A common form of irregular heartbeat called atrial fibrillation can cause blood clots, putting people at increased risk of stroke. The anticoagulant drug warfarin is used to reduce that risk, but since people respond to it very differently, it requires careful monitoring to avoid the risk of heavy bleeding. Now, researchers say a new drug called rivaroxaban looks to be as good as warfarin in preventing strokes. About 2.3 million people in the United States have atrial fibrillation, so drug manufacturers have been eager to come up with oral anti-clotting drugs to replace warfarin -- drugs that don't require blood tests and frequent fiddling with doses. Rivaroxaban, one of those new drugs, is just as good as warfarin at reducing the risk of stroke, according to a new study published online in the New England Journal of Medicine. That study followed 14,264 patients who took either rivaroxaban (brand name Xarelto) or warfarin (sold as Coumadin and other brand names). None of the study participants knew which drug they were given.