Warfarin: Purpose, Side Effects, and Management Medically reviewed by Douglas A. Nelson, MD - Verywell Health

Warfarin is a prescription anti-coagulant medication used to help treat and prevent certain kinds of blood clots. Though it can be very helpful for some people, it does carry some risks. Drugs like warfarin are sometimes called "blood thinners." In reality, 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.

Illustration by Brianna Gilmartin, Verywell

What Is a Blood Clot?

Blood normally flows through your blood vessels in a liquid form. A blood clot is a group of specific blood components and proteins that have joined together in a semi-solid state. 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 flow of blood out of 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. For example, a blood clot in one of the main arteries of the heart can lead to a heart attack. A blood clot in a vessel in the brain can cause a stroke. Thrombosis and embolus are both types of blood clots that can cause serious and even life-threatening problems.

The blood in your body can form necessary blood clots through a series of complex and highly coordinated physiological reactions. These involve specific proteins and blood components. Certain proteins (called "clotting factors") become activated through a series of reactions that ultimately help to form a blood clot. These coordinated reactions help make sure that the blood forms clots only when really needed.

How It Works

Vitamin K refers to 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 by the presence of vitamin K. A specific enzyme enables vitamin K to activate these clotting factors.

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.

This means that fewer clotting factors will become activated. Overall, this makes it harder for the blood to form a clot, which makes it less likely that a dangerous clot will form. However, this also means that a person is somewhat more likely to experience a dangerous bleeding episode.

Uses

Warfarin is sometimes prescribed for a variety of medical conditions. Warfarin is commonly used to treat people with 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)

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

Warfarin can also be used to prevent blood clots in people who are at high risk of them. For example, it can be used in preventing:

• Venous thrombosis
• Pulmonary embolism
• Stroke from a blood clot that originated elsewhere in the body (thromboembolic stroke)

People with certain medical conditions are at higher risk of such clots, and they may need warfarin therapy. This may include people with a variety of conditions.

People at Higher Risk for Blood Clotting

• Those with atrial fibrillation
• Those who have an artificial mechanical heart valve
• Those with genetic conditions that make it more likely to get a blood clot
• Those with a history of a recent heart attack
• Those with a history of a recent stroke
• Those with a history of a recent surgery

Administration

Warfarin is an oral medication that is usually taken daily, ideally at the same time each day. You need to take exactly the amount prescribed by your doctor. Different dosages of warfarin often come in different-colored tablets, making it easier to take exactly the right amount. The drug can be stored at room temperature.

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

Talk to your doctor 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. Do not double a dose the next day if you miss a dose the day before. When in doubt, contact your physician.

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

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. In some cases, this may help lower your risk of complications from surgery. If you have a medical procedure or surgery planned, be sure that your physician knows that you are taking the drug. That way, they can let you know if you should stop taking it temporarily.

Dosing

Usually, people take between 1 mg and 10 mg of warfarin daily. Dosage is based on a specific individual's needs; in other words, some people will need more than other people. This will vary based on many factors, including your diet, your age, and your medical conditions. If your warfarin dose isn't high enough for you, you may not lower your risk for a blood clot. However, if you take too much warfarin, you might be at risk for a bleed.

To help find the right dose, you will need to take blood tests that assess how easily your blood clots. The main tests used are an international normalized ratio test and a prothrombin time, also known, respectively, as an INR test and PT test. These are both blood tests that measure how easily and quickly your blood can clot. 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 right range. For example, if your INR is too low, you may need to take a higher dosage of warfarin every day. If your INR is too high, you may need to decrease your daily dosage. Eventually, your doctor will find a stable dose for you. After that, you will still need to have INR tests (although less frequently) to make sure your blood clotting is in the right range, perhaps around once a month.

Factors That Might Change Your Dose

Many factors can change the amount of warfarin you might need. These include changes in other medications or changes in diet.

Some foods will change how well your body responds to warfarin. Foods with a lot of vitamin K will tend to decrease your INR. Leafy green vegetables like spinach, kale, and collard greens have high amounts of vitamin K and should be monitored the most closely. Other foods contain moderate levels of vitamin K, such as green tea, broccoli, and asparagus. Your doctor can provide detailed information about the foods you need to keep in mind.

Other foods can increase your chances of bleeding due to warfarin. These include alcohol, grapefruit juice, and cranberry juice. One should limit the use of these while on warfarin therapy.

Medications can also alter the way your body responds to warfarin. For example, certain antibiotics might change how your body responds. If you add or change medications, you may need to get a new INR test. When you start a new medication, make sure your medical provider knows that you are already taking warfarin—this might change how much warfarin you need. Also make sure your doctor knows about all the other medications you are taking, including over-the-counter medications and herbal supplements. These can also interfere with warfarin.

Possible Side Effects

The most common possible side effect of warfarin therapy is unwanted bleeding. This is most likely to occur if, for some reason, a person's INR is too high. Sometimes this bleeding is minor. For example, people often find that they bruise more easily while taking warfarin, or that they bleed more heavily from a small cut. Other times, a person might suffer from a more significant bleed called a hemorrhage. This can happen to almost any part of the body, including the brain, the gastrointestinal tract, or the joints.

In some cases, these side effects can be life-threatening. This is uncommon, but it does happen. If a person suffers a dangerous bleed, doctors 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.

Be sure to tell your doctor right away if you have any signs that you might be taking too much warfarin.

Potential Signs of Warfarin Overdose

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

Other potential side effects of warfarin include:

• Nausea and vomiting
• Abdominal pain
• Bloating
• Altered sense of taste

Uncommonly, warfarin can cause painful lesions or skin death. Talk to your doctor right away if you notice any darkened skin, ulcers, severe pain that appears suddenly, or color or temperature changes in your body. You may need urgent medical care.

Contraindications

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
• Cerebral aneurysms
• Pericarditis
• Bacterial endocarditis

People who have recently had certain diagnostic or therapeutic procedures are also usually not prescribed warfarin. For example, taking warfarin may not be advised to a person after a spinal tap or other procedure that might cause uncontrolled bleeding. Warfarin is also not recommended for people who have recently had certain surgeries, like eye surgeries or brain surgery.

Warfarin is usually not the right choice for people who have difficulty taking their medicines as prescribed. This is due to the fact that it's important to take warfarin in exactly the way your doctor recommends each day. Caution should also be used in giving warfarin to elderly people. Due to a variety of factors, these individuals often have an increased risk of bleeding complications. These people need to be closely monitored, and they may need lower doses of the drug for optimum safety.

People with certain medical conditions may be able to take warfarin, but only cautiously. For example, someone with kidney problems may be more likely to suffer from bleeding problems from warfarin. Your doctor will help you weigh the risks and benefits of the treatment in your particular situation.

Pregnancy and Breastfeeding

Except in very unusual situations, warfarin should not be taken by pregnant women. Warfarin is known to cross the placental barrier, which means that an unborn fetus will be exposed to the drug. Warfarin can cause the fetus to bleed, and it is also associated with spontaneous abortion, preterm birth, and stillbirth. Another syndrome, warfarin embryopathy, occurs when exposure to warfarin early in pregnancy causes birth defects.

For pregnant women with mechanical heart valves, warfarin may be worth considering in spite of these risks. These women are also at risk, however, of dangerous blood clots because of their artificial valve. Such women should talk with their doctor to weigh the risks and potential benefits in their particular situation.

If you are already taking warfarin and find out you are pregnant, contact your doctor'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 from the mother's blood into her breastmilk. For this reason, it is considered a safe medication to take while breastfeeding.

Alternatives

Doctors have been prescribing warfarin for many years. It was previously the only oral anti-coagulation drug available. However, in recent years, other options have become available for many people who need such therapies. These drugs include apixaban, rivaroxaban, enoxaparin, and dabigatran.

These treatments don't have as many interactions with diet or other medications as compared to warfarin. You may want to ask your doctor about whether one of these other treatments might work in your situation.

A Word From Verywell

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