I've got to think that it's kind of hard to gain traction selling a drug that treats side effects affecting patients who are about to die. Fortunately for Wyeth (NYSE:WYE) and Progenics Pharmaceuticals (NASDAQ:PGNX) they might be able to expand their drug to patients who will use it for longer.

The duo's Relistor treats constipation caused by some prescription pain killers and is currently approved for use by terminally ill patients. On Wednesday, the companies announced results from a clinical trial testing the drug in patients taking opiods for chronic pain. No details were given except that the drug met its primary and key secondary efficacy endpoints.

That was enough news for Progenics investors. They sent the stock up more than 13% before breaking for turkey and stuffing. The big question now is how much a label expansion to additional patients will really help sales of the drug.

Relistor has to be injected, so it's never going to take precedence over laxatives like Novartis' (NYSE:NVS) Ex-Lax for many people, but the drug is headed into uncharted territory here. Which will patients find more bothersome: drug-induced constipation or having to give themselves shots?

Investors might be able to get a hint of the answer when the full details of the trial are unveiled. After the placebo-controlled portion of the trial, all subjects were allowed to go onto Relistor -- called an "open-label phase" because the subjects know they're getting the drug. If a large percentage of the subjects stayed to the end of the trial and entered the open-label phase, that'll be a sign that the drug could be in high demand.

The companies are working on an oral version, which would obviously have a much larger appeal. The drugmakers had also tried to get the drug approved to treat post-operative ileus (POI), a constipation-like condition after abdominal surgery. Unfortunately, the drug failed to show an effect, so now Entereg from Adolor (NASDAQ:ADLR) and GlaxoSmithKline (NYSE:GSK) has that market all to itself.

With a market cap under $300 million, Progenics could be cheap right now, and it's not likely to run out of money soon, since it has $104 million in the bank. But future sales of Relistor still seem too much like a foggy night , so I'll stay away until there's a little more clarity on how well the drug is going to do in this new market.

Let us know what you think in Motley Fool CAPS. Make an out- or underperform call on these companies; post a pitch about whether you think Relistor is bound for greatness. It's free. It's fun. And, it's Foolish. 

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