Anticoagulants are nonhabit-forming medications that keep existing blood clots from growing larger and prevent the formation of new blood clots.

Blood clots are an essential part of the body’s defense against injury. Without blood clots, a person would bleed to death from a relatively minor wound. However, when blood clots form in inappropriate locations, such as inside arteries, the result can be catastrophic. The danger is that a blood clot will prevent the flow of oxygen-rich blood through the vessel, thus resulting in the death of tissue beyond the clot. If this occurs in the heart or brain, a heart attack or stroke may result. If it occurs in peripheral arteries, it may result in the death of muscle and tissue beyond the clot. In internal organs, such as the kidneys, serious organ damage can result. Blood clots can also occur in the veins. In some situations, there is a risk that blood clots that occur in large veins can break off and travel through the circulatory system until they reach the heart or lungs, resulting in a heart attack or pulmonary embolism. To prevent these medical problems, anticoagulants may be prescribed for heart patients or other high-risk patients to prevent new blood clots from forming and to keep existing blood clots from growing larger.

The process of blood clotting is complex, but can be roughly understood to include three phases: coagulation, platelet aggregation and formation of the fibrin clot. Anticoagulants deal only with the first phase of blood clotting, or the cascade of chemical reactions that begins the blood clotting process. Other medications, called antiplatelets or clot-busting drugs, interfere with the platelet aggregation and clot-formation phase, respectively. Each of the three classes of drugs has a different clinical use.