After months of speculation and a pair of Food and Drug Administration advisory panels, FDA decisions for Arena Pharmaceuticals' (Nasdaq: ARNA) lorcaserin and VIVUS' (Nasdaq: VVUS) Qnexa are finally upon us.

Arena's PDUFA date is Oct. 22, while VIVUS' date with destiny is six days later on the 28th. The Prescription Drug User Fee Act (PDUFA) sets the goal for the FDA to make a decision by then, but the FDA is free to make a decision at any point before or after that date. A decision on lorcaserin especially could be delayed since the FDA advisory panel met just last month.

What's the chance of lorcaerin and Qnexa gaining FDA approval? I'd put it somewhere between unlikely and nonexistent.

Give me more
The FDA advisory panels tell the whole story. The panel voted 10-6 against recommending Qnexa, and 9-5 against recommending lorcaserin. In both cases, the panels wanted more safety data before recommending approval of the drugs. Fortunately, both companies already have trials in process that could potentially produce such data.

In Qnexa's case, there's two-year safety data from an extension trial. The generally positive data was released after the advisory panel, but it most likely won't be considered for this FDA decision. For lorcaserin, there's a trial in diabetics, although it's not clear if it's large enough to really answer the questions the panel of experts has.

Blame it on the ancestors
The agency isn't bound to follow its advisory panel's recommendation -- there are plenty of examples of when it hasn't -- but for the obesity drugs, that avenue is all clogged up by the remnants of previous obesity drugs.

The most famous is Wyeth's fen-phen. The drug was pulled from the market because of potential heart problems. Abbott Labs' (NYSE: ABT) Meridia was recently pulled from the market after a post-approval study revealed potential heart problems as well.

sanofi-aventis' (NYSE: SNY) Acomplia was never given the thumbs-up stateside, but the drug was approved in Europe. The suicidal behaviors and depression that U.S. regulators were worried about turned out to be problematic enough to justify pulling Acomplia from the market in Europe.

Everyone -- patients, doctors, and investors (especially investors) -- would like a new drug to help shed the pounds. But with diet and exercise available as relatively safe option, the agency is likely to error on the side of caution.

If the advisory panel had voted in favor of the drugs, I could see a reason to be somewhat optimistic, but the agency isn't going to go against its panel of experts. Imagine the outcries if the FDA approved the drugs against the advisory panels' recommendations, and then safety issues cropped up after they were on the market for a while.

Another red flag?
I'm not sure we need one, but here's some more bad news for investors hoping to sneak in an approval: A study of FDA decisions a few years ago revealed that applications submitted in the fourth quarter had the lowest approval rate by far. It could be that companies rushing to submit applications before self-imposed end-of-the-year deadlines tend to turn in sloppier work. It's also possible that the rush just makes the agency busy enough that it errs on the side of caution.

Not every drug expecting an FDA decision this month fits the first category. Amylin Pharmaceuticals (Nasdaq: AMLN) and Eli Lilly's (NYSE: LLY) Bydureon was submitted in April, but since it's a response to a previous rejection by the FDA, the agency gives itself six months to review the decision.

The third contender
What will rejections of Qnexa and lorcaserin mean for the third obesity drug that's currently under review? It's hard to say. Orexigen Therapeutics' (Nasdaq: OREX) Contrave has side effect issues of its own, but Orexigen will have the advantage of seeing how the other advisory panels and FDA rejections go before Contrave goes in front of the FDA advisory panel on Dec. 7.

Orexigen could learn enough from the first two to craft its presentation in such a way to get positive recommendation, but I'm inclined to think the FDA and its advisors are predisposed to reject any obesity drug on the first try. By requiring more data before approval, the FDA can somewhat cover their butts in case safety issues do crop up.

What do you think? Is the FDA willing to go against its advisors? Take the poll below and then fill in the details in the comment box below.

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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.