Warfarin is used to prevent or treat blood clots, including deep venous thrombosis or pulmonary embolism. It is also used for blood clots that may be caused by certain heart conditions, open-heart surgery, or after a heart attack. Warfarin is an anticoagulant (blood thinner) that decreases the clotting ability of the blood.

This medicine is available only with your doctor's prescription.

Take this medicine only as directed by your doctor. Your dose may need to be changed several times in order to find out what works best for you. Do not use more of it, do not use it more often, and do not use it for a longer time than your doctor ordered.

This medicine should come with a Medication Guide. Read and follow these instructions carefully. Ask your doctor if you have any questions.

Carefully follow your doctor's instructions about any special diet. This medicine works best when you eat about the same amount of vitamin K in your food every day. Tell your doctor before changing your diet. Avoid big changes in how much vitamin K you eat. Some foods that have a high amount of vitamin K are asparagus, broccoli, brussels sprouts, cabbage, green leafy vegetables (such as collards, turnip greens, mustard greens, spinach, and salad greens), plums, rhubarb, and certain vegetable oils (such as soybean oil and canola oil).

You may take the tablets on a full or empty stomach.

Do not drink grapefruit juice while you are using this medicine. Grapefruit juice may change the amount of this medicine that is absorbed in the body.

Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur:

Less common

Bleeding gums

blood in the urine

bloody stools

blurred vision

burning, crawling, itching, numbness, prickling, "pins and needles", or tingling feelings

chest pain or discomfort

confusion

coughing up blood

difficulty with breathing or swallowing

dizziness, faintness, or lightheadedness when getting up suddenly from a lying or sitting position

excessive bruising

headache

increased menstrual flow or vaginal bleeding

nosebleeds

paralysis

peeling of the skin

prolonged bleeding from cuts

red or black, tarry stools

red or dark brown urine

stomach pain with cramping

sweating

unexplained swelling

unusual tiredness or weakness

Rare

Arm, back, or jaw pain

blue-green to black skin discoloration

blue or purple toes

change in consciousness

chest tightness or heaviness

chills

clay-colored stools

diarrhea

dizziness

fainting or loss of consciousness

fast or irregular breathing

fast or irregular heartbeat

fever

itching or skin rash

light-colored stools

loss of appetite

nausea and vomiting

pain in the toes

pain, redness, or sloughing of the skin

pale skin

purplish red, net-like, blotchy spots on the skin

skin blisters

small red or purple spots on the skin

stomach pain

swelling of the eyes or eyelids

troubled breathing with exertion

unpleasant breath odor

unusual bleeding or bruising

upper right stomach pain

vomiting of blood

yellow eyes and skin

Some side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

Less common

Joint pain

muscle pain

Rare

Bloated

change in taste, or bad, unusual, or unpleasant (after) taste

cold intolerance

excess air or gas in the stomach or intestines

full feeling

general feeling of discomfort or illness

hair loss or thinning of the hair

hives or welts

lack or loss of strength

pain

passing gas

red, sore, or itching skin

sores, welting, or blisters

unusual drowsiness, dullness, or feeling of sluggishness

Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.

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

It is very important that your doctor check your progress at regular visits to make sure this medicine is working properly. Blood tests, such as an INR, are needed to check for proper dosage and unwanted side effects. Be sure to keep all appointments.

Using this medicine while you are pregnant can harm your unborn baby. Use an effective form of birth control to keep from getting pregnant during treatment and for at least 1 month after the last dose. If you think you have become pregnant while using this medicine, tell your doctor right away.

Make sure any doctor or dentist who treats you knows that you are using this medicine. You may need to stop using this medicine several days before having surgery or medical tests.

Check with your doctor immediately if you have diarrhea, fever, or any symptoms of an infection.

This medicine may cause skin necrosis or gangrene. Call your doctor right away if you have pain, a color change, or a temperature change to any area of your body. Call your doctor right away if you have pain in your toes and they look purple or dark in color. These could be signs of a serious medical problem.

Calciphylaxis or calcium uremic arteriolopathy may occur in patients with or without end-stage kidney disease. Tell your doctor right away if you have purplish red, net-like, blotchy spots on the skin.

Warfarin may increase your risk of having kidney problems, including acute kidney injury. Check with your doctor right away if you have blood in the urine, decreased urine output, muscle twitching, nausea, rapid weight gain, seizures, stupor, swelling of the face, ankles, or hands, or unusual tiredness or weakness.

This medicine may increase your chance of bleeding. Check with your doctor right away if you notice any unusual bleeding or bruising, black, tarry stools, blood in the urine or stools, or pinpoint red spots on your skin. Avoid picking your nose. If you need to blow your nose, blow it gently.

Be careful when using a regular toothbrush, dental floss, or toothpick. Your medical doctor, dentist, or nurse may recommend other ways to clean your teeth and gums. Check with your medical doctor before having any dental work done.

Be careful not to cut yourself when you are using sharp objects, such as a safety razor or fingernail or toenail cutters. Avoid contact sports or other situations where bruising or injury could occur.

It is recommended that you carry identification that says you are using warfarin. If you have any questions about what kind of identification to carry, check with your doctor.

Do not take other medicines unless they have been discussed with your doctor. This includes prescription medicines, nonprescription (over-the-counter [OTC]) medicines, and herbal or vitamin supplements.