Last weekend, Pfizer (NYSE: PFE) and Bristol-Myers Squibb (NYSE: BMY) were at the same conference boosting their stocks after presenting data on their separate cancer drugs. Today there was more boosting of their stocks, but it's because of the same drug: an anti-clotting drug called apixaban.

The duo said yesterday that they were stopping a trial testing apixaban in patients with atrial fibrillation, a type of irregular heartbeat, because it was clear that the drug worked better than aspirin at reducing strokes and blood clots. They're saving the full data for a medical presentation, but you can bet that the results will be substantially positive because the trial was stopped early.

Passing the trial is certainly a positive for Pfizer and Bristol, but the true test for apixaban is still to come. Yesterday's results were for a trial testing patients who couldn't take warfarin, a common drug used to treat prevent blood clots, but the largest market will be in patients who can take warfarin. The two companies are running a trial testing apixaban against warfarin, which should wrap up next year -- assuming it doesn't get stopped early as well.

Unlike other drugs that have tried to go after large indications but failed -- Eli Lilly's (NYSE: LLY) Effient trying to unseat Bristol-Myers and sanofi-aventis' (NYSE: SNY) Plavix, for instance -- drugmakers should have no problem unseating warfarin. The drug is ripe for getting picked off from its first-line status because it's so hard for doctors to find the right dose for patients. The window between effective and toxic is fairly small, and the food a patient eats can change warfarin's effectiveness.

Pfizer and Bristol-Myers aren't the only one to notice the need for a new drug. Bayer and Johnson & Johnson (NYSE: JNJ) are developing Xarelto to prevent blood clots, and Boehringer Ingelheim also has an anti-clotting drug in development. Investors won't know how much of the warfarin market each drug will take until the data is out.

Until then, coagulate in the comments and discuss the possibilities.

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