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Warfarin (By injection)

Warfarin (WAR-far-in)

Prevents and treats blood clots. May lower the risk of serious complications after a heart attack. This medicine is a blood thinner.

Brand Name(s):

There may be other brand names for this medicine.

When This Medicine Should Not Be Used:

This medicine is not right for everyone. You should not receive it if you had an allergic reaction to warfarin, if you are pregnant, or if you have health problems that could cause bleeding.

How to Use This Medicine:


  • A nurse or other health provider will give you this medicine.
  • Your doctor will prescribe your dose and schedule. This medicine is given through a needle placed in a vein.
  • Your doctor may give you a few doses of this medicine and then switch you to an oral medicine that works the same way.
  • This medicine should come with a Medication Guide. Ask your pharmacist for a copy if you do not have one.

Drugs and Foods to Avoid:

Ask your doctor or pharmacist before using any other medicine, including over-the-counter medicines, vitamins, and herbal products.

  • Many medicines and foods can affect how warfarin works and may affect the PT/INR test results. Tell your doctor before you start or stop any medicine, especially the following:
    • Ginkgo, echinacea, goldenseal, or St John's wort
    • Another blood thinner, including apixaban, argatroban, bivalirudin, cilostazol, clopidogrel, dabigatran, desirudin, dipyridamole, heparin, lepirudin, prasugrel, rivaroxaban, ticlopidine
    • Medicine to treat depression or anxiety, including citalopram, desvenlafaxine, duloxetine, escitalopram, fluoxetine, fluvoxamine, milnacipran, paroxetine, sertraline, venlafaxine, vilazodone
    • Medicine to treat an infection
    • NSAID pain or arthritis medicine, including aspirin, celecoxib, diclofenac, diflunisal, fenoprofen, ibuprofen, indomethacin, ketoprofen, ketorolac, mefenamic acid, naproxen, oxaprozin, piroxicam, sulindac. Check the labels for over-the-counter medicines to find out if they contain an NSAID.
    • Steroid medicine, including dexamethasone, hydrocortisone, methylprednisolone, prednisolone, prednisone
  • Warfarin works best if you eat about the same amount of vitamin K every day. Foods high in vitamin K include asparagus, broccoli, brussels sprouts, cabbage, green leafy vegetables, plums, rhubarb, and canola oil. Talk to your doctor before you make changes to your normal diet.
  • Do not eat grapefruit or drink grapefruit juice while you are using this medicine.

Warnings While Using This Medicine:

  • It is not safe to take this medicine during pregnancy. It could harm an unborn baby. Tell your doctor right away if you become pregnant. Use an effective form of birth control to keep from getting pregnant during treatment and for at least 1 month after your last dose.
  • Tell your doctor if you are breastfeeding, or if you have kidney disease, liver disease, diabetes, heart disease, heart failure, high blood pressure, an infection, a stomach ulcer, or protein C deficiency. Also tell your doctor if you had recent surgery or an injury, or a history of stroke, anemia, severe bleeding or bruising, or problems caused by heparin use.
  • This medicine may cause the following problems:
    • Bleeding, which may be life-threatening
    • Gangrene (skin or tissue damage)
    • Calciphylaxis or calcium uremic arteriolopathy
    • Purple toes syndrome
  • You must have a PT/INR blood test while you use this medicine to check how well your blood is clotting. Your doctor will use the test results to make sure the medicine is working properly. Keep all appointments for the PT/INR blood tests.
  • You may bleed or bruise more easily with warfarin. To prevent injury or cuts, do not play rough sports, be careful with sharp objects, and brush and floss your teeth gently. Blow your nose gently, and do not pick your nose.
  • Carry an ID card or wear a medical alert bracelet to let emergency caregivers know that you use warfarin.
  • Tell any doctor or dentist who treats you that you are using this medicine. You may need to stop using this medicine several days before you have surgery or medical tests.

Possible Side Effects While Using This Medicine:

Call your doctor right away if you notice any of these side effects:

  • Allergic reaction: Itching or hives, swelling in your face or hands, swelling or tingling in your mouth or throat, chest tightness, trouble breathing
  • Bleeding from your gums or nose, coughing up blood, heavy monthly periods
  • Bleeding, pain, or swelling where the needle is placed
  • Bleeding that does not stop, bruising, or weakness
  • Dizziness, fainting, or lightheadedness
  • Pain, brown or black skin, or skin that is cool to the touch
  • Purple toes or feet, or new pain in your leg, foot, or toes
  • Purplish red, net-like, blotchy spots on the skin
  • Red or dark brown urine, or red or black stools
  • Vomiting blood or material that looks like coffee grounds

If you notice other side effects that you think are caused by this medicine, tell your doctor

Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088

Last Updated: 7/4/2018

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