Warfarin: uses, side effects, and dosage

Warfarin is a generic prescription drug that has been used to treat and prevent blood clots since the 1950s. 

This drug is often referred to as a blood thinner, but that description doesn’t quite capture how warfarin works.

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Warfarin is a prescription medication approved by the U.S. Food and Drug Administration (FDA) to treat blood clots. It was previously available under the brand name Coumadin, which is no longer available in the United States and Canada. It’s now sold as a generic drug or under the brand name Jantoven (FDA, 2011). 

Warfarin is an anticoagulant, which is a type of drug that prevents your blood from clotting. While it’s widely known as a blood thinner, warfarin doesn’t actually thin the consistency of your blood. It does, however, stop blood clots from forming. 

If you have a blood clot or are at risk of one, your healthcare provider may prescribe you warfarin. This medication can prevent clots from developing or stop existing ones from getting bigger. Warfarin can also stop blood clots from moving to other parts of the body, where they can be harmful.

A healthcare provider may recommend that you use warfarin if you have any of the following conditions (Patel, 2021):

• A blood clot, often in the leg (venous thrombosis)

• Heart attack 

• A blood clot in the lungs (pulmonary embolism)

• Atrial fibrillation (rapid or irregular heart rhythms that can cause clots)

If they think it’s the right treatment for you, a healthcare provider may prescribe warfarin off-label to treat the following:

• Certain types of heart attacks

• Blood problems in people who receive a kidney transplant 

• Certain types of chronic kidney disease

• Stable coronary artery disease (partial blockage of the blood vessels that supply the heart)

Warfarin is a once-daily oral medication. It’s available in doses of 1 mg, 2 mg, 2.5 mg, 3 mg, 4 mg, 5 mg, 6 mg, 7.5 mg, and 10 mg. What dosage of warfarin you’re given depends on your underlying condition and medical history. 

To determine the proper dosage, your healthcare provider can perform a prothrombin time (PT) test or international normalized ratio (INR) blood test. These tests check how long it takes your blood to clot with or without the medication. Dosages are then adjusted based on the results. 

The typical starting dose of warfarin is 5–10 mg, but responses to the same dose differ from person to person. Your dosage can change along the treatment journey depending on blood test results, symptoms, side effects, and if you’re taking other medications. 

PT or INR blood tests may be performed every month to see how the drug is working––especially if your dose is changed (Hull, 2021).

Warfarin carries a severe warning from the FDA as it can cause severe and life-threatening bleeding. Bleeding is more likely to occur when you first start taking it and with higher doses. 

Those at higher risk of bleeding (like people with a history of falls or people who’ve experienced bleeding from their digestive system) should be carefully monitored. They may also benefit from a change in dosage or shorter period of treatment. 

The most common side effects of warfarin are (Hull, 2021):

• Easy bruising

• Gum bleeding

• Nosebleeds

While minor bleeds don’t require immediate medical attention, your healthcare provider should know about them. Let your healthcare provider know if your gums bleed after brushing or flossing your teeth, or if you’ve experienced excessive menstrual bleeding or bleeding between periods (Hull, 2021).

More serious bleeds are possible while taking warfarin. These are most likely to happen when first starting the medication or if your dosage is increased. Major internal bleeds aren’t that common (roughly 1–3% per year), but everyone’s risk factors are different. Maintaining a stable dose for at least six months puts your chances of a major bleed closer to 1% (Hull, 2021).

Some prevention measures you can take at home to minimize the risk of bleeding include monitoring your diet to ensure your vitamin K intake is consistent and avoiding sports or activities that may cause injury (DailyMed, 2017). 

Unless directed by your healthcare provider, never take vitamin K supplements at the same time as warfarin. That’s because vitamin K reverses the effects of warfarin. That, and significant changes to your vitamin K levels increases your risk for developing a dangerous blood clot.

Tell a healthcare professional immediately if you fall or are involved in an accident while taking warfarin, even if you have no symptoms. Get immediate medical attention if you experience any of these adverse effects:

• Severe headaches

• Dizziness

• Weakness

• Persistent nausea

• Vomiting blood or brown or black substances

• Dark red or brown urine

• Bloody or dark-colored stool

Warfarin increases your risk of bleeding. This risk is higher in people over age 65 and those with blood disorders. 

Talk to a healthcare provider if you’re pregnant or plan on becoming pregnant. Warfarin can cross through the placenta but does not enter breastmilk. This means it’s safe to breastfeed on this medication, but may not be safe to take while pregnant. Your healthcare provider may switch you to another medication during pregnancy and return to warfarin after you’ve given birth (Hull, 2021).

Be sure to tell your healthcare provider about any other drugs you’re taking before starting warfarin.

Combining warfarin with certain over-the-counter (OTC) medications can increase the risk of serious bleeding. Examples of these include nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin, ibuprofen (brand name Advil or Motrin), and naproxen (brand name Aleve). This includes many OTC antacid medications since a lot contain aspirin (FDA, 2016). 

Certain medications also change the effectiveness of warfarin. If you take more than the daily limit of acetaminophen (brand name Tylenol), it may affect how warfarin works. Otherwise, acetaminophen is a safer pain relief option than NSAIDs for people on warfarin (Hull, 2021).

Let your healthcare provider know about all vitamins, supplements, and medications you’re taking. Certain supplements, such as St. John's Wort and ginkgo biloba, may interact with warfarin. Your healthcare provider may decide to do more frequent blood tests if you’re taking certain dietary supplements or may recommend that you adjust your regiment to ensure that your medications work as intended.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

March 03, 2022

Written by

Linnea Zielinski

Fact checked by

Yael Cooperman, MD

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About the medical reviewer

Yael Cooperman, MD, Ro

Yael Cooperman is a physician and works as a Senior Manager, Medical Content & Education at Ro.