Last week, tiny drugmaker VIVUS (Nasdaq: VVUS) said it had completed enrollment for a phase 3 study of its anti-obesity drug Qnexa.

Qnexa is VIVUS' latest attempt at developing the next fad drug to tap into a potential multibillion-dollar market. For years, VIVUS has been developing compounds to treat erectile dysfunction, hot flashes due to menopause, and now obesity.

Muse, an erectile dysfunction product, hasn't been much of a hit since its approval in 1996. Compared with other erectile dysfunction therapies from Eli Lilly (NYSE: LLY) and Pfizer (NYSE: PFE), Muse is an unwieldy compound to use (it's not taken orally). VIVUS did get a nice break last year, though, when the Food and Drug Administration approved hot-flash treatment Evamist, triggering a $140 million payment from KV Pharmaceutical (NYSE: KV-A) that added to the $10 million VIVUS received when it signed away its rights to the drug.

Now there's Qnexa. This weight-loss drug is a combination of two already-approved compounds. The first part is phentermine, which is available in generic form, has long been used as an appetite suppressant, and is best known for forming part of the infamous fen-phen weight loss drug formerly marketed by Wyeth (NYSE: WYE). The second part of Qnexa is a formulation of Topamax, which is marketed by Johnson & Johnson (NYSE: JNJ) -- not for any weight-loss related conditions -- and will soon be available in generic form.

VIVUS is running three phase 3 studies for Qnexa, and in March it enrolled enough people for two of them. Undoubtedly, getting several thousand patients into these phase 3 studies so rapidly after the studies were announced late last year can be considered a success for the company.

Results from the first of these studies are expected "before the end of the year," with results from the second-largest of these trials due in the "second half of 2009." Anti-obesity drug candidates from VIVUS and other drugmakers like Arena Pharmaceuticals (Nasdaq: ARNA) definitely have a large potential pool of patients. The big question, really, is: If Qnexa is a success and wins FDA approval, what would stop patients from purchasing the (presumably) cheaper generic equivalents?

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. Pfizer, Eli Lilly, and Johnson & Johnson are Income Investor picks. Pfizer is also a recommendation of Inside Value. The Fool has an A+ disclosure policy.