Anticoagulant drugs help prevent harmful clots from forming in blood vessels by decreasing the blood's ability to clot. Although these drugs are sometimes called blood thinners, they do not actually thin the blood. Furthermore, this type of medicine will not dissolve clots that already have formed, although the drug stops an existing clot from worsening.

Anticoagulant drugs are used in several situations. For example, they may be given to prevent blood clots from forming after the replacement of a heart valve or to reduce the risk of a stroke or another heart attack after a first heart attack. They are also used to reduce the chance of blood clots forming during open heart surgery or bypass surgery. Low doses of these drugs may be given to prevent blood clots in patients who must stay in bed for a long time after certain...