Approval for the pill, to be sold under the brand name Qsymia, follows last month's go-ahead for Arena Pharmaceuticals' Belviq, but the approvals come after a 13-year dry spell for long-term weight loss drugs due to safety problems with past medicines.

Vivus was set to head a pack of three companies vying to get new drugs approved, but the U.S. Food and Drug Administration delayed its decision for three months to review the company's plan for reducing risks once the drug is sold, such as birth defects and elevated heart rate.

Shares of Vivus fell nearly 8 percent, or $2.24, to close at $26.46 on the NASDAQ, after the FDA failed to come out with a public decision during regular trading hours. After hours, shares rose to $24.65.

Despite the delay, analysts say Vivus may still capture greater market share than Arena because its pill helped patients on average lose at least 10 percent of their weight after a year of treatment in pivotal clinical trials - compared to the average 5 percent weight loss seen in trials of Arena's Belviq.

"We believe that Qsymia has a significant advantage over Belviq, given its considerably better efficacy," Cowen and Co analyst Simos Simeonidis wrote in a research note.

Wall Street sees a successful new diet drug raking in billions of dollars.

The FDA, knowing that millions of Americans would be tempted to take weight-loss pills, set a high bar for approval because of safety problems and even deaths seen with past medicines. The most notorious, known as the "fen-phen" diet-drug combo, was pulled from the market in 1997 after being linked to serious heart valve damage.

But with some two-thirds of Americans considered overweight or obese - and the related costs to public health and the economy skyrocketing - the FDA has been under mounting pressure to approve new weight loss treatments.

"The degree and severity of obesity and the lack of effective pharmacological interventions that we face as a society were two primary reasons for the development of Qsymia," Vivus president Peter Tam said in a statement.

Qsymia was expected to be available in the fourth quarter of 2012, he said.

The FDA approved the pill for use in obese adults, and in overweight adults who have at least one weight-related condition such as high blood pressure type 2 diabetes, or high cholesterol.

"Qsymia, used responsibly in combination with a healthy lifestyle that includes a reduced calorie diet and exercise, provides another treatment option for chronic weight management in Americans who are obese or are overweight and have at least one weight-related comorbid condition," Dr. Janet Woodcock, director of FDA's Center for Drug Evaluation and Research, said in a statement.

An advisory panel to the FDA in February voted 20 to 2 to recommend an earlier version of Qsymia, saying that its benefits outweighed the potential risk of birth defects.

Qsymia combines the anti-seizure drug topiramate with the appetite suppressant phentermine, which was part of the infamous "fen-phen" combination.

Some doctors have prescribed the combination "off-label," without formal FDA approval, since both ingredients are already sold in the market. Phentermine, a generic drug, is approved for short-term weight loss.

Vivus said its pill worked in a different way from "fen-phen" and did not have the same heart valve problems.

Vivus will still have to carry out a lengthy study to prove that the drug does not cause heart attacks or strokes.

Orexigen Therapeutics Inc which also is hoping to have a diet drug approved, was ordered by the FDA to conduct a heart safety study as a condition for approval.

The most common side effects of Qsymia are tingling of hands and feet (paresthesia), dizziness, altered taste sensation, insomnia, constipation and dry mouth.

(Additional reporting by Bill Berkrot; Editing by Gary Hill and Richard Pullin)

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