Investors are getting fat on the news that VIVUS (Nasdaq: VVUS) has a plan, and a backup plan, to get its obesity drug Qnexa on the market. Shares gained more than 6% yesterday on the news.

I'm not sure the news is so sweet.

You'll recall that the Food and Drug Administration is concerned that topiramate, one of the components of Qnexa, causes congenital malformations and oral cleft in the offspring of women exposed to the drug during pregnancy. The company plans to use medical records to run a retrospective study comparing babies from mothers exposed to topiramate, the active ingredient in Johnson & Johnson's (NYSE: JNJ) Topamax, to a control group that wasn't exposed to the drug.

If the relative risk is low enough, the FDA might let Qnexa on the market. How low does it have to be? VIVUS isn't saying, but it's certainly lower than it would be for migraines and epilepsy, the conditions Topamax is approved for. Obesity is an epidemic, but the FDA believes the standard for risk-benefit is diet and exercise, which doesn't carry all that much risk. In addition to Qnexa, it also rejected Arena Pharmaceuticals' (Nasdaq: ARNA) lorcaserin and Orexigen's (Nasdaq: OREX) Contrave for similar risk-benefit reasons.

VIVUS' backup plan is to ask for approval in just men and women who can't have children. That takes care of the potential harm to babies, but it removes most of the patients that would likely take the drug. It's not even clear whether the clinical trials are still positive if women of childbearing age are excluded from the studies.

The company plans to submit data -- one way or another -- to the FDA in the fourth quarter. If the retrospective data is complete and strong enough, VIVUS will submit that, otherwise it'll request approval in the limited population. Either way, we're looking at a six-month review cycle, so if there's any cake to be had, it'll come about a year from now. Don't start licking your lips quite yet.

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