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Blood thinners or anticoagulants describe any one of a number of drugs designed to prevent the blood in the body from clotting. They are appropriate in treating any disorder which may be the result of obstruction of the blood vessels and are routinely used to treat heart attacks and stroke.

However, administering a blood thinner to a patient who is suffering from a bleeding disorder is often fatal, so they must not be administered on patients who may be suffering from internal or external bleeding, or patients who are ready to undergo or have recently undergone surgery.

Heparin and Warfarin are the two most commonly used blood thinners.  However, other medications have side effects that include blood thinning, such as The pill and aspirin.

Anticoagulant at Wikipedia