Vivus Inc, hoping to win approval to sell the first new prescription diet drug in more than a decade, told U.S. medical advisers on Thursday its weight-loss pill gives patients a safe option for shedding pounds and improving their health.

Vivus is seeking Food and Drug Administration support to sell its drug, Qnexa, to adults to use once a day to slim down. But first it must prove its pill avoids side effects that have sidelined previous products.

The stakes are huge for Vivus and its investors, who have pushed the stock up more than 130 percent in the past year, drawn by the potential for huge sales to millions of overweight Americans.

FDA's advisory panel will weigh Vivus' data before deciding later on Thursday whether to back the drug. The FDA will weigh the advisers' vote before making a final decision.

Investors are also watching the panel meeting for clues on the approval potential for two rival drugs from Arena Pharmaceuticals Inc and Orexigen Therapeutics Inc, whose shares have also soared.

Arena shares were up more than 30 percent in early afternoon trading on the Nasdaq, while Orexigen was up 7 percent.

FDA staff largely support the effectiveness of Qnexa, the first of three potential new diet drugs up for review this year. But they are concerned about its effect on fetuses, mental health and heart rate. Memory loss and other cognitive problems and body acid changes are also important issues.

That assessment, released earlier this week, was cheered by investors who sent Vivus shares up nearly 20 percent, betting the safety issues were not serious enough to keep Qnexa from pharmacy shelves. Trading in the shares was halted on Thursday morning ahead of the FDA advisers' decision.

The safety risk associated with Qnexa are known ... we did not observe any surprises, said Vivus consultant Neil Gesundheit, adding most patients saw at least 10 percent weight loss when taking the pill along with a better diet and more exercise.

Those who took the highest dose for a year and did not quit over side effects lost a median of 15 percent of their body weight, or roughly 35-40 pounds, Gesundheit told panelists.

These levels of weight loss are unprecedented, he said.

If approved, Qnexa would offer a potential fat-fighting jump-start to the more than two-thirds of Americans who are overweight -- and would boost a nearly 20-year-old biotech company that has not had a U.S. product approved since 1996.

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

If Qnexa gets the FDA's green light, that figure could soar. Analysts estimate it could see $689 million in sales by 2014, according to consensus forecast data from Thomson Reuters.

Current weight-loss drugs have not gained much traction, as most only trim a few pounds, but carry serious or unpleasant side effects such as heart risks or gas.

It's clear that we need new medical therapies to manage the epidemic of obesity, said Louis Aronne, a Weill-Cornell Medical College professor who spoke on behalf of Vivus.

Weight can be managed with diet and exercise and surgery is also an option.

'FOCUS ON SAFETY'

In general, the FDA is in agreement with the company with respect to the weight-loss effects of the drug, said Eric Coleman, deputy director of the FDA division that oversees metabolic drugs. The focus is primarily on safety.

Qnexa's potential to cause congenital defects in unborn babies is a particular concern, especially since women of child bearing age are the pill's target customer.

So far, analysts have remained confident about Vivus' ability to overcome safety hurdles.

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

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 withdrawn in 1997 when serious heart valve problems emerged.

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

At least one panelist, University of California Los Angeles medical professor Sanjay Kaul, said he wanted to make sure the possible risks did not overshadow pounds lost.

Current diet pills 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.

(Additional reporting by Susan Kelly in Chicago; editing by Bernard Orr, John Wallace and Andre Grenon)

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