safety concerns such as the drug's effect on fetuses, mental health and heart rate.

The FDA will weigh the panel's recommendation before later making its final decision.

Although more than two-thirds of Americans are overweight or obese, weight-loss drugs have not gained much traction. Most only trim a few pounds but carry serious or unpleasant side effects such as heart risks or gas.

The critical factor in looking at the data is the enormous benefit that (Qnexa) provides, said Ira Loss, a healthcare analyst for Washington Analysis Corp who follows the FDA for financial clients. The risks are not insurmountable. They can be managed.

Last year, prescription and over-the-counter diet drugs took in just $381.5 million in 2009, according to IMS Health.

If Qnexa wins the FDA's green light, that could more than double. Analysts estimate Vivus' drug could take in nearly $689 million in sales by 2014, according to consensus forecast data from Thomson Reuters.

It would be the second U.S. drug for the biotech, which saw its last drug -- an erectile dysfunction suppository -- approved 14 years ago. Its efforts to market a sexual dysfunction cream for women flopped in 2006.

Investors, buoyed by the potential for wider sales among millions of overweight Americans (http://link.reuters.com/vup37m), have been lured to the company. Shares of Vivus have more than doubled in the last year, and are up more than 35 percent so far in 2010.

FDA staff have said the company's data clearly shows the drug works and largely meets the agency's new guidelines for obesity drugs issued in 2007.

Patients taking Qnexa saw between 3 and 9 percent weight loss over placebo depending on how much of the drug they took, the staff said. They also benefited from improved blood pressure, and better fats and sugar levels in the blood.

But first Vivus must convince FDA's panel of outside experts that it does not harbor significant safety issues that have sidelined past options.

Qnexa attempts to improve on the infamous fen-phen diet drug. It combines one of fen-phen's ingredients -- the appetite suppressant phentermine -- with the anti-seizure drug topiramate. Fen-phen's other ingredient, fenfluramine, was yanked in 1997 when serious heart valve problems emerged.

FDA staff earlier this week said they were particularly focused on five potential risks: the potential for congenital defects, psychiatric problems such as depression, memory loss and other cognitive issues, increased body acids, and higher heart rates.

I think it's pretty well established that Qnexa has strong efficacy ... The real question will be how the members of the panel view those safety concerns against the efficacy benefits, Canaccord Adams analyst Adam Cutler told Reuters.

The agency plans to ask its panelists to weigh each area of concern before voting on whether to back the drug.

FDA officials usually follow their panelists' advice. A decision is expected by October 28.

Industry watchers and competitors will also be looking at clues on the FDA's thinking ahead of similar reviews on two rival drugs later this year by Arena Pharmaceuticals and Orexigen Therapeutics.

Diet pills already on the market include Abbott Laboratories' Meridia, which carries several heart-related risks, and Roche Holding AG's Xenical, which causes liver problems and uncontrolled bowel movements.

Imagine what a drug can do that doesn't have side effects and removes more weight, said Leerink Swann analyst Steve Yoo.

On Wednesday, shares of Vivus closed down 2.7 percent at $12.11. Shares of Orexigen closed up more than 11 percent, while Arena closed up nearly 2 percent.

(Reporting by Susan Heavey; additional reporting by Susan Kelly in Chicago; Editing by Bernard Orr)