Warfarin (Oral)

Warfarin is an anticoagulant drug which thins the blood to prevent it from clotting. It helps blood to flow smoothly through the body by decreasing the amount of clotting proteins in your blood.


Warfarin is an anticoagulant drug which is prescribed by doctors to thin the blood of patients with blood clots such as deep venous thrombosis or pulmonary embolism. It is also used to treat blood clots that have occurred from heart conditions, from open-heart surgery or as a result of a heart attack. As well as treat blood clots, it can also reduce the risk of developing blood clots. In the US it is sold and prescribed under the trade names Coumadin and Jantoven.

Preventing and/treating blood clots can help reduce the risk of a stroke or heart attack. Conditions that increase the risk of developing blood clots in the first place include an atrial fibrillation (irregular heart rhythm), recent heart attack, heart valve replacement and certain surgeries such as knee or hip replacements. Some patients may be prescribed warfarin in the form of Coumadin due to one of these reasons, such as atrial fibrillation, even though they have not yet developed a blood clot.

Because this medicine thins the blood, it can be dangerous for certain individuals, such as those prone to bleeding, those with cancer, with diabetes or those prone to accidents. It can also interact with other drugs, tobacco or foods in a way that undermines its efficacy. As a result, great care should be taken for those who have been prescribed a course of warfarin and it's important for patients to be as open and honest with their doctor as possible. Your doctor should know about every drug or medicine you currently take, both prescribed and over-the-counter, as well as any you have recently taken. This includes recreational drugs and herbal remedies.

Clotting, or thickening, is a complicated biological process which involves a number of substances/clotting factors. These clotting factors are produced in the liver and are made in order to help control bleeding. They work at a cellular level to assist cells to trigger the clotting process of platelets. This ensures that blood clots effectively and therefore prevents humans from losing too much blood.

In order to produce these clotting factors, the liver needs a steady supply of vitamin K.

Warfarin works by blocking one of these enzymes/proteins, the one that utilises vitamin K in order to produce clotting factors. The desired effect of the drug is that it disrupts the clotting process, making it take a longer time for the blood to clot.

Although it is also available as an intravenous drug, oral Warfarin comes in tablet form and is generally taken in single doses. Tablets can contain anything between 1-10 mg of active warfarin sodium.

Conditions treated

  • Blood clots such as deep venous thrombosis or pulmonary embolism.
  • Blood clots resulting in heart conditions, open heart surgery or after a heart attack.
  • Prevention of blood clots in those with atrial fibrillation or those who have had heart surgery.

Type of medicine

  • Anticoagulant

Side Effects

In addition to the desired effects, any medicine may also cause some unwanted side effects. One or more of the following side effects may occur, and if they do occur, they will require urgent medical attention. Seek advice from your doctor immediately if any of the following side effects occur:

Less common side effects:

  1. Abdominal/stomach pain with/without cramping
  2. Bleeding from the gums
  3. Blood in the urine
  4. Blood in the stools
  5. Blurry vision
  6. Burning, itching, crawling, numbness, prickling (pins and needles) or general tingling feelings
  7. Chest pain/discomfort
  8. Confusion
  9. Coughing-up blood
  10. Difficulty with breathing or swallowing
  11. Dizziness/faintness/lightheadedness when suddenly rising from a lying or sitting position
  12. Excessive bruising to skin
  13. Headache
  14. Increase in menstrual flow or vaginal bleeding
  15. Nosebleeds
  16. Paralysis
  17. Peeling of skin
  18. Prolonged bleeding from cuts
  19. Red/black tarry stools
  20. Red/dark brown urine
  21. Sweating
  22. Unexplained swelling
  23. Unusual tiredness/weakness

Rare side effects:

  1. Arm/back/jaw pain
  2. Blue-green to black skin discoloration
  3. Blue/purple toes
  4. Changes in consciousness
  5. Chest tightening or heaviness
  6. Chills
  7. Clay-colored stools
  8. Diarrhea
  9. Dizziness
  10. Fainting/loss of consciousness
  11. Fast/irregular breathing
  12. Fast/irregular heartbeat
  13. Feverishness
  14. Itching or skin rash
  15. Light-colored stools
  16. Loss of appetite
  17. Nausea and vomiting
  18. Pain in toes
  19. Pain/redness of the skin
  20. Pale skin
  21. Purplish red, blotchy spots on the skin
  22. Skin blisters
  23. Small red/purple spots on the skin
  24. Stomach pain
  25. Swelling of eyes or eyelids
  26. Tightness in the chest
  27. Troubled breathing with exertion
  28. Unpleasant breath/odor
  29. Unusual bleeding or bruising
  30. Upper right abdominal/stomach pain
  31. Vomiting blood
  32. Yellowing of the eyes and/or skin

Some of the side effects that may occur from taking warfarin do not require medical attention. These side effects may move along naturally during the course of treatment as your body adjusts and learns to accommodate the medicine. Your health care professional may be able to advise you on ways to reduce or even prevent some of these side effects. Check with your health care professional if any of the following side effects are bothering you or even if you simply have any questions about them:

Less common side effects:

  1. Joint pain
  2. Muscle pain

Rare side effects:

  1. Bloatedness
  2. Changes in taste, or bad, unusual, or unpleasant (after) tastes
  3. Cold intolerance
  4. Excess air or gas in the stomach or intestines
  5. Full-up feeling
  6. General feeling of discomfort/illness
  7. Hair loss or hair thinning
  8. Hives or welts appearing on skin
  9. Loss/lack of strength
  10. Pain - general
  11. Passing of excess gas
  12. Red, sore, or itchy skin
  13. Sores, welting, or blisters
  14. Unusual levels of drowsiness, dullness or a general feeling of sluggishness

There may also be some other side effects which occur in some patients and are not listed here. If you notice any other symptoms which you suspect may be side effects of the medicine, check with your healthcare professional.

Contact your doctor for medical advice about side effects. You can also report side effects to the FDA at 1-800-FDA-1088.


The dosage of this medicine will differ according to each patient. Always follow the orders given by your doctor or the directions for dosage found on the label.

The amount of medicine you take will depend on the strength of the medicine as well as the number of doses taken each day, the time between doses, and the length of time for which you have been prescribed the medicine. The following information simply demonstrates average doses of this medicine when taken orally. If your dose differs to the following information, continue to do as your doctor tells you unless you are told differently.

Average dosage of oral Warfarin is as follows:

For oral dosage (tablet) form in the prevention/treatment of blood clots:

Adult = To start, 2 to 5 milligrams (mg) as a single dose once a day. (Your doctor may need to adjust your dose. However, the dose is rarely higher than 10 mg per day.)

Children = Usage and dosage must only be determined by your doctor.

Missed Dose

If you miss a dose of oral warfarin, take it as soon as possible. However, if it is almost time for your next dose, then patients are advised to skip the missed dose and return to their regular dosing schedule. Never double dose.

During the course of taking this medicine, you must always alert your doctor or relevant healthcare provider immediately should you:

  • Take too much (more than the specified dose within a specified time period).
  • Fall sick with diarrhea, an infection, or have a fever.
  • Take a fall or suffer an injury, most importantly, should you hit your head.


Major Drug Interactions

It's important to note that any drug can interact with another drug inside the human body. This interaction can cause one or both medications to be less active or, worse, cause undesirable effects on the human body. This is why it's extremely important to keep a list of all medications and to inform your GP or prescribing doctor. This is not limited to prescription drugs, as your GP or prescribing doctor will also need to know if you are taking any over the counter medicines or drugs.

Taking Warfarin with any of the following drugs is not advisable and as a result, your doctor may choose not to prescribe Warfarin, or may change the other drugs you are taking.

  • Amifampridine
  • Defibrotide
  • Tamoxifen

Other possible Drug Interactions

Taking Warfarin with the following drugs is usually not recommended, however there may be cases when patients require both drugs. In these instances, your doctor may change the way you take one or more of them or adjust the dosage of one or both prescriptions.

  • Abciximab
  • Aceclofenac
  • Acemetacin
  • Acenocoumarol
  • Alefacept
  • Alipogene Tiparvovec
  • Alteplase, Recombinant
  • Amiodarone
  • Amoxicillin
  • Ampicillin
  • Amtolmetin Guacil
  • Anagrelide
  • Anistreplase
  • Apixaban
  • Aprepitant
  • Aspirin
  • Azithromycin
  • Bemiparin
  • Bivalirudin
  • Blinatumomab
  • Bromfenac
  • Bufexamac
  • Cabozantinib
  • Capecitabine
  • Carbenicillin
  • Carboplatin
  • Cefadroxil
  • Cefdinir
  • Cefepime
  • Cefixime
  • Cefotaxime
  • Cefpodoxime
  • Ceftaroline Fosamil
  • Ceftazidime
  • Ceftibuten
  • Ceftizoxime
  • Celecoxib
  • Cephalexin
  • Cephalothin
  • Cephapirin
  • Cephradine
  • Ceritinib
  • Chamomile
  • Choline Salicylate
  • Ciprofibrate
  • Ciprofloxacin
  • Citalopram
  • Clarithromycin
  • Clonixin
  • Clopidogrel
  • Cloxacillin
  • Cobicistat
  • Collagenase, Clostridium histolyticum
  • Cyclophosphamide
  • Dabigatran Etexilate
  • Dabrafenib
  • Dalteparin
  • Danaparoid
  • Dapsone
  • Deferasirox
  • Desvenlafaxine
  • Dexibuprofen
  • Dexketoprofen
  • Diclofenac
  • Dicloxacillin
  • Diflunisal
  • Dihydroartemisinin
  • Dipyridamole
  • Dipyrone
  • Doxifluridine
  • Doxorubicin
  • Doxorubicin Hydrochloride Liposome
  • Dronedarone
  • Drotrecogin Alfa
  • Droxicam
  • Econazole
  • Edoxaban
  • Efavirenz
  • Elvitegravir
  • Enoxacin
  • noxaparin
  • Entacapone
  • Enzalutamide
  • Eptifibatide
  • Erlotinib
  • Erythromycin
  • Escitalopram
  • Etodolac
  • Etofenamate
  • Etoposide
  • Etoricoxib
  • Etravirine
  • Felbinac
  • Fenofibrate
  • Fenofibric Acid
  • Fenoprofen
  • Fepradinol
  • Feprazone
  • Fish Oil
  • Floctafenine
  • Fluconazole
  • Flufenamic Acid
  • Fluorouracil
  • Fluoxetine
  • Flurbiprofen
  • Fluvoxamine
  • Garlic
  • Gatifloxacin
  • Gemifloxacin
  • Ginkgo
  • Golimumab
  • Ibuprofen
  • Iloprost
  • Imatinib
  • Indomethacin
  • Infliximab
  • Influenza Virus Vaccine
  • Itraconazole
  • Ketoconazole
  • Ketoprofen
  • Ketorolac
  • Leflunomide
  • Lepirudin
  • Levocarnitine
  • Levofloxacin
  • Levomilnacipran
  • Linezolid
  • Lomitapide
  • Lornoxicam
  • Loxoprofen
  • Lumiracoxib
  • Lycium
  • Marijuana
  • Mechlorethamine
  • Meclofenamate
  • Mefenamic Acid
  • Meloxicam
  • Mercaptopurine
  • Methicillin
  • Methotrexate
  • Methyl Salicylate
  • Metronidazole
  • Miconazole
  • Milnacipran
  • Mirtazapine
  • Morniflumate
  • Moxalactam
  • Moxifloxacin
  • Nabumetone
  • Nadroparin
  • Nafcillin
  • Nalidixic Acid
  • Nandrolone
  • Naproxen
  • Nepafenac
  • Niflumic Acid
  • Nilotinib
  • Nimesulide
  • Nimesulide Beta Cyclodextrin
  • Nintedanib
  • Norfloxacin
  • Noscapine
  • Obeticholic Acid
  • Ofloxacin
  • Oritavancin
  • Orlistat
  • Oseltamivir
  • Oxacillin
  • Oxandrolone
  • Oxaprozin
  • Oxyphenbutazone
  • Papaya
  • Parecoxib
  • Paroxetine
  • Peginterferon Alfa-2b
  • Penicillin G
  • Penicillin V
  • Pentosan Polysulfate Sodium
  • Phenindione
  • Phenprocoumon
  • Phenylbutazone
  • Piketoprofen
  • Piperacillin
  • Piracetam
  • Piroxicam
  • Pixantrone
  • Posaconazole
  • Pranoprofen
  • Prasugrel
  • Procarbazine
  • Proglumetacin
  • Proguanil
  • Propyphenazone
  • Proquazone
  • Regorafenib
  • Reteplase, Recombinant
  • Rivaroxaban
  • Rofecoxib
  • Ropinirole
  • Roxithromycin
  • Secnidazole
  • Secukinumab
  • Sertraline
  • Siltuximab
  • Simvastatin
  • Sitaxsentan
  • St John's Wort
  • Streptokinase
  • Sulfamethoxazole
  • Sulfisoxazole
  • Sulindac
  • Sultamicillin
  • Tan-Shen
  • Tegafur
  • Telithromycin
  • Tenecteplase
  • Tenoxicam
  • Teriflunomide
  • Testosterone
  • Tiaprofenic Acid
  • Ticarcillin
  • Ticlopidine
  • Tinzaparin
  • Tirofiban
  • Tocophersolan
  • Tolfenamic Acid
  • Tolmetin
  • Torsemide
  • Treprostinil
  • Urokinase
  • Valdecoxib
  • Valproic Acid
  • Venlafaxine
  • Vilazodone
  • Vincristine
  • Vincristine Sulfate Liposome
  • Vindesine
  • Vorapaxar
  • Voriconazole
  • Vortioxetine

Other Interactions

Certain foods or recreational drugs such as alcohol and tobacco can also interact negatively with any medicine, including warfarin. For example, diets heavy in Vitamin K, which is found in dark green leafy vegetables, may need to be adapted as vitamin K can reduce the effectiveness of warfarin. It is always recommended that you tell your GP and/or prescribing consultant about your diet and whether you smoke tobacco, drink alcohol or take any other recreational drugs. The following are not recommended to be taken or consumed with Warfarin and as a result, your doctor may change the dosage or the way you take your medicine and/or ask you to cut or reduce the following from your lifestyle.

  • Cranberry Juice
  • Grapefruit
  • Pomegranate
  • Tobacco
  • Green leafy vegetables (or other sources of Vitamin K)


Before taking warfarin, your doctor should know if you suffer from any of the following; bleeding problems, cancer, low blood count, have a tendency to fall often, liver problems, kidney problems or undergoing dialysis, high blood pressure, congestive heart failure, diabetes or if you are planning to have any surgery or a dental procedure in the future.

It's imperative that your doctor can check the progress of your health during regular visits in order to see whether the medicine is working as desired. Blood tests, for example an INR, are essential to check that patients have been given the proper dosage and to avoid unwanted side effects. Be sure to attend appointments.

Using this medicine during pregnancy may harm your unborn baby. Aim to use an effective form of birth control in order to prevent getting pregnant during the treatment and for at least one month after you have taken the last dose. If you suspect that you've become pregnant during the course of the medicine, it's important to tell your doctor straight away.

Make sure any medical professional such as a doctor or dentist who treats you is aware that you are taking this medicine. You may need to cease taking warfarin several days ahead of having surgery or certain medical tests.

Check in with your doctor immediately should you suffer from diarrhea, fever or any other symptoms that may indicate infection.

This medicine could cause skin necrosis or gangrene. Call your doctor straight away if you experience pain, a color change or temperature change to any part of your body. Call your doctor immediately if you experience pain in your toes and they also look purple or dark in color. These could all be signals of a serious medical problem.

Calciphylaxis (calcium uremic arteriolopathy) can occur in patients with, or without, end-stage kidney disease. Inform your doctor immediately should you experience purplish red, net-like/blotchy spots on your skin.

This medicine could increase your chances of bleeding. Consult your doctor immediately should you notice any unusual bleeding, bruising, black/tarry stools, blood in the urine or the stools or pinpoint red spots anywhere on your skin. Try to avoid picking your nose while taking this medicine. Should you need to blow your nose, try to blow it gently.

Be mindful of using regular toothbrushes, dental floss or toothpicks. Your general physician dentist or nurse may choose to recommend alternative, gentler ways to clean your teeth and gums. Ensure you consult your doctor before having any dental work carried out.

Be mindful to avoid cutting yourself when using sharp objects, such as nail cutters, razors, scissors or kitchen knives. Try to avoid engaging in any sort of contact sports or indeed other situations where injury or bruising could occur.

It is recommended that those taking Warfarin carry identification which states this fact. Check with your doctor or nurse should you have any queries regarding the kind of identification that you should carry around with you.

Never stop taking your medicines or start any new over-the-counter OR prescription medicines unless you have discussed this with your general practitioner/family doctor. Retain a list of your current and recent medicines with you all the time. This list must include both prescription medicines and non-prescription/over-the-counter medicines. Ideally, it should also include details of vitamin supplements, herbal medicines and/or complementary therapies such as Chinese Medicine, Homeopathy or Bachs Flower Remedies.


Store your Warfarin prescription in a sealed container and ideally keep it at room temperature. Make sure it is kept well away from heat sources, away from moisture and away from sources of direct light. Also try to prevent the medicine from freezing.

Always keep medicines out of the reach of children.

Never keep out of date medicine or medicine that is no longer needed. If you are not sure about how to get rid of old/unnecessary medicine, then ask your healthcare professional as to how you should dispose of any Warfarin that you no longer need or has passed its use by date.


Warfarin, prescribed as Coumadin or Jantoven in the US, is an anticoagulant drug which is widely used to treat or prevent further blood clots or to prevent blood clots in those with increased chances of one occurring. Bodies need blood to clot in order to prevent loss of blood, but in some instances, this clotting process can cause potentially dangerous blood clots. Warfarin Sodium is the active ingredient in the medicine, which works by reducing the number of clotting proteins in the blood which allows blood to flow more freely around the body. This medicine can only be prescribed by a doctor and use of it should be strictly regulated and monitored. Those who are predisposed to bleeding or have other underlying medical conditions may not be able to use Warfarin. This includes those with kidney disease, cancer, diabetes and The drug can be used to treat clots such as DVT as well as pulmonary embolisms. Clots that have arisen from heart conditions, strokes, heart attacks or heart surgery, as well as knee or hip surgery, can also be treated with Warfarin. Patients with Atrial Fibrillation may also be prescribed warfarin as a means of preventing the chances of a blood clot, which is more likely in these individuals blood can build in pools in the heart. Also patients with thrombophilia, a blood clotting disorder, as well as those with a mechanical heart valve.

Untreated blood clots can cause strokes or heart attacks, which means treating and preventing blood clots may represent lifesaving medical treatment.

It's essential that anyone prescribed with this drug informs their doctor of ALL other drugs they have or are taking and of any pre-existing condition/s. Warfarin can interact with other drugs, foods and tobacco in a way that reduces its effectiveness. While taking warfarin it's essential that patients avoid instances where they may be cut or injured, as well as dental work or surgery. It's also important to take effective birth control and to avoid getting pregnant. Any patient who develops signs of kidney failure, or notices possible changes in kidney function, develops signs of skin necrosis or gangrene, or notices purpling of the toes or who suffers from heavy bleeding or has an injury should alert their doctor immediately. Those prescribed with this medicine will need to have their condition constantly monitored.

Last Reviewed:
December 25, 2017
Last Updated:
April 04, 2018
Content Source: