A stroke occurs when blood flow is cut off to an area of brain, depriving its cells of oxygen.  This can leads to serious health problems. One of the most serious risks associated with atrial fibrillation (AFib) is stroke—around 15 percent of all strokes occur in people with AFib.  Anticoagulants—medications that make blood less likely to clot—can reduce the risk of stroke in AFib patients by as much as 80 percent.