Anticoagulants are medicines that reduce blood clotting in an artery, vein or the heart. Clots can block the blood flow to your heart muscles and cause a heart attack. They can also block blood flow to your brain, causing a stroke. They can keep harmful clots from forming in your heart, veins or arteries. Blood clots are made up of red blood cells, platelets, fibrin, and white blood cells. Anticoagulants prevent these components from sticking together and forming a clot.