Warfarin: Purpose, Side Effects, and Management

Pros and Cons of This Blood Clot Treatment

Warfarin is a prescription anticoagulant (blood-thinning) medication used to treat and prevent blood clots. Though it can be beneficial for some people, it does carry some risks.

Drugs like warfarin are sometimes called “blood thinners.” These drugs don’t really “thin” your blood, but they do make it less likely to clot. Coumadin and Jantoven are common brand names of the drug warfarin.

This article explains how warfarin works. It also covers its uses, dosage, side effects, and alternatives.

Benefits of warfarin
Illustration by Brianna Gilmartin, Verywell

What Is a Blood Clot?

Blood normally flows through your blood vessels in a liquid form. On the other hand, a blood clot is blood in a semi-solid state. A blood clot occurs when specific blood components and proteins join together. The process of forming a blood clot is called "coagulation."

Sometimes blood clots are helpful and necessary. For example, they are needed to reduce the blood flow from a wound.

However, blood clots can also be very dangerous. They may block a blood vessel and reduce the flow of oxygenated blood to an organ. These kinds of clots can lead to life-threatening health complications, including:

  • A heart attack, caused by a clot in one of the main arteries of the heart
  • A stroke, caused by a blood clot in a vessel in the brain
  • Thrombosis, caused by a clot in a large vein (often the lower legs)
  • Embolus, a thrombosis that travels to the heart and lungs

Blood clotting involves specific proteins and blood components. Proteins called "clotting factors" activate through a series of reactions that ultimately help form a blood clot. These coordinated reactions help ensure that the blood forms clots only when necessary.


A blood clot is blood in a semi-solid state. Blood clotting is essential for reducing bleeding from an injury. However, blood clots can also lead to life-threatening complications, including heart disease and stroke.

Risk Factors for Blood Clots

People with certain medical conditions are at higher risk of clots, and they may need warfarin therapy. These conditions include:

  • Atrial fibrillation (irregular heartbeat, causing blood to pool in the upper chambers)
  • Organ transplants or implanted mechanical devices
  • Family or personal history of blood clots
  • Recent heart attack
  • Recent stroke
  • Recent surgery
  • Smoking
  • Excess weight and obesity
  • Pregnancy
  • Hormonal birth control pills
  • Hormone replacement therapy
  • Cancer
  • Diabetes
  • Heart disease
  • Long car rides, plane rides, or bedrest

How Warfarin Works

Warfarin belongs to a group of medications called “vitamin K antagonists.” These drugs block the enzyme that enables vitamin K to activate certain clotting factors.

What Is Vitamin K?

Vitamin K is a closely related group of compounds found in some foods. The "K" comes from the German word for coagulation (koagulation).

Certain clotting factors can only be activated when vitamin K is present. A specific enzyme enables vitamin K to activate these clotting factors.

Vitamin K antagonists, including warfarin, ensure that fewer clotting factors will become activated. Overall, this makes it harder for the blood to form a clot, making it less likely that a dangerous clot will form.

However, this also means that a person is more likely to experience a dangerous bleeding episode.


Healthcare providers prescribe warfarin for a variety of medical conditions. For example, warfarin is commonly used to treat and prevent different kinds of blood clots, such as:

  • Venous thrombosis (a blood clot in the veins of the leg)
  • Pulmonary embolism (a blood clot that lodges in the lungs)
  • Thromboembolic stroke (stroke from a blood clot that originated elsewhere in the body)

Unlike some other drugs, warfarin is not good at dissolving clots. However, it can help prevent clots from getting bigger.


Warfarin is an oral medication that you usually take daily, ideally at the same time each day. You need to take exactly the amount your healthcare provider prescribes.

Different dosages often come in different-colored tablets, making taking precisely the right amount easier. Store warfarin at room temperature.

Missed Dose

Talk to your healthcare provider about what to do if you forget a dose. In most cases, you should take the dose as soon as possible on the same day. However, do not double a dose the next day if you miss a dose the day before. When in doubt, contact your healthcare provider.

If you take more warfarin than prescribed, contact your healthcare provider or a poison-control line right away.

Short- vs. Long-Term Use

Sometimes people taking warfarin only need to take it temporarily (for example, after surgery). Other people need to take it long-term.


There may be situations in which you need to stop taking warfarin temporarily. For example, you might need to stop taking it before a planned surgery or medical procedure. Discontinuing warfarin may help lower your risk of complications from surgery in some cases.

If you have a medical procedure or surgery planned, be sure that your healthcare provider knows that you are taking the drug. They can let you know if you should stop taking it temporarily.


Warfarin is a pill that you take every day. Some people take it temporarily, such as after surgery, while others take it long-term. If you are having a planned medical procedure, let your healthcare provider know so they can advise you on how or if to stop taking it temporarily.


Usually, people take between 1 mg and 10 mg of warfarin daily. However, the dosage is based on a specific individual's needs; in other words, some people will need more than other people.

Dosage varies based on many factors, including:

  • Diet
  • Age
  • Health conditions

If your warfarin dose isn't high enough for you, it may not work to lower your blood clotting risk. However, if you take too much warfarin, you might be at risk for bleeding.

Blood Tests

Blood tests can help assess how easily your blood clots, which can help a healthcare provider find the correct dose for you. The main tests used for this purpose are an international normalized ratio test and a prothrombin time, also known, respectively, as an INR test and PT test.

In a person not taking warfarin or another anti-coagulation medication, the result of a typical INR test is around 1.0. The goal for people taking warfarin is usually to get an INR between 2.0 and 3.0.

You will need to take these tests several times so that your healthcare provider can raise or lower your dose accordingly to get your INR in the acceptable range. For example, if your INR is too low, you may need to take a higher warfarin dosage every day. On the other hand, if your INR is too high, you may need to decrease your daily dosage.

Eventually, a healthcare provider will find a stable dose for you. After that, you will still need to have INR tests (although less frequently) to ensure your blood clotting is in the suitable range, perhaps around once a month.


Warfarin dosages vary depending on health and lifestyle factors. In addition, healthcare providers use blood tests to determine how your blood clots, which can help them determine the appropriate dosage for you.


Many factors can change the amount of warfarin you might need, including dietary changes and medications.


Some foods will change how well your body responds to warfarin. For example, foods with a lot of vitamin K will tend to decrease your INR. These include:

  • High vitamin K foods, like spinach, kale, and collard greens
  • Moderate vitamin K foods, such as green tea, broccoli, and asparagus

Other foods can increase your chances of bleeding due to warfarin. Therefore, you should limit how much of these you consume while on warfarin therapy. These foods include:

  • Alcohol
  • Grapefruit juice
  • Cranberry juice


Medications can also alter the way your body responds to warfarin. For example, certain antibiotics might change how your body responds. So, if you add or change medications, you may need to get a new INR test.

When you start a new medication, make sure your healthcare provider knows that you are already taking warfarin since this might change how much warfarin you need. Also, make sure your healthcare provider knows about all the other medications you are taking, including over-the-counter (OTC) medications and herbal supplements. These can also interfere with warfarin.


Certain foods and medications can interact with warfarin and affect how much of the drug you should take.

Possible Side Effects

As with all medications, warfarin may produce side effects in some people. However, the most common side effect of warfarin therapy is unwanted bleeding. Bleeding is most likely to occur if a person’s INR is too high.


Sometimes this bleeding is minor. For example, you may find that you bruise more easily while taking warfarin or bleed more heavily from a small cut.

Other times, a person might suffer from a more significant bleed called a hemorrhage. This complication can occur in almost any body part, including the brain, gastrointestinal tract, or joints.

In the U.S., 65,000 people are treated annually in emergency rooms for warfarin-related hemorrhage.

In some cases, these side effects can be life-threatening. Therefore, if you suffer a dangerous bleed, healthcare providers will usually stop warfarin therapy temporarily. Depending on the situation, they may also give other treatments to help your blood clot more effectively, including vitamin K.


Tell your healthcare provider right away if you have any signs that you might be taking too much warfarin. Symptoms of an overdose include:

  • Bright red or tarry stool
  • Pinkish or dark brown urine
  • Heavy bleeding with menstruation
  • Coughing up blood
  • Unusual bruising or bleeding of any sort


Uncommonly, warfarin can cause painful lesions or necrosis (skin death). Talk to your healthcare provider right away if you notice any of the following:

  • Darkened skin
  • Ulcers
  • Severe pain that appears suddenly
  • Body temperature changes

Other Side Effects

Other potential side effects of warfarin include:

  • Nausea and vomiting
  • Abdominal pain
  • Bloating
  • Altered sense of taste
  • Hair loss
  • Chills


The most common side effect of warfarin is bleeding. Sometimes the bleeding risk is minor; other times, it can result in life-threatening hemorrhage. In addition, necrosis, overdose, nausea, abdominal pain, hair loss, and chills may occur.


It isn't safe for some people to take warfarin. For example, warfarin usually shouldn't be prescribed to anyone with:

  • A hypersensitivity allergy to warfarin
  • An ongoing bleed in any part of the body
  • Recent eye, central nervous system, or traumatic surgery
  • Threatened miscarriage
  • Pregnancy complications, like preeclampsia
  • Medical procedures with the risk of uncontrolled bleeding
  • Pregnancy
  • Cerebral aneurysms (a bulging artery in the brain)
  • Pericarditis (inflammation of the sac around the heart, the pericardium)
  • Bacterial endocarditis (heart inflammation)

In addition, warfarin is usually not the right choice for people who have difficulty taking their medicines as prescribed. That is because it's important to take warfarin in precisely the way a healthcare provider recommends each day.

Older People

Caution should also be used in older people because the risk of bleeding complications increases as you age. If you are older and take warfarin, a healthcare provider will monitor you closely. To take warfarin safely, you may need lower doses of the drug.

Some Health Conditions

If you have certain medical conditions, you may be able to take warfarin, but only cautiously. For example, if you have kidney problems, you may be more likely to experience bleeding problems from warfarin. A healthcare provider can help you weigh the risks and benefits of the treatment in your particular situation.

Pregnancy and Breastfeeding

Except in very unusual situations, you should not take warfarin during pregnancy. Warfarin is known to cross the placental barrier, which means that a fetus can be exposed to the drug.

Warfarin can cause the following when taken during pregnancy:

  • Fetal bleeding
  • Spontaneous abortion
  • Preterm birth
  • Stillbirth
  • Birth defects

If you are pregnant and have a mechanical heart valve, warfarin may be worth considering despite these risks. The risk of dangerous blood clots due to the artificial valve usually outweighs other risks. However, talk with your healthcare provider to weigh the risks and potential benefits in your particular situation.

If you are already taking warfarin and find out you are pregnant, contact your healthcare provider's office right away to tell them. They can advise you whether you should stop taking the medication. Don't wait for your appointment to ask.

Warfarin does not pass through breastmilk. For this reason, it is considered a safe medication to take while breastfeeding.


Warfarin is contraindicated in situations where you are at increased risk of bleeding. In addition, if you have trouble remembering to take medications, warfarin may not be the best choice for you. Healthcare providers should closely monitor older people and those with some medical conditions while on warfarin. The drug is safe while breastfeeding but should not be taken while pregnant.


Healthcare providers have prescribed warfarin for many years. It was previously the only oral anti-coagulation drug available. However, in recent years, newer blood thinner options have become available. These drugs include:

  • Eliquis (apixaban)
  • Xarelto (rivaroxaban)
  • Lovenox (enoxaparin)
  • Pradaxa (dabigatran)

These treatments don't have as many interactions with diet or other medications compared to warfarin. So, you may want to ask your healthcare provider whether one of these different treatments might work in your situation.


Warfarin is a blood-thinning drug used to prevent and treat blood clots. It is vitamin K antagonist, which means it works by blocking the enzyme that enables vitamin K to activate clotting. You take the medication orally, and dosage depends on your health and lifestyle factors. Blood tests help healthcare providers establish and maintain a correct dose.

The most common side effect is bleeding, which can sometimes be life-threatening. Therefore, people with risks of bleeding should not take warfarin. In addition to warfarin, newer blood thinners are available that have fewer interactions with food and other medications.

A Word From Verywell

Properly taken, warfarin can reduce the risk of dangerous blood clots in people with certain health conditions. However, you must take the drug correctly to be safe and effective. Don't hesitate to talk through all your concerns with your healthcare provider before you start taking the medication. Contact your healthcare provider right away if you experience any potentially serious signs or symptoms. 

7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. National Library of Medicine. Blood clots.

  2. American Heart Association. Understand your risk of excessive blood clotting.

  3. Patel S, Singh R, Preuss CV, Patel N. Warfarin. StatPearls.

  4. Food and Drug Administration. Coumadin (warfarin sodium) tablets label

  5. Tideman PA, Tirimacco R, St John A, Roberts GW. How to manage warfarin therapy. Aust Prescr. 2015;38(2):44-8. doi:10.18773/austprescr.2015.016

  6. Harter K, Levine M, Henderson SO. Anticoagulation drug therapy: A review. West J Emerg Med. 2015;16(1):11-7. doi:10.5811/westjem.2014.12.22933

  7. National Library of Medicine. Warfarin.