Drugmaker Vivus Inc. said Monday that regulators will take another three months to make a decision on its closely-watched obesity drug Qnexa, which could become the first new prescription weight loss pill to reach the market in more than a decade.

Vivus said the Food and Drug Administration pushed back its target date to July 17 to consider a new drug safety plan submitted by the company. Previously the agency was scheduled to make a decision by April 17. Vivus said the three-month extension is standard when companies submit additional risk management information for a drug.

The FDA has rejected three experimental drugs for obesity in the last three years, including Qnexa, raising questions about whether any new weight loss drugs can win approval. The agency has not approved a new prescription diet pill since 1999.

An effective and safe diet pill would likely be a blockbuster product in the U.S.: With more than 75 million obese adults, the nation's obesity rate is nearing 35 percent.

Qnexa was previously rejected in 2010 over concerns that it can cause heart palpitations and birth defects if taken by pregnant women.

But Vivus resubmitted the drug and it received a surprisingly positive 20-2 favorable vote from a panel of medical advisers in February.

The group emphasized the drug's benefits, which include weight loss of nearly 10 percent for most patients taking the drug over a year — the highest reduction reported with any recent diet pill. But panelists stressed that Vivus should be required to conduct a large, follow-up study of the pill's heart effects, to assure its safety.

Vivus said it submitted new materials for Qnexa's risk management plan to the FDA last Wednesday. Such plans often include materials like patient brochures that explain a drug's safety risks. Side effects reported with Qnexa include birth defects, heart palpitations, suicidal thoughts, and memory lapses. Vivus has asked the FDA to approve Qnexa for patients who are obese and who have related health problems like high blood pressure or type 2 diabetes.

Shares of the Mountain View, Calif., company dropped $1.37, or 6 percent, to $21.55 in after-hours trading. The stock rose 39 cents to close at $22.92 during regular trading.

Qnexa is a combination of two older drugs that have long been available. The amphetamine phentermine, which is approved for short-term weight loss, and topiramate, an anti-seizure drug sold by Johnson & Johnson as Topamax. Phentermine helps suppress appetite, while topiramate is believed to make patients feel more satiated.

While several drugs are available for short-term weight loss, there is only one FDA-approved prescription drug for long-term weight loss: Xenical from Roche, which is seldom prescribed due to unpleasant digestive side effects and modest weight loss.

Experts say the challenge of weight loss drug development lies in safely turning off one of the body's fundamental directives: to eat enough food to maintain its current weight.

The decadeslong search for an effective diet drug is littered with failures, most notably the fen-phen combination, which was linked to heart damage in 1997. The cocktail of phentermine and fenfluramine was a popular weight loss combination prescribed by doctors, though it was never approved by FDA.

Other safety failures for diet pills have continued to pile up in recent years.

Four years ago Sanofi-Aventis SA discontinued studies of its highly anticipated pill Acomplia due to psychiatric side effects, including depression and suicidal thoughts. In 2010, Abbott Laboratories withdrew its drug Meridia after a study showed it increased heart attack and stroke.

Qnexa is generally regarded as the most promising of three experimental obesity drugs to be considered because it was the most effective at helping lose weight. The competing drug candidates are Arena Pharmaceuticals Inc.'s lorcaserin, and Orexigen Therapeutics Inc.'s Contrave.