Warfarin

Say it: WAR-far-in · Brand names: Jantoven

Warfarin is a blood thinner pill that helps prevent and treat harmful blood clots.

What it is

Your liver uses vitamin K to make proteins that help blood clot. Warfarin blocks vitamin K from doing this job. With fewer clotting proteins, dangerous clots are less likely to form or grow.

Often used for

• treating blood clots in the legs or lungs
• lowering the chance that blood clots come back
• lowering the chance of stroke in people with an irregular heartbeat called atrial fibrillation
• preventing clots in people with a heart valve replacement
• lowering the chance of another heart attack or stroke after a heart attack

How you get it

It is a pill you take by mouth once a day, at the same time each day. Your dose is set by a blood test called an INR (international normalized ratio). The dose often changes, especially at first. Your schedule may be different.

Common side effects and what helps

• Bruising more easily. Use a soft toothbrush and an electric razor.
• Small bleeding, like from your gums or nose. Press firmly for 10 minutes. Call if it does not stop.
• Heavier periods. Tell your team if your periods change.
• Upset stomach, gas, or a change in taste. Tell your team if it bothers you.
• Feeling cold or tired. Tell your team if it is new.

Call your care team right away if you have

• Bleeding that will not stop, such as a nosebleed or a cut that keeps bleeding
• Red, pink, or dark brown urine
• Red or black stools that look like tar
• Coughing up blood, or throwing up blood or something that looks like coffee grounds
• A sudden bad headache, confusion, weakness, or trouble seeing or speaking
• Pain, color change, or sores on your toes or skin, especially purple or dark areas
• A fall or a hit to the head, even if you feel fine

Before you start, tell your team

• All medicines, vitamins, herbs, and supplements you take. Many can change how warfarin works, including antibiotics.
• If you have ever had bleeding problems, stomach ulcers, or a stroke
• If you are pregnant or may become pregnant. Warfarin can cause birth defects.
• If you are breastfeeding
• If you have a surgery, dental work, or a procedure planned

Good to know

• Warfarin can cause serious bleeding. You will have regular INR blood tests to keep your dose in a safe range.
• Foods with vitamin K, like leafy greens, affect warfarin. You do not need to avoid them, but try to eat about the same amount each week.
• Tell your team before starting or stopping any medicine, including over the counter ones and herbs.
• Rarely, warfarin can damage skin or cause painful purple toes.
• Alcohol and big changes in diet or health can change your INR.

At home

• Take it at the same time each day. Some tablet colors mean different strengths, so check your pills.
• Keep all INR blood test visits.
• If you miss a dose, ask your team what to do. Do not take two doses at once.
• Store it at room temperature, away from children.
• Pregnant people should not handle crushed or broken tablets.

Questions to ask

• What INR range is right for me?
• How often will I need INR tests?
• Are there foods or medicines I should be careful with?
• What should I do before surgery or dental work?

Side effect guides

• Fatigue (tiredness)
• Taste changes
• Low platelets and bleeding

Last updated: 2026-10-27. Medically reviewed by Veli Bakalov, MD, medical oncologist (October 2026). General education from Step by Step, the patient portal of HemeOncBuddy.