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Warfarin is one of the most commonly used medications in the United States for anticoagulant therapy. It has been sold under brand names such as Coumadin® and Jantoven®. Anticoagulant medications are often referred to as “blood thinners,” but they do not actually thin the blood. Instead, they interfere with the body’s ability to form blood clots.

Certain proteins must circulate in the blood in order for clotting to occur. These proteins are formed in the liver, and vitamin K is required for their production. Warfarin acts as a vitamin K antagonist, meaning it interferes with vitamin K–dependent clotting factor synthesis.

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Warfarin was developed in the 1950s from spoiled sweet clover

Warfarin was developed in the 1950s from spoiled sweet clover. Scientists observed that cattle consuming large amounts of this spoiled clover developed a tendency to bleed excessively, which led to the identification of anticoagulant compounds and eventually the development of warfarin as a medication.

Warfarin is used to treat or prevent blood clots in veins or arteries, thereby reducing the risk of stroke, heart attack, or other serious conditions. It should be taken at the same time each day to maintain consistent blood levels.

Precautions and Warnings

Before using warfarin, patients should inform their doctor if they have any of the following conditions:

  • Bleeding disorders or blood cell disorders

  • Blood in the urine or stools

  • Stomach or intestinal bleeding

  • Very high blood pressure

  • Infection of the heart lining (bacterial endocarditis)

  • Recent or upcoming surgery

  • Need for a spinal tap or epidural

Patients with celiac sprue, diabetes, congestive heart failure, overactive thyroid, connective tissue disorders (including autoimmune diseases such as Marfan syndrome, Sjögren’s syndrome, scleroderma, rheumatoid arthritis, or lupus), or a history of low platelet counts after receiving heparin should also notify their physician.

Warfarin increases the risk of bleeding, which can be severe or life-threatening. Patients taking warfarin require frequent blood tests to monitor clotting time. A physician should be contacted immediately if bleeding does not stop, if there is blood in the urine, black or bloody stools, or if coughing up blood or vomiting material that resembles coffee grounds.

Common Side Effects

Warfarin may cause unusual bleeding or bleeding that will not stop. Internal bleeding may occur, particularly in the stomach or intestines.

Other possible side effects include:

  • Pain or swelling; hot or cold sensations

  • Abdominal pain or cramping

  • Unusual bleeding, including bleeding gums, nosebleeds, blood in urine, bloody stools, coughing up blood, or increased menstrual bleeding

  • Blurred vision

  • Tingling, burning, or itching sensations

  • Chest discomfort or pain

  • Confusion

  • Difficulty breathing or swallowing

  • Dizziness or lightheadedness, especially when standing up suddenly

  • Easy or excessive bruising

  • Headaches

  • Red, black, or tarry stools

Rare Side Effects

Less common but serious side effects may include:

  • Pain in the arm, back, or jaw

  • Tightness in the chest

  • Skin discoloration (black, blue-green, blue, or purple), including purple toes or small red or purple skin spots

  • Clay-colored stools

  • Diarrhea

  • Rapid or irregular breathing or heart rate

  • Fever

  • Bloating or excessive gas

  • Bad taste in the mouth

  • Hair loss or thinning

  • Hives or welts