Want some proof of the unforgiving nature of biotech investing? Shares of Adolor (NASDAQ:ADLR) are down nearly 50% today, based on mostly positive clinical trial results announced this morning for Entereg, a drug from Adolor and and partner GlaxoSmithKline (NYSE:GSK) to treat opiod-induced bowel dysfunction (OBD).

The results of the two phase 3 clinical trials announced today were mixed but mostly positive. One of the two trials showed statistical significance and succeeded in the primary endpoint, but the other trial failed the primary endpoint by some margin, yet succeeded on the secondary endpoint. Adolor tried to put a positive spin on the second trial, calling it "supportive" -- a fair description.

The mixed results are somewhat confounding, though, since the trials were designed to be identical. Then again, the difficulty of running clinical trials in this space is well-known, as evidenced by plenty of other companies in the past. One of my favorite development-stage pharmaceuticals, Pain Therapeutics (NASDAQ:PTIE) has had similar problems running clinical trials in this area.

Entereg has had an interesting development history. Numerous clinical trials have been run for the drug, which first received an approvable letter in July 2005 for postoperative ileus (POI). Adolor subsequently ran another successful trial of Entereg in this indication; it's scheduled for another FDA review Nov. 9.

Why the big decline in share price today? OBD is a much larger indication than POI, and currently there are no good treatments for the disorder.

The biggest beneficiary today of the mixed Entereg results was Progenics Pharmaceuticals (NASDAQ:PGNX). Shares of Progenics were up nearly 13% this morning. Wyeth (NYSE:WYE) and Progenics have a competing drug to treat opioid-induced constipation, but the companies don't expect to even file for drug approval until late 2008 at the earliest.

The aforementioned Pain Therapeutics is another beneficiary of the Entereg announcement. Pain's drug, Oxytrex, works similarly to the Progenics and Wyeth drug, but it's farther along in development, and it remains unpartnered.

The nearly 50% haircut in shares of Adolor today is a huge overreaction. Based on past trials for Entereg, my initial reaction is that the trial results reported today don't show the drug to be a failure. Instead, they're probably a result of the statistical noise inherent in trials for drugs to treat OBD.

Investors willing to take a gamble on Adolor at this price may be in for huge rewards if the company and Glaxo can convince the FDA of Entereg's viability. A drug that successfully treats OBD could have a market of more than a billion dollars.

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Fool contributor Brian Lawler owns shares of PTIE. He welcomes your feedback. The Motley Fool has a disclosure policy .