Warfarin is an anticoagulant medication that is used to slow down the blood-clotting process. Blood clots may cause vein blockages, heart attack, and stroke. Anticoagulants can prevent blood clots.

Brand names for warfarin are Warfant, Jantoven, Coumadin, Lawarin, Marevan, and Waran.

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Warfarin prevents blood clots from forming.

Warfarin is useful for people who are at risk of forming blood clots. This includes people with deep vein thrombosis (DVT), where clots form in the legs, and pulmonary embolism, where there is a clot in the lung.

If a clot forms and breaks loose, it can travel through the bloodstream and block a vessel in another part of the body, such as the lungs, the leg, the kidneys, or the gut. In the brain, a clot can cause a stroke. The consequences of a clot can be fatal.

Warfarin can help to prevent stroke in older patients with atrial fibrillation, or irregular heartbeat. People with an artificial heart valve may also use warfarin.

In the 1920s, farmers in the northern United States and Canada were troubled by a hemorrhagic disease that was killing their cattle. It was traced to moldy hay, and the disease became known as sweet clover disease.

When one of the farmers presented an agricultural scientist named Wilhelm Schoeffel with a can of unclotted blood 10 years later, to see what he could suggest, Schoeffel experimented with it that same day. From there, warfarin was discovered.

Warfarin was first used in 1948 as a pesticide to kill mice and rats. It is still used in pest control today. In high doses, it can cause multiple hemorrhages.

In the 1950s, researchers found that it helped prevent thrombosis and embolism in people with a range of conditions. In 1954, it was approved in the U.S. for clinical use in humans.

President Eisenhower was prescribed warfarin, following a heart attack in 1955. Some historians claim that the Soviet leader, Joseph Stalin, died after being poisoned with warfarin.

Warfarin remains the most commonly prescribed anticoagulant in the U.S., although other drugs, such dabigatran, are becoming more widespread.

The name "warfarin" refers to the organization that funded key research, the Wisconsin Alumni Research Foundation (WARF), and coumarin.

Coumarin is commonly used as a precursor molecule for producing many synthetic anticoagulants, including warfarin.

Vitamin K enables blood to clot. Warfarin slows down vitamin K production in the body. In this way, it slows down the blood clotting process.

This helps the blood to flow more freely around the body, and it reduces the risk of clots forming in the heart or blood vessels.

Warfarin is an effective medication, but it has several disadvantages. Many prescription drugs and foods interact with it.

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A number of common foodstuffs and supplements can interact with warfarin.

Foods with high levels of vitamin K interact with warfarin.

These include liver, broccoli, Brussels sprouts, and green leafy vegetables, such as coriander, cabbage, collards, spinach, and Swiss chard. Anyone using warfarin should check with their doctor before making a radical change in their diet.

Researchers from the Intermountain Medical Center Heart Institute have found that over two thirds of the 100 most popular over-the-counter herbal or dietary supplements interfere with how Warfarin works. Out of 100 people who were using warfarin for atrial fibrillation, 35 were using supplements, and over half were unaware that they could interact.

The supplements compete with the anticoagulant medication in the liver, causing warfarin to become either under- or over-active. The blood can either become too thin, increasing the risk of bleeding, or too thick, leading to a higher chance of stroke.

Health Canada lists (resource no longer available at www.hc-sc.gc.ca) a number of mineral, herbal, and vitamin products that can alter levels of warfarin in the bloodstream or that might directly affect blood clotting on their own.

These are: vitamin K, vitamin A, St. John's Wort, papaya extract with papain, methyl salicylate, Lycium barbarum, horse chestnut, green tea, ginseng, Ginkgo Biloba, fish oil supplements containing EPA and DHA, feverfew, Fenugreek with boldo, Dong Quai (Angelica Sinensis), Devil's Claw, danshen, coenzyme Q10, and chondroitin with glucosamine.

Conventional medications that are known to interact with warfarin include:

  • Acetaminophen, including paracetamol and Tylenol
  • Antibiotics such as tetracycline azithromycin, and erythromycin
  • Antidepressants, for example, paroxetine, fluoxetine, sertraline
  • Antifungal agents, specifically itraconazole
  • Lipid lowering agents, such as fibrates and statins
  • Non-steroidal anti-inflammatory agents, including celecoxib and acetylsalicylic acid
  • Stomach ulcer and acid-reducing agents, for example, cimetidine, ranitidine, omeprazole.

Alcohol also has an antiplatelet effect, and this can interfere with the action of warfarin.

Patients on warfarin have to be monitored carefully. This is done by measuring their international normalized ratio (INR). They need regular blood tests for their INR to make sure the dosage is exactly right.

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People who take Warfarin will need regular blood tests.

If the INR is high, there is a risk of bleeding. A low INR increases the likelihood that a clot, or thrombus, will form.

Initially, the patient's blood will probably be tested daily. Later, the tests will be at least once a month, but as often as twice a week for some people.

There are effective alternative anticoagulant medications to warfarin that do not require regular drug testing, and that are less likely to interact with other drugs. The U.S. Food and Drug Administration (FDA) provides more information about this.

Warfarin remains popular because it is more cost-effective than the other options, but researchers predict that other options will become more widespread.

A new drug, named andexanet alpha (AndexXa) is now in the pipeline. It has performed well in trials, and FDA approval is expected to be given, once additional information is given to support approval for all the factor Xa inhibitors. This could ultimately lead to a decline in prescriptions for warfarin.

People should not use warfarin if they have severe hypertension, or high blood pressure, or a peptic ulcer.

During pregnancy, women should avoid warfarin because it passes through the placental barrier, and it may cause bleeding in the fetus. It also increases the risk of preterm birth, neonatal death, stillbirth, and spontaneous abortion.

Warfarin should be used with caution in the following cases:

  • Hemophilia and other bleeding problems
  • Liver problems
  • Kidney problems.

Older people and those who have recently had surgery need to use warfarin with care.

The two most serious side effects are gangrene and internal bleeding.

Patients who use anticoagulant medications need to look out for signs of excessive bleeding, or hemorrhage, and seek immediate medical advice if these appear.

The signs include:

  • Vomiting or coughing up bright red blood
  • Passing blood in urine or feces
  • Excessive bruising
  • Bleeding gums
  • Heavy or excessive menstrual bleeding
  • Bleeding from the vagina that is not related to menstruation.

The following side effects are also possible, but less common:

Patients who experience jaundice should tell their doctor immediately.

To reduce the risk of bleeding, the American Heart Association (AHA) suggests using a soft bristle tooth brush, an electric razor instead of a blade, and avoiding contact sports.

In a study of over 1.2 million patients who were admitted to trauma centers between 2002 and 2007, those who used warfarin faced a higher risk of traumatic death after an injury.

Following a traumatic event, 9.3 percent of warfarin users died from their injuries after a traumatic event, compared with 4.8 percent of people of the same age who were not using Warfarin.

Warfarin use was found to increase the mortality from severe head injuries by 50 percent in patients younger than 65 years.

Patients using warfarin were also more likely to have blunt mechanism injuries and to sustain their injuries at home or in a residential institution. They also experienced more severe injuries overall.

The researchers caution that warfarin should not be used for longer than it is needed.

The AHA warn against purchasing drugs, including warfarin or Coumadin, online, because the ingredients of drugs with similar names in other countries may be slightly different from those in the U.S.