Get ready, Fools; we're headed toward a battle royal of epic proportions. Boehringer Ingelheim's Pradaxa was recently approved to treat patients with an erratic heartbeat, and Bayer and Johnson & Johnson (NYSE: JNJ) have a second compound, Xarelto, that isn't far behind.

Data presented at the American Heart Association showed Xarelto lowered the incidence of stroke or a blood clot to just 1.7%, compared with 2.2% of patients taking the current standard of care, warfarin. When you add in the patients that dropped out of the study, the results weren't as impressive: 2.1% for those on Xarelto versus 2.4% for those on warfarin.

But even if Xarelto is only viewed as being as good as warfarin, that should be enough to capture much of the market. Warfarin is a notoriously difficult drug to find the optimal dose for, and it interacts with other foods and medications that patients take.

Is Xarelto better than Pradaxa? It's a little hard to know. Since the inclusion criteria for participating in the trial are a little different, comparing the results between trials is difficult. Until someone does a head-to-head trial comparing the two, we won't know for sure.

But it may not matter much. There's plenty of room for multiple players. This is a multibillion-dollar market after all. Take a look at the anti-inflammatory drugs as a model of how the heart drugs might be able to play well together: There are three drugs that all register sales north of $4 billion in sales annually.

Boehringer Ingelheim, Bayer, and Johnson & Johnson had better hope so. There are multiple companies looking to enter the market behind the two leaders. Pfizer (NYSE: PFE) and Bristol-Myers Squibb (NYSE: BMY) are next with data due out next year for their competing drug, apixaban. Merck (NYSE: MRK) and Japan's Daiichi-Sankyo each have a drug that's farther back.

With very few untapped mega markets left and plenty of blockbusters going off patent, this is one area investors should keep their eye on.

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