MOUNTAIN VIEW, Calif. ( TheStreet) --The U.S. Food and Drug Administration rejected Vivus' ( VVUS - Get Report) weight-loss drug Qnexa Thursday. Getting Qnexa approved eventually hinges on Vivus convincing regulators that the drug is safe enough for use by potentially millions of obese Americans.

An impossible task? Perhaps it is given the FDA's current risk-averse stance on diet drugs. The agency similarly rejected Arena Pharmaceuticals' ( ARNA - Get Report) lorcaserin and forced Abbott ( ABT - Get Report) to yank Meridia from the market.

Yet Vivus still has a chance because Qnexa's "path forward" appears to be navigable. The Qnexa worst-case scenario didn't play out Thursday. FDA didn't ask Vivus for pre-approval studies to rule out the possibility Qnexa raises the risk of heart attacks and strokes; or that the drug puts pregnant women at higher risk for giving birth to babies with birth defects.

Instead, Vivus plans to respond to FDA's safety questions outlined in the complete response letter by crunching through existing Qnexa data to come up with new analyses of the drug's cardiovascular and birth-defect risk. The response will be submitted to FDA within six weeks, Vivus said, which sets up the potential for another FDA approval decision date for Qnexa in the middle of 2011.

"We believe no additional studies are required," said Vivus CEO Leland Wilson on a conference call Friday morning.

All in, Vivus came out of Thursday's big event in decent shape. The FDA didn't throw any surprise roadblocks in its complete response letter and Vivus was clearly prepared for the questions the agency did raise, which is why the company is ready to respond with such alacrity. Unlike Arena's lorcaserin, FDA isn't calling Qnexa's weight-loss efficacy "marginal."

This should be enough for Vivus' stock price to move higher, and early Friday, the stock was up about 30% to $8 in the pre-market session. At a minimum, Vivus' downside looks limited for now.

None of this guarantees Qnexa's approval. Let's be clear about that. Interpreted through the lens of Vivus' press release, the FDA's complete response letter is open-ended enough to worry that new clinical trials could be ordered if Vivus' response falls short of assuaging the agency's concerns.

If we've learned anything about the FDA this fall, it’s that regulators have no qualms about throwing up new roadblocks to approval, even asking for stuff that seems to come from nowhere. (Just ask Amylin Pharmaceuticals about that.)

According to Vivus, FDA wants "evidence" to rule out cardiovascular risk tied to Qnexa. On its conference call Friday, Vivus said this evidence will come from existing Qnexa data, including a two-year safety study that was completed after the FDA advisory panel in July and which has not yet been reviewed by regulators.

Across the entire Qnexa clinical trial program that includes 4,323 patients, Vivus says that serious cardiovascular and neurovascular adverse event rates in patients taking Qnexa were similar to placebo, with a 41% reduction in the relative risk favoring Qnexa. In addition, no birth defects were observed in any of the women who participated in the Qnexa clinical trials.

The challenge for Vivus is that FDA has seen a lot of the data tied to this argument already and it wasn't strong enough to get the drug approved on the first go-around. While studying Qnexa in almost 4,400 patients seems thorough, the number heart attacks and strokes observed in the studies is small and may not provide enough of the "evidence" that FDA requires. The same can be said about pregnancies.

If that's the case, FDA could ask for new studies to gather the evidence -- clinical data -- required to really judge Qnexa's safety. Vivus, on its call Friday, sounded very confident that this will not happen.

The only thing for certain, however, is that race to develop a new obesity drug -- once expected to cross the finish line in 2010 -- is dragging well into 2011.

--Written by Adam Feuerstein in Boston.

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Adam Feuerstein writes regularly for TheStreet.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet.com. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.