It's not often that safety data pushes a stock up more than 8%, as Pozen (Nasdaq: POZN) experienced yesterday. Sure, data that hints at safety problems can crush a stock -- ViroPharma (Nasdaq: VPHM) last summer being a prime example -- but a company's announcement that a drug is safe rarely has the opposite effect.

The exception comes when investors think it's the last hurdle before a drug can be approved. Pozen announced yesterday that it had submitted safety data to the FDA that showed its migraine treatment doesn't cause DNA damage that might lead to cancer. In August, the FDA sent Pozen and its marketing partner, GlaxoSmithKline (NYSE: GSK), an approvable letter requesting more data, because the agency was worried about a preclinical test that showed the drug causing DNA damage in cells grown in the laboratory.

I have to hand it to Pozen; the company got the clinical trial done quickly. In October, the company took a calculated risk by submitting a response to the FDA without data, hoping that the agency would buy its explanation that the DNA damage in the laboratory was an artifact that wouldn't occur in humans -- which the new clinical data confirms. I made the argument that because of time constraints, Pozen had nothing to lose by doing this but that the FDA probably wouldn't give the drug a thumbs-up without the safety trial data. We don't have a window on the FDA decision-making process, so I guess we'll never know whether my guess was right. 

Pozen also announced that the FDA has tentatively accepted its new name -- Treximet -- for the drug, so we at least know the file hasn't been just sitting on a desk gathering dust. Whether it means that Pozen will get a response well before its April 15 action date or not is anyone's guess.

It seems as though Pozen has jumped through all of the FDA's hoops, but then again, that's what it thought after it responded to its first approvable letter -- only to get a second letter for the DNA-damage issue. If the FDA would be more open about its decision-making process, investors could be certain that an approval was imminent. Instead, they'll have to wait a few months to find out whether Pozen has covered all of its bases.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Glaxo is a selection of the Income Investor newsletter. The Fool has a disclosure policy.