Anticoagulants are nonhabit-forming medications that are used to treat and prevent vascular diseases caused by blood clots. Under normal conditions, blood should not clot unless the body has been injured. However, there are conditions, such as atherosclerosis and venous diseases, that cause blood clots to form even when a person has not been wounded. These blood clots can be dangerous if they are large enough to block a blood vessel (a thrombus), or if either a whole clot or pieces of the clot break off, travel through the bloodstream, and block a blood vessel in another part of the body (an embolism). Either of these situations could lead to heart attack or stroke. Therefore, anticoagulants may be prescribed to reduce the risk of these dangerous events.

Anticoagulants prescribed for regular use (e.g., warfarin) are generally vitamin K antagonists taken in pill form. Vitamin K is an essential cofactor in the blood clotting process. They typically take effect within 48 hours, but may take a week or longer to be stabilized. Other anticoagulants (e.g., herparin, fibrin and thrombin inhibitors) are administered through intravenous (I.V.) injection in a medical setting, where they quickly take effect. Patients will sometimes be given anticoagulants in both forms when first beginning anticoagulant treatment and will then continue to take only the vitamin K inhibitors by mouth.