Rates of serious bleeding among patients taking warfarin are much higher than those found in clinical trials of the drug and are even greater during the first month of use, according to a five-year study of 125,195 seniors with atrial fibrillation.

The findings complicate an already difficult decision-making process for patients with atrial fibrillation and highlight the importance of careful management of warfarin, especially during the first month of use, experts said. Warfarin is a generic anticoagulant that is marketed under brand names such as Coumadin and Jantoven.

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