GlaxoSmithKline's (NYSE: GSK) fish oil pill, Lovaza, really stank up its latest clinical trial.

Results presented at the American Heart Association showed the prescription-strength drug didn't do anything to help an irregular heartbeat condition called atrial fibrillation. Earlier trials hinted that the omega-3 fatty acids from fish oil might help atrial fibrillation, but this larger 663-patient trial has debunked that theory.

This is a big loss for Lovaza; there are some 2.2 million Americans with atrial fibrillation. Glaxo sold $590 million worth of Lovaza through the first nine months of 2010, up 19% year over year, but grabbing just a piece of the atrial fibrillation market could have skyrocketed sales.

It's a little unclear how much this trial could hurt sales of Lovaza. The drug is approved for decreasing fat molecules called triglycerides. Doctors will likely continue to prescribe it for that reason; I can't see why the new data would give a doctor a reason to switch to a competitor such as Abbott Labs' (NYSE: ABT) TriCor.

But if doctors were prescribing Lovaza off-label to treat or even prevent atrial fibrillation, they're likely to cut those prescriptions based on the lack of evidence that Lovaza helps the condition. My guess is that's a minimal amount of the total prescriptions, but it's really hard to tell.

The big winners in Lovaza's loss are Boehringer Ingelheim, which recently got its atrial fibrillation drug Pradaxa approved, as well as Bayer, Johnson & Johnson (NYSE: JNJ), Pfizer (NYSE: PFE), Bristol-Myers Squibb (NYSE: BMY), and Merck (NYSE: MRK), which all have atrial fibrillation drugs in the works. There's a blockbuster battle royal in the works, but it looks like Lovaza wasn't invited.

And that just stinks for Glaxo's investors.

Brian Richards and Tim Hanson smell something fishy about a money manager recommending that you not manage your own money.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool's disclosure policy isn't sure what that green thing is in the back of the lunchroom fridge.