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What to Know When Taking Warfarin

Warfarin is a medicine that controls how your blood clots. It's used to help prevent blood clots that may cause serious health problems. You may be taking warfarin to reduce your risk for stroke or heart attack. Or you may be taking it to stop a blood clot passing to your lung. But warfarin can also increase your risk of bleeding. This can be dangerous. Because of this, you’ll need to take important steps when you’re on warfarin.

Closeup of arm showing medical alert bracelet.

Before you start warfarin

Before starting warfarin, tell your healthcare provider if you have ever had any of these:

  • Stomach ulcer

  • Vomiting of blood, or red or black stools

  • Any heart or blood vessel disease

  • A blood disorder

  • A stroke or transient ischemic attack (TIA)

  • Kidney or liver disease

  • Lupus or other collagen-vascular disease

Also tell your healthcare provider if you:

  • Are pregnant or breastfeeding

  • Are younger than 18 years old

  • Had a recent dental procedure or surgery

  • Have an upcoming dental procedure or surgery

  • Had a spinal puncture, recent spinal anesthesia or spinal surgery

Many medicines cause problems if you take them while you’re on warfarin. Tell your healthcare provider about every medicine you take. It may be dangerous for you to take some medicines and supplements, such as:

  • Medicines that thin your blood, such as aspirin, clopidogrel, ticagrelor, or prasugrel

  • Antibiotics

  • Heart medicines

  • Cimetidine

  • Ibuprofen

  • Naproxen, ketoprofen, or other arthritis medicines

  • Medicines for depression, cancer, HIV, diabetes, seizures, gout, high cholesterol, or thyroid

  • Vitamins that have vitamin K

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

This list may not include all medicines and supplements that can affect how your medicine works. Talk with your healthcare provider and pharmacist. Your healthcare provider may change or tell you to stop taking some of these before you take warfarin.

Taking your warfarin as directed

You’ll need to take the medicine exactly as directed by your doctor. Make sure to:

  • Take it at the same time each day.

  • Take it with a full glass of water. You can take it with or without food.

  • Use a pillbox to help keep track of your doses.

  • If you miss a dose, call your healthcare provider right away to find out how much to take.

  • Never take a double dose. If you take too much, it can cause bleeding on the outside and inside of your body.

Keeping certain foods steady in your diet

Some foods can affect how warfarin works.

Many foods contain vitamin K. Vitamin K is a substance that helps your blood clot. So eating foods that contain vitamin K can affect the way warfarin works. You don’t need to avoid foods that have vitamin K. But you do need to keep the amount of them you eat steady -- about the same day to day. Foods that have vitamin K include:

  • Asparagus

  • Avocado

  • Broccoli

  • Cabbage

  • Kale

  • Spinach

  • Some other leafy green vegetables

  • Oils such as soybean, canola, and olive

Other foods and drinks can affect the way your blood clots. You’ll also need to keep the amounts of these steady in your diet. These include:

  • Cranberries and cranberry juice

  • Fish oil supplements

  • Garlic, ginger, licorice, and turmeric

  • Herbs used in herbal teas or supplements

  • Alcohol

If you change your diet for any reason, such as because of illness or to lose weight, tell your healthcare provider.

Preventing injury

Once you’re on warfarin, you’ll need to be extra careful. Because it makes you bleed more, you’ll need to protect yourself from breaks in the skin. To do this:

  • Don’t go barefoot. Always wear shoes.

  • Don’t trim corns or calluses yourself.

  • Use an electric razor instead of a manual one.

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

You’ll also need to avoid any activities that may cause injury. If you fall or are injured, call your healthcare provider right away. You could be bleeding inside your body and not know it. Make sure to get medical attention right away if you have:

  • A serious fall

  • A blow to the head

  • Any other kind of injury

Getting your blood tested

You’ll need to have your blood tested on a regular schedule. Your healthcare provider will tell you how often you need to have your blood tested. This is to make sure you’re taking the right amount of warfarin. Too much can cause excess bleeding, which can be very serious. Too little may not prevent blood clots from harming you.

The blood tests check your international normalized ratio (INR) and prothrombin time (PT). These show how quickly your blood clots. Together the test is called PT/INR.\

You may need to visit a hospital or clinic to have your blood tested. Or, a nurse may come to your home and test your blood. In some cases, you may be able to test your blood at home with a small machine. Talk with your healthcare provider to find out what’s best for you. Do not miss any appointments to get your blood tested. If you have a blood test outside of your healthcare provider’s office, make sure to call him or her as soon as you get your test results.

After the blood test, your healthcare provider may tell you to change your dose of warfarin. Take the medicine exactly as directed. Don’t stop taking it unless your healthcare provider tells you to.

Important tips

While on warfarin:

  • Make sure you report any new medicines started by any of your providers to the team that manages your warfarin. You may need to be monitored more often after starting a new medicine, changing the dose or stopping a medicine you have been on.

  • Don't stop taking the medicine without talking with your provider. If you have an upcoming surgery or procedure your surgeon and your healthcare provider will give you specific instructions on how to change your dose to prevent excessive bleeding for your procedure or surgery.

  • Tell all of your healthcare providers that you take warfarin. This includes your dentist, chiropractor, nurses, physical therapist, and home health nurse.

  • Wear a medical alert bracelet or carry an ID card in your wallet that says you take warfarin.

  • Keep all appointments for your blood tests.

  • Talk with your healthcare provider before taking any new medicine. This includes any over-the-counter medicines. It also includes supplements, herbs, or vitamins.

  • Tell the healthcare provider who manages your warfarin if you change your contact information

Call 911

Warfarin increases your risk of bleeding. Call 911 and 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

  • Coughing up blood

  • Vomiting blood or what looks like coffee grounds

  • Nausea, bloating, or diarrhea

  • Dark red or brown urine

  • Red or black tarry stools

  • Dizziness, headache, weakness, or fatigue

  • Chest pain or trouble breathing

  • A serious fall or a blow to the head

  • Signs or symptoms of a stroke (facial drooping, difficulty speaking, weakness on one side of the body)

  • Signs of an allergic reaction such as swelling of the mouth, lips, throat, tongue, or face; rash, hoarseness, trouble breathing or speaking

  • Severe pain, loss of sensation, color change to the skin, and temperature change to a part of the skin or an extremity such as the arm or leg. This may indicate a rare but serious side effect of warfarin where the skin and tissue does not get blood flow.

When to call your healthcare provider

Other urgent or important signs and symptoms that you want to call your provider about include:

  • Swelling or pain after an injury

  • Swelling or pain at an injection site

  • Bleeding gums after brushing your teeth

  • A fever or an illness that gets worse

  • Bleeding hemorrhoids

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

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

[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.]

Online Medical Reviewer: Lu Cunningham
Online Medical Reviewer: Mandy Snyder APRN
Online Medical Reviewer: Steven Kang MD
Date Last Reviewed: 7/1/2019
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