Blood thinners are used to lengthen the time it takes for blood to form clots, or to prevent clots from forming. They are most used in those who are at risk for heart attack or stroke. A blood thinner can be composed of several different chemical formations, and there are two types: antiplatelet drugs and anticoagulants. The most common blood thinner is aspirin, which is often taken in doses of 81 mg per day.

Aspirin, an antiplatelet drug, has been recognized as a blood thinner for at least 50 years. Often, a daily dose of aspirin is the only required cardiac medicine. This is particularly the case with those children born with congenital heart defects who have undergone surgery. Aspirin helps prevent platelets from clumping together and forming clots.

In children, the use of aspirin as a blood thinner must be carefully monitored, and stopped if the child has stomach flu, influenza, or chicken pox. Aspirin use has been indicated in the development of Reye's syndrome, an illness that can prove fatal, causing damage to the major organs of the body. More recent studies indicate that the risk of Reye's syndrome increases with the amount of aspirin taken. Parents with children on aspirin are usually given a list of when to avoid giving their children aspirin as a blood thinner. Since the dose of aspirin given daily is so low, however, contracting Reye's is a very minimal risk.

In some cases, aspirin is not a strong enough blood thinner to address the risk of potentially life-threatening clots forming. The second most common blood thinner is warfarin or Coumadin®. Coumadin®, an anticoagulant, is a much stronger blood thinner, and use of this medication must be carefully monitored. Dosing is difficult because warfarin is highly reactive; properly titrating the dosage can take several months and require weekly blood tests before an ideal dose is achieved.

Patients taking warfarin and other anticoagulants are advised to monitor their intake of foods high in vitamin K; this vitamin can reduce the effectiveness of the drug. Those on warfarin are also cautioned about the risks regarding severe bleeding after bruising or cutting the skin. As a blood thinner for children, risk involves cranial bleeding if injury to the head occurs. Children on this blood thinner are often cautioned to wear helmets when engaging in any kind of rough play.

A more recently developed blood thinner is clopidogrel, an antiplatelet drug sold in the US under the name Plavix®. Unlike warfarin, clopidogrel is a more stable drug, with fewer side effects; serious side effects are still possible, however, and should be reported to a doctor immediately. In 2010, the US Food and Drug Administration added a black box warning to clopidogrel because some patients do not metabolize the drug effectively, leaving them still at risk for clots.

Another type of blood thinner, heparin, is injected directly into the blood stream. It is an anticoagulant, often used during certain types of surgery, blood transfusions, and dialysis, and may be used to help keep intravenous (IV) lines from clotting. Heparin may also slow the growth of blood clots, although — like all blood thinners — it does not dissolve clots after they have formed.

If a patient is taking a blood thinner regularly, his or her physician should be notified. Many blood thinners have interactions with other drugs. Additionally, the patient should receive directions regarding stopping a blood thinner before any type of surgical procedure. Usually physicians and dentists recommend stopping the use of a blood thinner about a week before surgery. Stopping use is only indicated if this does not increase risk of heart attack or stroke to the patient, however, and should only be done under the direction of a physician.