Warfarin is a blood thinner (anticoagulant). Anticoagulants help to prevent the formation of blood clots. They also help to stop the growth of blood clots.

Warfarin is prescribed for people who are at risk for developing harmful blood clots, such as people who have:

  • Surgically implanted mechanical heart valves.
  • Irregular heart rhythms (atrial fibrillation).
  • Certain clotting disorders.
  • A history of harmful blood clotting in the past. This includes people who have had:
    • A stroke.
    • Blood clot in the lungs (pulmonary embolism, or PE).
    • Blood clot in the legs (deep vein thrombosis, or DVT).
  • An existing blood clot.

Warfarin is a medicine that you take by mouth (orally). Warfarin tablets come in different strengths. Each tablet strength is a different colour, with the amount of warfarin printed on the tablet. If you get a new prescription filled and the colour of your tablet is different than usual, tell your pharmacist or health care provider immediately.

The goal of warfarin therapy is to lessen the clotting tendency of blood, but not to prevent clotting completely. Your health care provider will monitor the anticoagulation effect of warfarin closely and will adjust your dose as needed.

Warfarin is a medicine that needs to be closely monitored, so it is very important to keep all lab visits and follow-up visits with your health care provider. While taking warfarin, you will need to have blood tests (prothrombin tests, or PT tests) regularly to measure your blood clotting time. This type of test can be done with a finger stick or a blood draw.

The PT test results will be reported as the International Normalized Ratio (INR). The INR tells your health care provider whether your dosage of warfarin needs to be changed. The longer it takes your blood to clot, the higher the INR.

Your health care provider will tell you your target INR range. If your INR is not in your target range, your health care provider may adjust your dosage.

  • If your INR is above your target range, there is a risk of bleeding. Your dosage of warfarin may need to be decreased.
  • If your INR is below your target range, there is a risk of clotting. Your dosage of warfarin may need to be increased.
  • When you first start warfarin, you will usually have your INR checked every few days.
  • You may need to have INR tests done more than once a week until you are taking the correct dosage of warfarin.
  • After you have reached your target INR, your INR will be tested less often. However, you will need to have your INR checked at least once every 4–6 weeks for the entire time you are taking warfarin.

Too much warfarin can cause bleeding (haemorrhage) in any part of the body, such as:

  • Bleeding from the gums.
  • Unexplained bruises.
  • Bruises that get larger.
  • Blood in the urine.
  • Bloody or dark stools.
  • Bleeding in the brain (haemorrhagic stroke).
  • A nosebleed that is not easily stopped.
  • Coughing up blood.
  • Vomiting blood.

Warfarin use may also cause:

  • Skin rash or irritations
  • Nausea that does not go away.
  • Severe pain in the back or joints.
  • Painful toes that turn blue or purple (purple toe syndrome).
  • Painful ulcers that do not go away (skin necrosis).

Too little warfarin can increase the risk of blood clots in your legs, lungs, or arms.

Signs and symptoms of a DVT in your leg or arm may include:

  • Pain or swelling in your leg or arm.
  • Skin that is red or warm to the touch on your arm or leg.

Signs and symptoms of a pulmonary embolism may include:

  • Shortness of breath or difficulty breathing.
  • Chest pain.
  • Unexplained fever.

If you are taking too much or too little warfarin, you can have a stroke. Signs and symptoms of a stroke may include:

  • Weakness or numbness of your face, arm, or leg, especially on one side of your body.
  • Confusion or trouble thinking clearly.
  • Difficulty seeing with one or both eyes.
  • Difficulty walking or moving your arms or legs.
  • Dizziness.
  • Loss of balance or coordination.
  • Trouble speaking, trouble understanding speech, or both (aphasia).
  • Sudden, severe headache with no known cause.
  • Partial or total loss of consciousness.
  • Take warfarin exactly as told by your health care provider. Doing this helps you avoid bleeding or blood clots that could result in serious injury, pain, or disability.
  • Take your medicine at the same time every day. If you forget to take your dose of warfarin, take it as soon as you remember that day. If you do not remember on that day,do not take an extra dose the next day.
  • Contact your health care provider if you miss or take an extra dose.Do not change your dosage on your own to make up for missed or extra doses.
  • Wear or carry identification that says that you are taking warfarin.
  • Make sure that all health care providers, including your dentist, know you are taking warfarin.
  • If you need surgery, talk with your health care provider about whether you should stop taking warfarin before your surgery.
  • Avoid situations that cause bleeding. You may bleed more easily while taking warfarin. To limit bleeding, take the following actions:
    • Use a softer toothbrush.
    • Floss with waxed floss, not unwaxed floss.
    • Shave with an electric razor, not with a blade.
    • Limit your use of sharp objects.
    • Avoid potentially harmful activities, such as contact sports.
  • Warfarin is not recommended during the first trimester of pregnancy due to an increased risk of birth defects. In certain situations, a woman may take warfarin after her first trimester of pregnancy.
  • If you are taking warfarin and you become pregnant or plan to become pregnant, contact your health care provider right away.
  • If you plan to breastfeed while taking warfarin, talk with your health care provider first.
  • Avoid drinking alcohol, or limit alcohol intake to no more than 1 drink a day for nonpregnant women and 2 drinks a day for men. One drink equals 12 oz of beer, 5 oz of wine, or 1½ oz of hard liquor.
    • If you change the amount of alcohol that you drink, tell your health care provider. Your warfarin dosage may need to be changed.
  • Avoid street drugs while taking warfarin. The effects of street drugs on warfarin are not known.
  • Many prescription and over-the-counter medicines can interfere with warfarin. Talk with your health care provider or your pharmacist before starting or stopping any new medicines. This includes over-the-counter vitamins, dietary supplements, herbal medicines, and pain medicines. Your warfarin dosage may need to be adjusted.
  • Some common over-the-counter medicines that may increase the risk of bleeding while taking warfarin include:
    • Acetaminophen.
    • NSAIDs, such as ibuprofen or naproxen.
    • Vitamin E.
  • It is important to maintain a normal, balanced diet while taking warfarin. Avoid major changes in your diet. If you are going to change your diet, talk with your health care provider before making changes.
  • Your health care provider may recommend that you work with a diet and nutrition specialist (dietitian).
  • Vitamin K decreases the effect of warfarin, and it is found in many foods. Eat a consistent amount of foods that contain vitamin K. For example, you may decide to eat 2 vitamin K-containing foods each day.

Most foods that are high in vitamin K are green and leafy. Common foods that contain high amounts of vitamin K include:

  • Kale, raw or cooked.
  • Spinach, raw or cooked.
  • Collards, raw or cooked.
  • Swiss chard, raw or cooked.
  • Mustard greens, raw or cooked.
  • Turnip greens, raw or cooked.
  • Parsley, raw.
  • Broccoli, cooked.
  • Noodles, eggs, and spinach, enriched.
  • Brussels sprouts, raw or cooked.
  • Beet greens, raw or cooked.
  • Endive, raw.
  • Cabbage, cooked.
  • Asparagus, cooked.

Foods that contain moderate amounts of vitamin K include:

  • Broccoli, raw.
  • Cabbage, raw.
  • Bok choy, cooked.
  • Green leaf lettuce, raw
  • Prunes, stewed.
  • Pickles.
  • Kiwi.
  • Edamame, cooked.
  • Romaine lettuce, raw.
  • Avocado.
  • Tuna, canned in oil.
  • Okra, cooked.
  • Black-eyed peas, cooked.
  • Green beans, cooked or raw.
  • Blueberries, raw.
  • Blackberries, raw.
  • Peas, cooked or raw.
  • You miss a dose.
  • You take an extra dose.
  • You plan to have any kind of surgery or procedure.
  • You are unable to take your medicine due to nausea, vomiting, or diarrhoea.
  • You have any major changes in your diet or you plan to make any major changes in your diet.
  • You start or stop any over-the-counter medicine, prescription medicine, or dietary supplement.
  • You become pregnant, plan to become pregnant, or think you may be pregnant.
  • You have menstrual periods that are heavier than usual.
  • You have unusual bruising.
  • You develop symptoms of an allergic reaction, such as:
    • Swelling of the lips, face, tongue, mouth, or throat.
    • Rash.
    • Itching.
    • Itchy, red, swollen areas of skin (hives).
    • Trouble breathing.
    • Chest tightness.
  • You have:
    • Signs or symptoms of a stroke.
    • Signs or symptoms of a blood clot.
    • A fall or have an accident, especially if you hit your head.
    • Blood in your urine. Your urine may look reddish, pinkish, or tea-coloured.
    • Blood in your stool. Your stool may be black or bright red.
    • Bleeding that does not stop after applying pressure to the area for 30 minutes.
    • Severe pain in your joints or back.
    • Purple or blue toes.
    • Skin ulcers that do not go away.
  • You vomit blood or cough up blood. The blood may be bright red, or it may look like coffee grounds.

These symptoms may represent a serious problem that is an emergency. Do not wait to see if the symptoms will go away. Get medical help right away. Call your local emergency services. Do not drive yourself to the hospital.

  • Warfarin needs to be closely monitored with blood tests. It is very important to keep all lab visits and follow-up visits with your health care provider.
  • Make sure that you know your target INR range and your warfarin dosage.
  • Wear or carry identification that says that you are taking warfarin.
  • Take warfarin at the same time every day. Call your health care provider if you miss a dose or if you take an extra dose.Do not change the dosage of warfarin on your own.
  • Know the signs and symptoms of blood clots, bleeding, and a stroke. Know when to get emergency medical help.
  • Tell all health care providers who care for you that you are taking warfarin.
  • Talk with your health care provider or your pharmacist before starting or stopping any new medicines.
  • Monitor how much vitamin K you eat every day. Try to eat the same amount every day.

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Genomic Medicine UK is the home of comprehensive genomic testing in London. Our consultant medical doctors work tirelessly to provide the highest standards of medical laboratory testing for personalised medical treatments, genomic risk assessments for common diseases and genomic risk assessment for cancers at an affordable cost for everybody. We use state-of-the-art modern technologies of next-generation sequencing and DNA chip microarray to provide all of our patients and partner doctors with a reliable, evidence-based, thorough and valuable medical service.

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