Adolor (NASDAQ:ADLR) and partner GlaxoSmithKline (NYSE:GSK) announced yesterday that they will have to continue playing the FDA waiting game, as the agency has still not decided whether to allow clinical trials of Adolor's top drug to continue.

Adolor's lead drug -- Entereg, which treats constipation caused by surgery and by opioids -- has been under an FDA hold preventing new studies ever since June, after unexpected safety issues popped up in a long-term safety study earlier in the year. 

Even with the new safety issues, Adolor has persisted with its marketing application for Entereg as a treatment for post-operative ileus (POI) -- essentially constipation after lower abdominal surgery. The Prescription Drug User Fee Act (PDUFA) date has been scheduled for Feb. 10.

Since POI is not a chronic condition, Entereg does have a greater than zero chance of getting approved for this indication, whereas getting permission to use it to treat opioid-induced constipation looks like a long shot at best with the new safety issues.

The problem with Entereg in the POI indication is that the FDA already granted the drug an approvable letter asking for more safety data in November of last year, and since the new study results have not been favorable, I don't see any reason why the agency will change its mind on the drug. So investors should expect another approvable letter seeking more data in February.

The fact that the FDA has now extended its review of the clinical hold placed on any new studies of Entereg shows that there are still major unanswered safety issues the agency is concerned about.

Adolor does have one other compound in development, an opioid painkiller that just entered phase 2 studies. Little data is known about the drug, so it's near impossible to handicap its future one way or another, but if Entereg fails in POI and any possible use of the drug in other indications is years away, the painkiller may represent Adolor's last chance to bring a new compound to market.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. GlaxoSmithKline is an active Income Investor pick. No clinical holds have been placed on the Fool's disclosure policy.