Warfarin (brand name Coumadin) is one of the original anticoagulant medications, also referred to as blood thinners. They are used to prevent strokes and other serious complications. The drugs require regular blood testing and frequent dosage adjustments. They are also linked to increased bleeding risks, which can sometimes be fatal.

warfarin tablets
Warfarin Facts
  1. Used to Treat Patients at risk of blood clots and stroke, including those diagnosed with atrial fibrillation (AF or AFib)
  2. Related Drugs Jantoven, heparin, Pradaxa, Xarelto
  3. Manufacturer Bristol-Myers-Squibb
  4. Black Box Warnings Bleeding risk
  5. FDA Approval 1954

Warfarin (Coumadin) is an anticoagulant (blood thinner) that has been on the U.S. market for over six decades since its initial approval by the U.S. Food and Drug Administration (FDA) in 1954.

Warfarin prevents blood clots. The drug is also used to prevent strokes in patients with atrial fibrillation (AF or AFib) (a condition resulting in an irregular heart rate).

An estimated 2.6 million individuals suffer from AF in the U.S. That’s predicted to jump to 12 million by 2050.

This drug has a black box warning for bleeding. Users must regularly test their International Normalized Ratio (INR), a blood-clotting measurement.

The tests help determine dosage. Any changes to diet, the starting or stopping of other drugs, or certain lifestyle choices, such as alcohol intake or cigarette smoking, can all alter a patient’s INR.

What Does Warfarin Treat?

Warfarin fights blood clots. It prevents clots from forming in blood and blood vessels. The drug treats certain types of irregular heartbeats. It is also used in patients who have suffered a heart attack or have prosthetic heart valves.

Coumadin is used to prevent and treat venous thrombosis (a blood clot in a vein), as well as a related condition, pulmonary embolism. A pulmonary embolism happens when part of a blood clot breaks away and becomes lodged in an artery supplying blood to the lungs.

Warfarin is also used to prevent and treat strokes associated with atrial fibrillation (AF or AFib). AF is the most common heart-rhythm abnormality, according to the FDA.

The condition is characterized by an irregular and often rapid heart rate that can increase a patient’s risk of suffering from stroke among other serious and sometimes deadly conditions, such as heart failure and other heart-related complications.

Coumadin may also be prescribed to reduce the risk of death from heart attacks and thromboembolic events, such as stroke or system embolization (a blocked artery often caused by a blood clot) after a heart attack.

Warfarin’s Black Box Warning: Bleeding Risk

Coumadin contains a black box warning for bleeding risks. Since the drug is designed to prevent blood from clotting, it can also take longer than usual for patients to stop bleeding if they are cut or injured.

Bleeding linked to Coumadin-use can also result in increased menstrual flow or vaginal bleeding in women, nosebleeds and bleeding of the gums from teeth brushing.

The drug label advises patients to report any unusual bleeding, bruising or other signs and symptoms of internal bleeding, such as headache, dizziness or weakness to their health care provider.

Additionally, health care providers are urged by the warning label to perform regular monitoring of INR and PT levels for patients treated with Coumadin.

Warfarin Side Effects?

Aside from bleeding, Coumadin-use is associated with several other side effects. These side effects can range from mild adverse reactions, such as nausea or abdominal pain, to serious complications, such as calciphylaxis (calcium build-up in small blood vessels of the fat and skin tissues) and tissue necrosis (death of body tissue).

Serious side effects of Warfarin
  • Calciphylaxis (a serious, potentially deadly, condition characterized by calcification of blood vessels) – this condition can lead to blood clots and skin necrosis
  • Tissue necrosis or gangrene (death of skin or body tissues)
  • Hemorrhage (severe bleeding)
  • Acute kidney injury
  • Cholesterol embolism (blockage of an artery, typically by a clot) – this condition occurs when cholesterol is released, usually from plaque build-up in the arteries, and travels through the bloodstream becoming lodged in blood vessels in various parts of the body
  • Restriction of blood supply to limbs tissue death and gangrene in patients with heparin-induced thrombocytopenia (low blood platelet count) (HIT) and HITTS (heparin-induced thrombotic thrombocytopenia syndrome)
The most common side effects reported in patients taking Coumadin include:
  • Hypersensitivity or allergic reaction – can result in hives as well as anaphylaxis. When anaphylaxis occurs, the condition can become deadly and requires immediate emergency medical attention
  • Inflammation of the skin (dermatitis), including bullous eruptions (fluid-filled blisters)
  • Elevated liver enzymes
  • Nausea and/or vomiting
  • Diarrhea
  • Changes in the sense of taste
  • Abdominal pain
  • Hepatitis (inflammation of the liver) – cholestatic hepatitis has been found to occur in patients taking both Coumadin and ticlopidine (a drug that also helps to prevent blood clots), simultaneously
  • Vasculitis (an extreme reaction to a drug) – leads to inflammation and damage to blood vessels, primarily in the skin
  • Gas and/or bloating
  • Rash
  • Itchy skin
  • Hair loss
  • Feeling cold or chills

How Does Warfarin Work?

The drug works by slowing down the blood-clotting process. Clots form through chemical reactions involving various substances, including vitamin K. The drug weakens Vitamin K. That prolongs the time it takes to form clots.

The drug does not dissolve or break down blood clots. It can, however, assist in the maintenance of an existing clot to allow the body time to try to break it down on its own.

Blood Tests, Vitamin K

Vitamin K rich foods like kale, above, can impact INR/PT measurements as well as the effectiveness of warfarin

International Normalized Ratio (INR) and Prothrombin Time (PT) are laboratory test values obtained from measurements of the time it takes blood to clot. When patients are prescribed warfarin (Coumadin), they are required to undergo regular testing, usually once a month, to make sure INR and PT measurements stay within the desired testing range.

Vitamin K intake can affect the INR/PT measurement as well as the effectiveness of the drug. The National Institutes of Health (NIH) recommends that patients keep vitamin K intake “as consistent as possible.” Sudden increases of vitamin K may decrease Coumadin’s effect while lowering vitamin K intake can increase the effect of the medication.

One way to keep vitamin K intake consistent is to not make too many dietary changes, or modifications in food choices, and to carefully consider any dietary supplements or herbal products. Since dosing is dependent on each patient’s INR measurements, patients should not entirely stop eating vitamin K-rich foods that they normally eat, nor should they suddenly add foods or supplements rich in vitamin K to their normal daily diets.

Foods rich in vitamin K include:
  • Kale
  • Spinach
  • Collards
  • Swiss chard
  • Mustard greens
  • Turnip greens
  • Parsley
  • Broccoli
  • Brussels sprouts
  • Cabbage
  • Green leaf lettuce
  • Romaine lettuce
  • Prunes
  • Asparagus
  • Avocado
  • Endive
  • Tuna
  • Blueberries
  • Blackberries
  • Peas

Administration and Dosages

Warfarin (Coumadin) comes in a tablet form to be taken by mouth. The tablets are available in several different strengths. Health providers will likely start patients on a lower dose and gradually increase or decrease the dose based on results of blood tests. Drug labeling for warfarin advises patients not to stop taking the medication without first talking to their doctor.

Tablet Strengths and Descriptions

Warfarin Tablet 1 milligram Pink
1 milligram Pink
Warfarin Tablet 2 milligrams Lavender
2 milligrams Lavender
Warfarin Tablet 2.5 milligrams Green
2.5 milligrams Green
Warfarin Tablet 3 milligrams Tan
3 milligrams Tan
Warfarin Tablet 4 milligrams Blue
4 milligrams Blue
Warfarin Tablet 5 milligrams Peach
5 milligrams Peach
Warfarin Tablet 6 milligrams Teal
6 milligrams Teal
Warfarin Tablet 7.5 milligrams Yellow
7.5 milligrams Yellow
Warfarin Tablet 10 milligrams White
10 milligrams White

Warfarin’s Dosage and Duration

The dosage and administration of Coumadin is individualized for each patient according to their blood tests and conditions.

Lower initial doses are recommended for patients who are elderly, weakened or of Asian descent. Duration of treatment will also vary from one patient to the next with therapy generally being continued until the danger of blood clotting or embolism has passed.

Certain dental or surgical procedures may require a temporary interruption or change in warfarin (Coumadin) treatment due to bleeding risks.

Overdosage of Warfarin (Coumadin)

The primary concern with overdosage in patients taking Coumadin is the risk for bleeding. Taking too much of the anticoagulant can result in several signs and symptoms that a dose adjustment is needed. Many of these symptoms can signal life-threatening bleeding and require immediate medical intervention.

Signs and symptoms of Coumadin overdose may include:
  • Excessive bruising or persistent oozing from superficial injuries (any injury that does not affect underlying muscles or organs, typically only affecting the skin)
  • Blood in the stool or urine
  • Excessive menstrual bleeding in women
  • Unexplained fall in hemoglobin (the molecule in red blood cells that carries oxygen)
  • Dark, sticky feces containing partly digested blood
  • Pinpoint, round spots that appear on the skin

Treatment of overdose is based on the patient’s INR levels, the presence or absence of bleeding and other clinical circumstances. Reversal of the medication’s effects may be achieved by discontinuing the drug and administering vitamin K.

Warfarin Drug Interactions

Certain drugs, and even herbal or dietary supplements, can interact with Coumadin. Additionally, more frequent blood monitoring is needed when a patient starts or stops other drugs, including supplements, or when dosages of other drugs are changed.

Enzyme Inhibitors and Inducers

Certain drugs, such as those belonging to the class of inhibitors of CYP2C9, 1A2 and/or 3A4 (certain enzymes found in the body), also have the ability to increase the effectiveness of Coumadin. These medications include certain antifungals, decongestants, Xanax and caffeine. Other substances may have the opposite effect on Coumadin. These include certain anti-seizure drugs, antibiotics, sleep-disorder medicines and cigarettes.

Drugs that increase the risk of bleeding

These medications can include certain painkillers, such as aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen, as well as serotonin reuptake inhibitors (SRIs) used to treat depression and anxiety disorders.

Herbal and dietary supplements

Evidence suggests that vitamin E has blood-thinning effects. Aside from vitamin E, several other dietary supplements and herbal medications can alter a patient’s INR/PT levels.

Dietary supplements and herbal medications that can alter a patient’s INR/PT levels include:
  • Arnica
  • Bilberry
  • Butchers broom
  • Cat’s claw
  • Dong quai
  • Feverfew
  • Forskolin
  • Garlic
  • Ginger
  • Ginkgo
  • Insositol hexaphosphate
  • Licorice
  • Melilot (sweet clover)
  • Pau d’arco
  • Red clover
  • St. John’s Wort
  • Sweet woodruff
  • Turmeric
  • Willow bark
  • Wheat grass
  • Horse chestnut
  • Any vitamin/mineral supplements that list vitamin K on the label

Antibiotics and Antifungals

Some antibiotics can either lower vitamin K levels in the body or interfere with the effectiveness of warfarin (Coumadin). There have been reports of changes in INR in patients taking warfarin and certain antibiotics or antifungals, according to drug labeling for Coumadin.

Alcoholic Beverages

Alcohol can affect dosing of warfarin (Coumadin). It is best to avoid alcohol while taking Coumadin.

Please seek the advice of a medical professional before making health care decisions.

Related Pages
Kristin Compton
Written By Kristin Compton Writer

Kristin Compton's background is in legal studies. She worked as a paralegal before joining Drugwatch as a writer and researcher. She was also a member of the National Association of Legal Assistants. A mother and longtime patient, she has firsthand experience of the harmful effects prescription drugs can have on women and their children. Some of her qualifications include:

  • Bachelor of Arts in Legal Studies | Pre-Law from University of West Florida
  • Past employment with The Health Law Firm and Kerrigan, Estess, Rankin, McLeod & Thompson LLC
  • Personal experience battling severe food allergies, asthma and high-risk pregnancies
Edited By
Kevin Connolly
Kevin Connolly Managing Editor

11 Cited Research Articles

  1. DailyMed, NIH. (2017, August 22). Label: Coumadin – warfarin sodium tablet. Retrieved from: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=D91934A0-902E-C26C-23CA-D5ACCC4151B6#s17
  2. FDA. (2011). Full Prescribing Information: Coumadin. Retrieved from: https://www.accessdata.fda.gov/drugsatfda_docs/label/2011/009218s107lbl.pdf
  3. FDA. (2012). A Guide to Drug Safety Terms at FDA. Retrieved from: https://www.fda.gov/media/74382/download
  4. FDA. (2012, November 2). FDA Drug Safety Communication: Update on the risk for serious bleeding events with the anticoagulant Pradaxa (dabigatran). Retrieved from https://wayback.archive-it.org/7993/20170406044001/https://www.fda.gov/Drugs/DrugSafety/ucm326580.htm
  5. Fiumara, K. and Goldhaber, S. (2009, March 2). A Patient’s Guide to Taking Coumadin/Warfarin. Retrieved from: https://www.ahajournals.org/doi/full/10.1161/CIRCULATIONAHA.108.803957
  6. Kimmel, S. (2008). Warfarin therapy: in need of improvement after all these years. Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2855533/
  7. Kuruvilla, M. and Gurk-Turner, C. (2001). A review of warfarin dosing and monitoring. Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1305837/
  8. MedlinePlus, NIH. (2017, June 15). Warfarin. Retrieved from: https://medlineplus.gov/druginfo/meds/a682277.html
  9. Monson, K. (2017, January 1). Clinical Effects and Treatment Guidelines for Warfarin: How Does Warfarin Work? Retrieved from: http://heart.emedtv.com/warfarin/warfarin-p3.html
  10. National Institutes of Health. (2012, September 5). Important Information to know when you are taking: Warfarin (Coumadin) and Vitamin K. Retrieved from: https://cc.nih.gov/ccc/patient_education/drug_nutrient/coumadin1.pdf
  11. U.S. Department of Health and Human Services. (2014). National Action Plan for Adverse Drug Event Prevention: Section 5. Anticoagulants. Retrieved from: https://health.gov/hcq/pdfs/ade-action-plan.pdf
View All Sources
Who Am I Calling?

Calling this number connects you with a Drugwatch representative. We will direct you to one of our trusted legal partners for a free case review.

Drugwatch's sponsors support the organization's mission to keep people safe from dangerous drugs and medical devices. For more information, visit our sponsors page.

(888) 645-1617