Warfarin (Injection)


Warfarin (WAR-far-in)

Helps to prevent new blood clots from forming, and helps to keep existing blood clots from getting worse. This medicine is a blood thinner (anticoagulant).

Brand Name(s)


When This Medicine Should Not Be Used

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 your treatment to an oral medicine that works the same way. If you have any concerns about this, talk to your doctor.

Drugs and Foods to Avoid

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

  • Diet and other medicines can affect the PT/INR level.
    • Always tell your doctor before you start or stop any other medicine, even if you can buy the medicine without a prescription (over-the-counter). This also includes vitamins and herbs such as ginkgo, echinacea, and St John's wort.
    • Always tell your doctor before you make big changes to your regular diet. Food and drink changes that can change your PT/INR include cranberry, grapefruit, and anything that has high amounts of vitamin K. Asparagus, broccoli, brussels sprouts, cabbage, green leafy vegetables (such as collards, turnip greens, mustard greens, spinach, and salad greens), plums, rhubarb, and some vegetable oils (such as soybean oil and canola oil) are all high in vitamin K. Warfarin works best if you eat and drink about the same amount of vitamin K every day.
  • Do not take other medicines that also contain warfarin. Too much warfarin may cause serious bleeding problems.
  • There are many other medicines that you should not use while you are using warfarin. These include many herbs, supplements, and over-the-counter medicines, including NSAIDs (nonsteroidal anti-inflammatory drugs) such as aspirin, ibuprofen, naproxen, Advil®, Aleve®, or Motrin®. Check the labels of all medicines to be sure they do not contain NSAIDs.

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.
  • Make sure your doctor knows if you are breastfeeding, or if you have kidney disease, liver disease, congestive heart failure, high blood pressure, diabetes, any type of infection, or bleeding problems. Tell your doctor if you have had recent surgery or injury, protein C deficiency, or a history of problems caused by heparin.
  • You must have a blood test such as PT/INR to make sure this medicine is working and is not causing problems. Your doctor will tell you how often you need to have the test done. It is very important that your doctor checks your progress at regular visits. Keep all appointments.
  • Diet and other medicines can affect the PT/INR level. Do not start or stop other medicine or change your diet without telling your doctor first.
  • Tell your doctor right away if you get hurt or start to have diarrhea, fever, or any signs of infection.
  • Carry an ID card or wear a medical alert bracelet to let emergency caregivers know that you use warfarin.
  • This medicine may cause skin or tissue damage (gangrene). Call your doctor right away if you have pain, color change, or temperature change in any part of your body. Call your doctor right away if you have a pain in your toes and they look purple or dark. This problem must be treated right away to avoid permanent damage.
  • Make sure any doctor or dentist who treats you knows that you are using this medicine. Your doctor might tell you to change the dose or stop using this medicine for a short time. Do not change anything unless your doctor tells you to.
  • Keep a list of your medicines with you at all times.
  • You may bleed and bruise more easily while you are using this medicine. Stay away from rough sports or other situations where you could be bruised, cut, or injured. Brush and floss your teeth gently. Be careful when using sharp objects, including razors and fingernail clippers. Avoid picking your nose. If you need to blow your nose, blow it gently.

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, bruising easily, or coughing up blood
  • Blistering, soreness, or redness of the skin
  • Chest pain, trouble breathing
  • Decrease in how much or how often you urinate
  • Dizziness, fainting, or lightheadedness
  • Heavy menstrual bleeding, or bleeding from cuts or wounds that does not stop
  • Numbness, tingling, or weakness anywhere in your body
  • Pain, color change, or temperature change in any area of your body
  • Painful, prolonged erection of your penis
  • Purple discoloration of your toes or the soles of your feet, or new pain in a leg, foot, or your toes
  • Red or dark brown urine, or red or black stools
  • Swelling, pain, or bleeding where the needle is placed
  • Swelling in your hands, ankles, or feet
  • Unusual bleeding, bruising, or weakness
  • Vomiting blood or material that looks like coffee grounds
  • Yellow skin or eyes

If you notice these less serious side effects, talk with your doctor:

  • Headache, hair loss
  • Mild nausea

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

Version Info

  • Last reviewed on 8/4/2014

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