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What: Shares of drug developer Pain Therapeutics (Nasdaq: PTIE) were aching today as they fell as much as 47% in intraday trading on heavy volume.

So what: At this point, all Pain Therapeutics shareholders know is that the Food and Drug Administration didn't approve its Remoxy drug. The company was working with Pfizer (NYSE: PFE) on the drug, which is an oxycodone formulation that's designed to prevent abuse. Of the rejection, the company simply said, "Pfizer is working to evaluate the issues described in the Complete Response Letter and plans to have further discussions with FDA around them."

Now what: Without details, it's tough to say exactly how bad this news is for Pain Therapeutics, though this isn't the first time that Remoxy has been dinged. Back in 2008, the FDA pushed back approval when it asked for more lab data. This could, however, be good news for Acura Pharmaceuticals (Nasdaq: ACUR). Pfizer is working with Acura on a pain pill called Oxecta, which was approved earlier this week. Some investors are speculating that Pfizer could divert some of the resources that were going toward Remoxy to Oxecta. Acura's shares went in the opposite direction of Pain Therapeutics today, gaining as much as 18%.

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Fool contributor Matt Koppenheffer does not have a financial interest in any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or Facebook. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.