Pain drugs are a big pain on Wall Street. Problems with prescription drugs Vioxx at Merck (NYSE:MRK) and Celebrex at Pfizer (NYSE:PFE) have led to their parent company's stocks falling like rocks. Heck, even over-the-counter medication Aleve (naproxen) from Bayer (NYSE:BAY) has recently won dubious distinction, thanks to one clinical trial finding that Celebrex was safer than high doses of naproxen.

Today's news is that biotech company Adolor (NASDAQ:ADLR) and partner GlaxoSmithKline (NYSE:GSK) are -- once again, mind you -- back in the fray with another painful clinical trail for Adolor's lead drug, Entereg.

In January, fellow contributor David Nierengarten covered the knockout blow Entereg delivered to Adolor's stock when it failed to meet primary end points to treat post-operative ileus -- a paralysis of the intestines after surgery. As David noted at the time, "Trials for Entereg continue for other applications, such as painkiller-induced constipation and chronic constipation/irritable bowel syndrome."

The latest trail results, and the words "not statistically significant," have sent the stock down over 50% today.

This second knockout punch is not going to be a fatal blow. A Glaxo drug-development senior vice president said, "I was pleased to see that, although the study did not achieve statistical significance in the primary endpoint, it did achieve statistical significance in some key secondary endpoints."

Adolor is standing and ready to fight at its next big date for Entereg. In late April, the Food and Drug Administration is expected to review the drug's ability to manage post-operative ileus by accelerating the time to recover gastrointestinal functions after abdominal or pelvic surgeries.

Will the FDA deliver knockout punch No. 3? No one knows. Those finding it too painful to watch Adolor catching naps on its back on the canvas might want to take a look at two competitors Nierengarten mentioned -- Pain Therapeutics (NASDAQ:PTIE) and Progenics (NASDAQ:PGNX).

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Fool contributor W.D. Crotty does not own stock in any of the companies mentioned.

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