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(Reuters) - U.S. health regulators have extended by three months their deadline for making an approval decision on Vivus Inc's  diet drug Qnexa, the company said on Monday, marking the latest delay to bring a new obesity treatment to market.

Vivus shares fell 6.4 percent after the delay was announced, slipping to $21.45 in extended trading from their Nasdaq close at $22.92.

The Food and Drug Administration informed Vivus that it will decide whether to approve Qnexa by July 17 as it needs more time to review the company's risk evaluation plan for the drug. The previous FDA action date for Qnexa had been April 17.

Vivus's Qnexa is one of three experimental new weight loss drugs seeking FDA approval after initial rejections by the agency. Arena Pharmaceuticals Inc  and Orexigen Therapeutics  are also hoping for better news from the FDA after rejections of their diet pills.

After an advisory panel to the FDA in February voted overwhelmingly to recommend approval of Qnexa, saying that its benefits outweighed the risks, hopes that the first new obesity treatment would be approved for sale in the United States in more than a decade soared -- along with Vivus's share price.

But the FDA requested a Risk Evaluation and Mitigation Strategy (REMS) plan from the company, which the agency considers a major amendment to the approval application requiring further review time.

"The Qnexa REMS submission is comprehensive, with materials based on ongoing feedback from the FDA since our advisory committee meeting in February," Vivus Chief Executive Leland Wilson said in a statement.

"We look forward to finalizing our REMS with the FDA while we move forward with our commercialization plans," Wilson added.

Qnexa, which combines the appetite suppressant phentermine and the anti-seizure drug topiramate, helped patients on average lose at least 10 percent of their weight after a year of treatment in pivotal clinical trials.

Obesity has become a major health crisis as it can lead to diabetes, heart disease and a host of other preventable serious health problems.

With about a third of Americans deemed obese and some two thirds classified as overweight, the FDA has been under mounting pressure to approve a new weight loss treatment. But high profile safety problems with past diet drugs and the knowledge that a huge portion of the population will be tempted to take any new diet pills has left the FDA reluctant to approve them.

The infamous diet drug cocktail known as "fen-phen" was pulled from the market in 1997 after it was found to cause serious heart valve damage.

Meridia, another diet pill, was pulled from the U.S. market in 2010 after being linked to heart problems, leaving Roche's  Xenical as the only prescription obesity drug approved for long term use. Its side effects and limited weight loss has made it far less than an optimal choice to tackle the obesity epidemic.

The FDA could still require Vivus to conduct additional studies to rule out heart problems as it has done with Orexigen's Contrave, or could allow Vivus to conduct such studies after the drug is already on the market.

(Reporting By Bill Berkrot; Editing by Gary Hill, Bernard Orr)