With the big guns -- Pfizer (NYSE:PFE), Merck (NYSE:MRK), and Sanofi-Aventis -- all out of the picture, the obesity treatment market is wide open.

And tiny VIVUS (NASDAQ:VVUS) is hoping to fill it with its obesity treatment, Qnexa.

The drug looked OK in the first of three phase 3 trials. Subjects taking Qnexa lost 9.2% and 8.5% of their body weight at two different doses compared to a 1.7% loss for the placebo group. The percent of patients that lost 5% of body weight -- a pretty good benchmark compared to current obesity treatment options from GlaxoSmithKline (NYSE:GSK) or Abbott Labs (NYSE:ABT) -- also looked good. The two doses allowed 66% and 62% of subjects to hit that target compared to just 15% for placebo.

While the data looks good, Qnexa is far from home free. It'll need to perform well in the year-long studies -- this one was just 28 weeks -- without the various side effects that have dogged so many other diet drugs. Results from those two studies are expected in the middle of next year.

Even if Qnexa is successful in its clinical trials, success in the market isn't guaranteed. Orexigen Therapeutics and Arena Pharmaceuticals (NASDAQ:ARNA) both have data on their obesity drugs due out next year, and Qnexa could run into generic competition before it even makes it to market.

The drug is a combination of phentermine, an appetite suppressant best known for forming part of the infamous fen-phen weight loss drug formerly marketed by Wyeth, and the active ingredient in Johnson & Johnson's (NYSE:JNJ) epilepsy and migraine drug Topamax. Phentermine is already available as a generic, and Topamax will be available as a generic shortly. VIVUS has made some changes like having the drug contain 92 milligrams of Topomax rather than the 100 mg that will be available in generic form, but it's not clear whether that'll be enough to keep doctors from prescribing the two generics individually rather than the likely-higher-priced Qnexa.

With a market cap around $450 million, VIVUS looks accurately priced to me. There's a lot of risk involved in developing drugs for obesity, and its stock isn't likely to take off until the company has eliminated those risks.

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Fool contributor Brian Orelli, Ph.D., wouldn't mind losing 5% of his body weight. He doesn't own shares of any company mentioned in this article. Pfizer, Johnson & Johnson, and GlaxoSmithKline are Motley Fool Income Investor picks. Pfizer is also an Inside Value recommendation and the Fool owns shares. The Fool's disclosure policy is as thin as a piece of paper.