Anticoagulants — drugs that reduce the blood’s ability to clot — are used to treat clots in the lungs and legs and to prevent strokes in people with the heart rhythm abnormality called atrial fibrillation. The anticoagulant warfarin has been used for these purposes for many years. But it is difficult and time-consuming to find the optimal warfarin dose, and it carries a risk of difficult-to-control bleeding. Newer anticoagulants offer an easier and equally effective way to control the blood’s clotting ability, but until recently, there was no way to reverse the effects of these drugs if necessary. The approval of new antidotes to these newer anticoagulants will enable doctors to prescribe these drugs with increased confidence.