Vivus (VVUS) has been approved by U.S. officials to sell its diet drug Qsymia, setting up a battle for the prescriptions of millions of obese Americans.

Vivus will compete with Arena Pharmaceuticals (ARNA), which is preparing its own launch of a diet drug.

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The announcement came after USA Today inadvertently put out a story online Tuesday that reported the drug was already approved -- before it actually had been. The story was pulled and USA Today made a statement about the goof. In that article, Vivus President Peter Tam was quoted as saying he didn't believe insurance companies would initially cover the cost of the drug. Without revealing a price, he told the paper it won't be "outrageous." The FDA did approve the drug later in the day.

Shares of Vivus bounced around all day Tuesday. They closed down 8% to $26.46 but were back up over 11% Wednesday in early afternoon trading.

The competition will certainly depend heavily on the marketing message for the drugs' safety and effectiveness. Vivus may claim it has a more effective drug but its product also carries safety risks. The approval comes with an agreement that Vivus conduct a so-called risk evaluation and mitigation strategy to inform women of potential birth defects caused by taking the drug.

One of the ingredients in Qsymia (formerly known as Qnexa) is topiramate, which poses the risk of cleft lips in newborns of women taking the drug, according to the FDA and Vivus' own studies. Topiramate already is on the market to treat epileptic seizures and migraine headaches.

Vivus also must perform post-approval safety studies, including research to assess cardiovascular risk of taking the diet drug.

Last month, the FDA approved Arena's diet pill Belviq for sale in the U.S., making it the first new weight-loss drug approved in this country in more than a decade.

Both Vivus and Arena won backing of expert advisers to the FDA earlier this year. The companies and rival Orexigen Therapeutics (OREX) argue that obesity has become an epidemic in this country, contributing to a number of health issues such as diabetes and heart disease.

Vivus is the only company among the three diet pill makers who doesn't have a development partner, a fact that has led to speculation that it may be acquired. Arena is partnered with Japanese drug maker Eisai, while Orexigen is linked up with another company from Japan, Takeda Pharmaceutical (TKPYY). Orexigen is more than two years away from even being considered again for approval but the company believes it can compete.

It hasn't been an easy road for the diet drug companies. The FDA rejected Vivus and Arena's new drug applications in 2010 and Orexigen's in early 2011 because of fear of side effects. Abbott Laboratories (ABT) pulled its diet pill Meridia from the market in 2010 due to concerns about heart attack and stroke risk. The diet pill combination known as fen phen was taken off the U.S. market in 1997 because of risk of heart valve damage.

Vivus, Arena, and Orexigen drugs all carry safety risks. When the FDA approved Belviq, it said obesity is a "major public health concern," and that the benefits outweighed the risks of the drugs. Last night, the agency made similar remarks.

Shares of Arena fell more than 7% Wednesday to $10.23, while Orexigen was up 1.7% in afternoon trading $7.26.

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