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Anticoagulants, warfarin

Page last reviewed: 13/07/2011

Warfarin is an anticoagulant. This is a medicine that stops blood from clotting.

It can be given to people if they are at risk of developing a blood clot (thrombosis) or if they have developed a clot and need treatment.

Warfarin is used to treat and prevent the following health conditions:

  • deep vein thrombosis (blood clots in the veins of the legs)
  • pulmonary embolism (a blood clot in the lungs)
  • some transient ischaemic attacks (TIA or 'mini-stroke') or strokes, especially those caused by the irregular heart beat known as Atrial Fibrillation

How long you will take warfarin depends on the condition for which it has been prescribed. If you are not sure, ask your GP.

You will also be given a yellow booklet on anticoagulants, which explains your treatment.

How it works

The blood needs vitamin K to be able to clot. Warfarin interrupts the activation of vitamin K in the body, which increases the time it takes for your blood to clot. This is sometimes described as 'thinning the blood'.

Warfarin helps your blood to flow freely around your body and stops any clots forming in the heart or in the blood vessels.


Warfarin is the main oral anticoagulant used in Ireland (oral means that it is taken by mouth). There are a number of newer oral anticoagulants such as Dabigatran (brand name Pradaxa), Rivaroxaban (brand name Xarelto) and Apixiban (brand name Eliquis) that have recently become available which may be used as an alternative for some indications.

Page last reviewed: 13/07/2011


While taking warfarin, you must have a regular blood test called an INR (international normalised ratio) test. This measures how long it takes your blood to clot. The results of the INR tests will determine the dose of anticoagulant you need to take (see box, right).

Because your warfarin dose can change after an INR test, you will be given different strengths of warfarin tablets to keep at home.

It is important that you are familiar with the different strengths and colours of these tablets and know which tablets you need to take to make up your dose.

The strengths and colours of warfarin tablets are:

  • 1mg: brown tablets
  • 3mg: blue tablets
  • 5mg: pink tablets

Warfarin is taken once a day, usually in the evening. It is important to take your dose at the same time each day.

When to avoid warfarin

Warfarin should be avoided if you have:

  • severe hypertension (high blood pressure)
  • a peptic ulcer (an ulcer anywhere in the digestive tract)
  • bacterial endocarditis (infection of the heart lining and heart valves)

Warfarin should also be avoided if you are pregnant (see below).

Using warfarin with caution

Warfarin should be taken with caution if you have:

  • bleeding problems, such as haemophilia (a blood clotting disorder)
  • liver problems
  • kidney problems

Warfarin should also be taken with caution if you are elderly or if you have recently had surgery.

Pregnancy and breastfeeding

Warfarin can affect the development of a baby in early pregnancy, so it is not routinely used during pregnancy.

If you are already taking warfarin and think you may be pregnant, speak to your GP urgently.

You can usually take warfarin when you are breastfeeding. However, discuss this with your GP or midwife.

Getting the dose right

  • If the warfarin dose is too low, it may not protect you against blood clots and you may need an injection of heparin. If a blood clot forms, it could break off and travel through the blood vessels to the lungs or brain. This clot can cause life-threatening problems.
  • If the warfarin dose is too high, you could be at risk of bleeding, which can cause serious or life-threatening problems. To reverse the effect of warfarin, you may need to take vitamin K.

Page last reviewed: 13/07/2011

The most serious side effect of warfarin is bleeding. You must seek medical attention and have an urgent blood test if you experience any of the following:

  • passing blood in your urine or faeces
  • passing black faeces
  • severe bruising
  • long nosebleeds (lasting longer than 10 minutes)
  • bleeding gums
  • blood in your vomit or coughing up blood
  • unusual headaches
  • (in women) heavy or increased bleeding during your period, or any other bleeding from the vagina

You must seek immediate medical attention if you:

  • are involved in a major accident
  • experience a significant blow to the head
  • are unable to stop any bleeding

Less common side effects

Less common side effects of warfarin include:

  • rashes
  • nausea (feeling sick)
  • vomiting
  • diarrhoea

Page last reviewed: 13/07/2011

When two or more medicines are taken at the same time, the effects of one of the medicines can be altered by the other. This is known as a drug-drug interaction. Warfarin can interact with many medicines.

If you want to check that your medicines are safe to take with warfarin, ask your doctor or local pharmacist, or read the patient information leaflet that comes with your medicine.

Lots of herbal medicines and supplements can interact with warfarin. Do not start taking any new herbal medicine or supplement without checking with your anticoagulant clinic, GP or pharmacist.

Aspirin and other painkillers

If you are on warfarin:

  • do not take aspirin or drugs that contain aspirin because bleeding may happen
  • do not take ibuprofen
  • you can take paracetamol but do not take more than the recommended dose

Interactions with food

Some foods, such as spinach and broccoli, contain vitamin K. This can interfere with warfarin if you eat lots of it. It is best to eat regular small amounts.

Losing or putting on weight can affect your INR (see Considerations when taking warfarin for more information), so you will need to be weighed regularly.

Interactions with alcohol

  • It is dangerous to binge drink or get drunk while taking warfarin. Doing this may increase the effect of warfarin and so increase the risk of bleeding.
  • the recommended weekly limit of alcohol consumption is 21 standard drinks for men and 14 for women.
  • One standard drink is roughly equivalent to half a pint of beer , a pub measure ( of a spirit such as vodka, or a small glass of wine.
  • People with liver disease who are taking warfarin should not drink alcohol.

Page last reviewed: 13/07/2011

Warfarin is taken once a day, usually in the evening. It is important to take your dose at the same time each day.

Missed dose: evening

If you forget to take your dose of warfarin in the evening but remember before midnight on the same day, take the missed dose. If midnight has passed, it is probably best to leave that dose.

Make a note that you have missed a dose and take your normal dose the next day at the usual time.

Missed dose: morning

If you usually take your warfarin in the morning and have forgotten to take it, the general advice is as follows:

  • If it is less than two hours late, take the dose as soon as you remember and then continue as normal.
  • If it is more than two hours late, take the dose as soon as you remember and then continue as normal. However, if it is time to take your next dose, leave the missed dose. Never take a double dose to catch up (unless your GP has specifically advised this).

If you are not sure what to do if you have missed a dose, ask your GP or anticoagulant clinic.

Extra doses or wrong doses

If you accidentally take an extra dose or take the wrong dose of warfarin, contact your GP or anticoagulant clinic for advice.

Check the patient information leaflet that comes with your medicine. This should give you advice about what to do.

Page last reviewed: 13/07/2011

What do I do if I have a nose bleed?

If you have a nosebleed, carry out normal first aid:

  • lean your head forward
  • pinch just below the bridge of your nose firmly for at least 10 minutes

If the nosebleed lasts longer than 15 minutes or you have regular nosebleeds, contact your anticoagulant nurse to get your INR checked.

What happens if I need an operation or teeth taken out?

Due to the risk of bleeding, your dose of warfarin may have to be lowered or stopped a few days before an operation or removal of teeth. You must tell the surgeon or dentist you are on warfarin. Also, tell your anticoagulant nurse if you need an operation as soon as possible so they can make arrangements.

Can I have normal vaccinations?

Yes, but injected vaccines may:

  • be given into the muscle with gentle pressure applied to the site for 2 minutes afterwards
  • be given under the skin

Can I play sports?

Yes, but due to the risk of bleeding:

  • sports such as football, rugby, cricket and hockey are best avoided if played competitively
  • martial arts and kickboxing must be avoided

Non-contact sports such as running, athletics, cycling and racquet sports can be played. Wear the right protective clothing, such as cycle helmets and knee padding.

Try to lead as normal a life as possible.

Do I need to tell my school?

You must tell your school if you are on warfarin so that they know:

  • that you should avoid contact sports
  • how to care for you if you start bleeding
  • when to contact your parents or guardian

Can I have a body piercing?

It is not a good idea to have a body piercing because of the increased risk of bleeding and the risk of infection.

Can I still go on holiday?

If you are going on holiday, in this country or abroad, tell your anticoagulant nurse and arrange to have your INR checked just before you go.

If you are away for longer than a month, you may need to arrange to have your INR checked locally.

Make sure you have enough warfarin tablets to last your trip.

Will girls have problems with periods and pregnancy?

Due to the risk of bleeding, periods may be heavy and last longer than normal. There are drugs that can help reduce blood loss during these times.

It is important that all girls taking warfarin are told about the importance of contraception as warfarin can be harmful to a baby, especially in the first 12 weeks of pregnancy.

If you are planning a family in the future, you must discuss it with your doctor. Special arrangements will be made for anticoagulant care.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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