What to Know When Taking Warfarin

November 21, 2017

What to Know When Taking Warfarin

Warfarin is a medicine that controls how your blood clots. It is used to help prevent blood clots that may cause serious health problems. These problems include heart attack (myocardial infarction), stroke, a blockage in an artery or vein (thrombus), or a blood clot that travels to the lungs (pulmonary embolism).

Closeup of arm showing medical alert bracelet.

Before you start taking this medicine, tell your healthcare provider if you have any of these conditions:

  • Stomach ulcer now or in the past

  • Vomited blood or had bloody stools (black or red color)

  • Aneurysm, pericarditis, or pericardial effusion

  • Blood disorder

  • Recent surgery, stroke, mini-stroke, or spinal puncture

  • Kidney or liver disease, uncontrolled high blood pressure, diabetes, vasculitis, heart failure, lupus or other collagen-vascular disease, or high cholesterol

  • You are pregnant or breastfeeding

  • You are younger than 18 years old

  • Recent or planned dental procedure

Although warfarin reduces the risk for blood clots, it increases your risk of bleeding. Because of this, you must take the medicine exactly as you are told by your healthcare provider. 

You will have blood tests often to check the level of warfarin in your blood. It is important to have just the right amount to balance preventing blood clots and causing excessive bleeding. If the level is too low, you are at risk for developing a blood clot. If it's too high, you are at risk for bleeding. Make sure you keep all scheduled appointments for blood testing. If you don’t, you will be at risk for bleeding problems. You also need to protect yourself from injury that may cause bleeding such as a fall or hitting your head.

Follow these tips

  • Take this medicine at the same time each day. Take it with a full glass of water, with or without food.

  • Make sure you know what to do if you miss a dose. If you are not sure what to do, call your healthcare provider or pharmacist. Don't take more warfarin than you were told to.

  • Tell all of your providers including your dentist that you take warfarin. If you will be taking warfarin for quite a while, carry an ID card or get a Medic-Alert bracelet. This will alert medical staff in case you aren’t able to do so yourself. Also carry with you the name and number of the person to contact in case of an emergency.

  • Keep your appointments for regular blood tests to measure the effects of warfarin. Your healthcare provider may need to change your dose based on the results of your blood test. Follow your provider’s advice exactly about how to take this medicine.

  • Don't stop taking the medicine without talking with your provider.


Monitoring your PT/INR blood levels

You will need to have a blood test called a PT/INR regularly. PT stands for pro thrombin. INR stands for international normalized ratio. The PT/INR blood test is done to make sure you are getting the right dose of this medicine. The PT/INR test tells your healthcare provider how your blood is clotting. Prothrombin time, commonly referred to as pro time or PT, measures the time it takes for blood to clot. International normalized ratio or INR is a way to compare results of the PT tests done at different labs.

  • Go for your blood (PT/INR) tests as often as directed. Note that diet and medicines can affect your PT/INR level.

  • Your next PT/INR blood draw is due on _________________ (date) at ________________ (time) by ____________________ (name of healthcare provider or clinic).

  • The name of the healthcare provider who is monitoring your anticoagulation therapy is ______________________ and the phone number is ___________________.

  • Follow up with your healthcare provider or as advised by his or her staff. It usually takes a few hours for your doctor to get the results of your clotting tests. Call to get your lab results and find out if your provider needs to make further changes to your warfarin dose.

  • If your labs (PT/INR) are drawn at a location other than your provider's office, remember to tell your provider as soon as you get your lab results.


What to do at home

  1. Adjust your warfarin dose as directed by your healthcare provider, ____________________ (name of healthcare provider).

  2. Have your blood clotting test done every _______ days at _____________________ (name of clinic) by ____________________ (name of healthcare provider).

  3. Prevent injuries and bleeding:

    • Don't go barefoot. Always wear shoes.

    • Don't trim corns or calluses yourself.

    • Use an electric razor to shave instead of a manual one.

    • Use a soft-bristled toothbrush and waxed dental floss.

  4. Some medicines can affect your blood clotting. Always talk with your healthcare provider before stopping, starting, or changing any prescription or over-the-counter (OTC) medicines. This includes herbal medicines and vitamins. Talk with your healthcare provider about the following medicines:

    • Some antibiotics. This is critical because some common antibiotics can cause severe bleeding if you take them along with warfarin. These antibiotics include ciprofloxacin and trimethoprim-sulfamethoxazole. 

    • Some heart medicines

    • Cimetidine

    • Aspirin or other anti-inflammatory medicines, such as ibuprofen, naproxen, ketoprofen, or other arthritis medicines

    • Some medicines for depression, cancer, HIV (protease inhibitors), diabetes, seizures, gout, high cholesterol, or thyroid replacement

    • Vitamins containing Vitamin K

    • Herbal products such as ginkgo, Q10, garlic, or St. John's wort

  5. Don't make major changes in your diet. Consuming high amounts of foods containing vitamin K can reduce the effectiveness of your warfarin.


Keep your diet steady

Keep your diet pretty much the same each day. That’s because many foods contain vitamin K. Vitamin K helps your blood clot. So eating overly high amounts of foods that contain vitamin K can affect the way warfarin works. You don’t need to stay away from foods that have vitamin K. But keep the amount of them you eat steady (about the same day to day). If you change your diet for any reason, such as for illness or to lose weight, tell your healthcare provider.

  • Foods that have vitamin K include asparagus, avocado, broccoli, cabbage, kale, spinach, and some other leafy green vegetables. Oils such as soybean, canola, and olive oils also contain vitamin K.

  • Other food products can affect the way warfarin works in your body:

    • Food products that may affect blood clotting include cranberries and cranberry juice, fish oil supplements, garlic, ginger, licorice, and turmeric.

    • Herbs used in herbal teas or supplements can also affect blood clotting. Keep the amount of herbal teas and supplements you use steady.

    • Alcohol can increase the effect of warfarin in your body.

Talk with your healthcare provider if you have concerns about these or other food products and their effects on warfarin.

When to call your healthcare provider

Warfarin increases your risk of bleeding. Call your healthcare provider right away before you take your next dose of warfarin if you have any of these problems:

  • Bleeding that doesn’t stop in 10 minutes. This might be from a bloody nose or a cut, for example.

  • A heavier-than-normal period or bleeding between periods

  • Coughing or throwing up blood or something that looks like coffee grounds

  • Nausea, bloating, or diarrhea

  • Bleeding hemorrhoids

  • Dark red or brown urine

  • Red or black tarry stools

  • Red or black-and-blue marks on the skin that get larger

  • A fever or an illness that gets worse

  • Dizziness, headache, weakness, or fatigue

  • Chest pain or trouble breathing

  • A serious fall or a blow to the head

  • Swelling or pain after an injury or at an injection site

  • Bleeding gums after brushing your teeth

What to watch for

If you have any of the following allergic reactions, call your doctor right away or go to the hospital.

  • Rash

  • Itching

  • Swelling

  • Trouble swallowing or breathing

[NOTE: This information topic may not include all directions, precautions, medical conditions, medicine/food interactions, and warnings for this medicine. Check with your doctor, nurse, or pharmacist for any questions that you may have.]


November 21, 2017


Hirsh, J., Foundation Guide to Warfa, Circulation ;1692-1711(2003); 107rin Therapy, Performance Measure Name D/C on Antithrombotic Therapy, Core Measures set: Stroke (STK-1), Performance Measure Name VTE Warfarin Therapy DC Core Measures set: Venous Thromboembolism (VTE) VTE-5)

Reviewed By:  

Fetterman, Anne, RN, BSN,Gandelman, Glenn, MD, MPH,Turley, Raymond Kent, BSN, MSN, RN