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Warfarin - an anticoagulant

Warfarin is an anticoagulant. It will be prescribed if you have an unwanted clot in your blood, or if you are at risk of having an unwanted clot.

You may be given a yellow anticoagulant treatment booklet; read this carefully and carry it with you at all times.

You will need to have regular blood tests to measure how quickly your blood clots.

What you eat and drink can affect your treatment. Do not change your diet without discussing it with your doctor first; do not drink cranberry juice; and only drink moderate amounts of alcohol.

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About warfarin

Type of medicine


Used for

Prevention and treatment of harmful blood clots

Available as

Tablets and oral liquid medicine

Warfarin is an anticoagulant, which means that it increases the time it takes for your blood to clot. It works by reducing the effects of vitamin K, which is a vitamin your body uses in the process of blood-clotting.

Warfarin is used to prevent unwanted clots from forming if you have a condition that puts you at risk of this happening, such as atrial fibrillation. It is also used to prevent any clots that may have already formed in the blood vessels of your legs, lungs or heart from becoming larger and causing problems.

Warfarin is currently the most prescribed anticoagulant although newer anticoagulants are increasingly being prescribed.

Before taking warfarin

Some medicines are not suitable for people with certain conditions, and sometimes a medicine may only be used if extra care is taken. For these reasons, before you start taking warfarin it is important that your doctor knows:

  • If you are pregnant, trying for a baby or breastfeeding.

  • If you have any cuts or wounds.

  • If you have a condition that increases your risk of bleeding, such as a stomach ulcer, or if you have had surgery recently, or if you have recently had a stroke.

  • If you have any problems with the way your liver works, or with the way your kidneys work.

  • If you have high blood pressure (hypertension).

  • If you have been told you have an infection of your heart, called bacterial endocarditis.

  • If you are taking or using any other medicines. This includes any medicines you are taking which are available to buy without a prescription, as well as herbal and complementary medicines. It is particularly important that you tell your doctor if you are also using an anti-fungal preparation containing miconazole, as this medicine can affect the way warfarin works. Miconazole is an ingredient in creams, ointments, sprays, and powders used for fungal skin infections such as athlete's foot, and also in gels and creams for thrush.

  • If you have ever had an allergic reaction to a medicine.

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How to take warfarin

  • Before you start the treatment, read the manufacturer's printed information leaflet from inside the pack of tablets and any additional information you have been given by your doctor. These will give you more information about warfarin and will provide you with a full list of the side-effects which you may experience from taking it.

  • Take warfarin exactly as you have been advised by your doctor or anticoagulant clinic. You should aim to take warfarin at the same time each day. This will help keep the levels of warfarin in your blood steady, and will also help you to avoid missing any doses. It is usually recommended that you take warfarin at six o'clock in the evening.

  • Warfarin tablets are available in different strengths. Your dose may be made up of more than one strength of tablet. Each strength of tablet is a different colour to help you tell the difference between them; 0.5 mg tablets are white, 1 mg tablets are brown, 3 mg tablets are blue and 5 mg tablets are pink.

  • If you have been prescribed the oral liquid medicine you will need to take 1 ml for every 1 mg of your dose - for example, a 3 mg dose is 3 ml of liquid. Use an oral syringe to measure your dose carefully.

  • If you forget to take a dose, take it as soon as you remember. If you do not remember until the following day, skip the missed dose. Do not take two doses together to make up for a forgotten dose, and remember to let your doctor know about any missed doses when you next have your blood test.

  • Continue to take warfarin tablets regularly until your doctor tells you to stop. A course of treatment typically lasts from six weeks to three months, although some people may be advised to continue taking warfarin for longer than this.

Getting the most from your treatment

  • You will be given a yellow anticoagulant treatment booklet; read this carefully as it gives you information about when you should contact a doctor for advice. Carry it with you at all times in case of an emergency and a doctor needing to know that you are on warfarin, and at what dose.

  • You will need regular blood tests to check on how quickly your blood clots when you are taking warfarin. Blood tests may be needed quite often at first, but should reduce in frequency quite quickly. The extent to which warfarin is working is measured by the International Normalised Ratio (INR), which is a measure of the ability of your blood to prevent clotting. The amount of warfarin that you need to take will depend upon the result of these blood tests, and this is why your dose may change from time to time. The aim is to get the dose of warfarin just right so your blood does not clot as easily as normal, but not so much as to cause bleeding problems.

  • Changing your diet suddenly can affect your INR, especially if you begin to eat more vegetables and salad. You should not begin a weight-reducing diet without discussing it with your doctor first. A major change in diet may mean that you need closer monitoring and may need a change in warfarin dose.

  • Only drink alcohol in small amounts, as this can affect the levels of warfarin in your body. Limit the amount of alcohol that you drink to a maximum of one or two units in any day, and never binge drink.

  • Drinking cranberry juice can interfere with warfarin and affect your INR, so it is best if you avoid cranberry juice altogether.

  • If you buy any medicines, check with a pharmacist that they are safe to take with warfarin. For example, you should not take some painkillers (such as aspirin and other anti-inflammatory painkillers) and some vitamin and herbal preparations while you are on warfarin. If you need to take a painkiller, you may take paracetamol, but you should let your doctor know if you need to take it regularly.

  • Because warfarin is used to prevent blood clots from forming, you should try to avoid activities that could cause you to cut or bruise yourself, such as contact sports. Let your doctor know if you have any falls or injuries.

  • You should avoid getting pregnant while you are taking warfarin. Make sure you have discussed with your doctor which types of contraception are suitable for you and your partner.

  • If you are due to have any injections, or any medical or dental treatment, tell the person carrying out the treatment that you are taking warfarin, and show them your anticoagulant booklet. It is important that they know you may take longer to stop bleeding. If you are due to have surgery, you may be advised by the clinic to stop taking warfarin for a few days beforehand.

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Can warfarin cause problems?

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the more common ones associated with warfarin - the most common are bleeding and bruising. You will find a full list in the manufacturer's information leaflet supplied with your medicine. Speak with your doctor or pharmacist if any of the following continue or become troublesome.

Warfarin side-effects

What can I do if I experience this?

Unusual bleeding or bruising, blood in your urine or stools (faeces)

See your doctor straightaway - your dose of warfarin will probably need to be reduced


Drink plenty of water to replace lost fluids

Skin rash, painful or 'purple' toes

Speak with your doctor about this

Hair loss

If this becomes troublesome, speak with your doctor

Feeling sick (nausea) or being sick (vomiting)

Stick to simple meals until this passes

Painful or tender abdomen, yellowing of your skin or the whites of your eyes (jaundice)

Speak with your doctor about this straightaway

If you experience any other symptoms which you think may be due to the tablets, speak with your doctor or pharmacist for further advice.

How to store warfarin

  • Keep all medicines out of the reach and sight of children.

  • Store in a cool, dry place, away from direct heat and light.

  • The oral liquid medicine lasts for 28 days after first opening, after that time you will need a fresh supply. It is a good idea to write on the label the date you opened the bottle.

Important information about all medicines

Important information about all medicines

Never take more than the prescribed dose. If you suspect that you or someone else might have taken an overdose of this medicine, go to the accident and emergency department of your local hospital. Take the container with you, even if it is empty.

This medicine is for you. Never give it to other people even if their condition appears to be the same as yours.

Do not keep out-of-date or unwanted medicines. Take them to your local pharmacy which will dispose of them for you.

If you have any questions about this medicine ask your pharmacist.

Report side effects to a medicine or vaccine

If you experience side effects, you can report them online through the Yellow Card website.

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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