So much for a fairly quick resubmission of VIVUS' (Nasdaq: VVUS) obesity drug Qnexa to the Food and Drug Administration after the agency turned down the drug in October. VIVUS announced today that the FDA wants the company to determine whether it's feasible to use existing health-care databases to determine the historical incidence of cleft lip in babies of women treated with topiramate.

Topirmate -- the active ingredient in Johnson & Johnson's (NYSE: JNJ) migraine and epilepsy drug, Topamax -- is also one of the components of Qnexa. There have been reports of women who take topirmate for migraines having an increased risk of having a baby with cleft lip.

The delay itself isn't that big of a deal. It's not like VIVUS announced a lengthy clinical trial. But the 15% drop today still looks justified because the news adds a layer of uncertainty to Qnexa's likelihood of approval:

  • Exactly how long is this going to take?
  • What happens if the databases aren't sufficient to answer the FDA's questions? I'm not sure how one could ethically test this in a clinical trial.
  • If they are acceptable, what level of cleft lip is acceptable? Certainly it's lower for a diet drug than for a migraine treatment.
  • What will the FDA do with this information? Limit Qnexa use to just men, cutting the market in half? Require regular pregnancy tests, which could indirectly lower sales?

Until these answers are known, Qnexa is looking a lot like rival lorcaserin from Arena Pharmaceuticals (Nasdaq: ARNA), which has a bunch of unknowns of its own.

For now, Orexigen's (Nasdaq: OREX) obesity drug Contrave is the front runner. If it gains FDA approval at the end of this month, it could have the market all to itself for a year -- potentially more. If not, it'll join the queue of obesity drugs with unknown timelines for approval.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.