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Warfarin is a drug that can be given if there is a risk of a blood clot forming or a clot has developed and treatment is needed to prevent further clots.

Warfarin is an anticoagulant tablet or liquid suspension taken by mouth which works by increasing the time taken for your blood to clot, therefore preventing formation of blood clots.

The coagulation (blood clotting) system

The human body depends on the blood circulating all the time to provide its organs such as the heart, brain and skin with oxygen and nourishment. To keep the blood flowing well, it needs to be of a certain consistency (not too runny and not too thick) and it needs to be able to plug any leaks that may occur, like when you cut your finger.

The wall of the damaged blood vessel is sealed off by a tiny blood clot (sometimes called a thrombus). This clot is formed as a result of proteins that circulate in the blood in a dissolved form. When an injury occurs they travel there to form a blood clot. This is a natural process and necessary to stay in good health.

If a clot forms when it shouldn't it can be carried in the blood to other parts of the body. This is called an embolism and this can then lead to serious problems, such as a stroke. This is the reason for the blood thinning treatment.

How warfarin works

Warfarin works by slowing the blood clotting process down. It is a 'Vitamin K antagonist' which means it prevents the clotting proteins from doing their job fully. Vitamin K is naturally present in food and milk (read more about diet below) and is used to treat overdoses of warfarin therapy or in people who have a high INR (clotting level).

You may have noticed that once you are on warfarin if you cut yourself it takes a little longer to stop bleeding. Everybody reacts differently to warfarin so the dose must be worked out each time for every patient.

The effects of warfarin can last quite a long time in the body but it takes about three to four days to see these effects. This means that a change in dose will not be reflected in the blood test until this time. All this needs to be taken into account when deciding on the dose of warfarin.

The only way to test how the treatment is working is by taking a blood sample and seeing how long it takes the blood to clot.

Why you might need to take warfarin

With some diseases or medical problems blood clots are produced more easily than normal. These diseases include:

  • cardiomyopathy (heart muscle problem)

  • following insertion of artificial heart valves

  • some heart operations, for instance the Fontan procedure or TCPC operation

  • previous clots or embolisms

The International Normalised Ratio (INR) test

Measuring the INR

The unit of measurement used is called the International Normalised Ratio (INR). This is simply a numerical scale.

Normal measurement – about 1.

The higher the number, the longer it takes the blood to clot.

Warfarin and target INR

We will try to keep your blood clotting to within a certain 'target' number depending on the medical condition requiring warfarin. This is known as the 'target INR'. The most common 'targets' are: two to three units of INR for Fontan type procedures (TCPC), and 2.5 to 3.5 units of INR for artificial valves. Many other medical conditions have a target of 2 to 3 units of INR. We will tell you the target INR required for your warfarin therapy.

Although we try to keep the INR within this target range, many things can affect how warfarin works in the body, such as other medications, diet and ill health. Many children and young people will not be in their range 100 per cent of the time – this is why regular blood tests are necessary.

Frequency of blood tests

A blood test is required regularly to check that your INR level is within the target range. Any adjustment in dose will require a blood test shortly afterwards. If the INR is too low, a dose may need to be increased or if the INR is too high, the anti-coagulation team may decide to miss a dose.

How is warfarin given?

Warfarin is given by mouth as a tablet. Tablets are scored and can be halved with a tablet cutter or a knife for half or quarter doses. For children, who cannot swallow tablets, or who need small dose increments, an oral liquid is available from Rosemont Pharmaceuticals. If your child is taking the liquid warfarin, shake the bottle well before use.

Warfarin tablets available:

  • 0.5mg white

  • 1mg brown

  • 3mg blue

  • 5mg pink

Please do not adjust your warfarin dose without the advice of the anticoagulation team managing your treatment.

What are the side effects of warfarin?

Too much warfarin

Haemorrhage (severe bleeding) is the major side effect of warfarin. Seek urgent medical attention for major bleeding.

  • a high INR (>5)

  • heavily bleeding gums

  • heavy periods

  • bruises for no reason

  • nosebleeds that last longer than 10 minutes or frequent nose bleeds

  • blood in the urine

  • 'coffee ground' vomit or passing of black faeces (poo)

If you see any of these signs or other unexpected or unexplained signs you should do an INR check and ask your healthcare provider/anticoagulation team for advice.

Too little warfarin (signs of clotting)

  • low INR (<1.5)

More serious symptoms would be:

  • pains in the legs or chest can be a symptom of a clot thrombosis or embolism

  • breathlessness

Other side effects of warfarin include:

  • hypersensitivity

  • rash

  • alopecia (hair loss)

  • diarrhoea

  • nausea (feeling sick)

  • vomiting (being sick)

  • changes to skin circulation resulting in ‘purple toes’ or skin necrosis (darkened skin)

  • jaundice (yellowish tinge to skin colour)

Effects of other drugs on warfarin

The drugs you take regularly that are prescribed will have already been taken into consideration. Any new medicines may interact with warfarin and affect the INR level, such as some antibiotics or medicines you buy from the pharmacy for a cold. 

Some other sorts of medicine you may buy over the counter at the chemist can also affect the INR level. For example, when you have a cold you may want to take some pain killers or other medicines. You must mention to the chemist that you already take warfarin as some medication may interact with it.

You should also check if you wish to use any herbal or homeopathic treatments as some also interact with warfarin. Some drugs stop warfarin from working as effectively, but some increase its effectiveness.

Some common drugs that will affect the INR

  • Some heart drugs such as amiodarone.

  • Some analgesics (pain killers); such as long term use of paracetamol.

  • Aspirin, although they should not be given to children under 16 years of age except on advice of a healthcare professional.

  • Ibuprofen.

  • Some antibiotics such as erythromycin and clarithromycin.

  • The flu vaccine - side effects of the flu vaccine can include flu-like symptoms and it is advisable to check your INR level a few days afterwards to check it has not been affected.

  • Vitamins.

  • Steroids.

This is not a complete list, it is important to check with a pharmacist if buying ‘over the counter’ medicines or your prescriber when starting any new medicines. Make sure your anticoagulation team are aware when you start any new medicines so that they can advise you if and when an INR check is needed.


Any change in lifestyle or wellbeing may also influence the effectiveness of warfarin therapy. An example of this may be going on holiday to a place where there is a big change in climate or diet. 

If you are unwell, for instance, with flu, sickness or diarrhoea, or start any new medication you should inform your anticoagulation team and check your INR. They will advise you about the best time to check it and any dose adjustments if necessary.

You should also try to stick to a fixed time of day for taking your warfarin medicine- between 6pm and 7pm is ideal.


Some foods that you eat every day contain vitamin K which is involved in the blood clotting process. It does not really matter how much vitamin K you eat when you start warfarin as the dose is worked out around it.

Foods rich in vitamin K include; green leafy vegetables such as cabbage, brussel sprouts, broccoli and spinach. Avocado, soya bean products, Swiss hard cheese and cranberry juice are also rich in Vitamin K. Cranberry juice is known to affect warfarin levels so we suggest you avoid it.

You do not need to limit your diet, however binge (short-term) diets are not recommended as major changes in diet may have an effect on warfarin levels. You should eat a healthy balanced diet like everyone else.

Babies who are breast fed may need lower doses of warfarin than those who are bottle fed with formula milk, as formula milk contains some Vitamin K and breast milk does not.


Contact sports such as rugby or martial arts are not advised due to the risk of injury.

Sports such as football, hockey or cricket are fine provided you remember that any contact injury sustained during play is likely to result in bruising or possibly bleeding. Wearing protective gear is essential.

Children may do PE and games at school, however care should be taken with other more robust activities Please make sure your teachers, especially your sports teacher, know you take warfarin in case you get an injury.

Please talk with the anticoagulation team about any sports or activities you wish to do for further advice as necessary.

Letting your school/college know

It is essential that staff, in particular sports teachers, are aware of students on warfarin therapy should an injury occur. You can give a copy of our information sheet (PDF, 325KB) to your school or college. Further information is also available from the contacts listed at the end of this page.

Think about what to do if you have an injury


  • Press for 10 minutes with a clean cloth.
  • If bleeding does not stop, go to your nearest Accident and Emergency Department (A&E) department.

Head injury/bump to the head

Be more vigilant, as cuts to the skin can be seen clearly. However bumps to the head which may cause internal bleeding are not so obvious. If you have more than a very minor bump it is advisable to get checked by a doctor.

We strongly recommend wearing cycle helmets to reduce the risk of a head injury when riding a bicycle.

Symptoms to look out for following a head injury include:

  • drowsiness

  • feeling dizzy when walking or standing up

  • vomiting

  • worsening headaches

  • problems with vision such as blurring

  • changes in behaviour

  • fits or convulsions

If any of these symptoms happen you must go to your nearest A&E department straight away.

Nose bleeds

  • If you have a nose bleed carry out normal first aid.
  • Lean your head forward.
  • Pinch your nose just above your nostrils for 10 minutes.
  • Leaning forward and breathing through your mouth will let blood drain out of your nose instead of down the back of your throat.
  • If nose bleed carries on for more than 10 minutes, you should go to your local A&E department for treatment and to get your INR checked.
  • Contact your anticoagulation team to inform them so that they can manage your INR levels as required.
  • When you go to A&E, make sure you tell the staff that you are taking warfarin.

General health and safety

Surgical or dental procedures

If you have any planned medical investigations, operation or dental procedures it may be necessary to interrupt the warfarin treatment for a few days. The doctor/surgeon will usually consult the doctor/ nurse responsible for your warfarin management and ask for the necessary advice.

Immunisations and vaccinations

All injections and vaccinations should be given under the skin (subcutaneously). Intramuscular injections (given into the muscle) should not be given to people on warfarin therapy as this may cause bruising.

Apply firm pressure to the site for 10 minutes afterwards.

Holidays and travel

If you are going on holiday in this country or abroad let your anticoagulation team know and arrange an INR check before you go. If you are away for more than one week you may need an INR checked locally.

Make sure you have enough warfarin tablets/suspension for your stay plus a few days’ extra and carry them in your hand luggage in case of lost suitcases! Travel insurance is essential if going abroad.

Diarrhoea and vomiting

If you are at home or in the UK, ensure you drink plenty of fluids to prevent dehydration, such as water, very dilute squash or rehydration sachets from the pharmacy. If symptoms worsen or persist for more than 24 hours, seek medical advice from your GP or nearest A&E department.

Diarrhoea and vomiting - which you may experience with foreign travel – will affect the absorption of your warfarin and your INR level very quickly. If you are abroad, the same advice applies about drinking plenty of fluid. However, it is important to ensure you drink only sealed bottled water in countries where tap water is not recommended for drinking purposes. 

If symptoms carry on or worsen seek medical advice. If you are unsure how to do this, speak to your holiday representative or travel insurance provider for further information and support.

Anticoagulation record book

Your anticoagulation team will give you a book to record your INR, warfarin dose and all your treatment details. Always bring this to every medical appointment.

Growing up and getting ready for adulthood

As you grow your needs will change.

Medic alert jewellery

This provides all relevant medical information and is particularly useful if you are away from home. Information is available at www.medicalert.org.uk


We advise electrical razors rather than blades.


Periods may be heavy and last longer than normal.

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

Pregnancy will affect your INR levels so we advise discussing this with a member of the team before you get pregnant.

You may find it helpful to discuss these issues with a member of the team managing your warfarin therapy for further advice.

Body piercing

This is not recommended due to the risk of bleeding and infection.


Alcohol can affect your blood test and should be taken in moderation. Excessive alcohol should be avoided as it increases the risk of bleeding. Vomiting after drinking too much leads to dehydration which will affect your INR.

Safe storage

Keep medicines in a safe place where children cannot reach them.

Keep medicines in a cool, dry place out of direct sunlight and away from heat.

If your doctor decides to stop treatment with warfarin, return any unused medicine to the pharmacist. Do not flush down the toilet or throw it away.

Important information about warfarin

If you forget to take a dose, take it as soon as you remember. Do not take a double dose.

If you vomit after taking the medicine, do not take a double dose.

Your family doctor (GP) will need to give you a repeat prescription for warfarin. Some medicines will need to be ordered by your community pharmacy (chemist) so arrange this in plenty of time.

More information

Who to contact:

  • The team managing your anticoagulation therapy. This should be written in the yellow anticoagulation record book, and include an 'out of hours' telephone number.

  • The Nurse Practitioner team at Great Ormond Street Hospital (GOSH) are on Bear Ward (020 7829 8829). You can also call them on 020 7405 9200 (ext 5774).

  • GOSH switchboard: 020 7405 9200

  • Pharmacy dept: 020 7829 8680

  • Pharmacy Medicines Information: 020 7829 8608

Last reviewed by Great Ormond Street Hospital: July 2012

Ref: 2012F0313 July 2012

Compiled by the Nurse Practitioner team in collaboration with the Child and Family Information Group

Please read this information in conjunction with any patient information leaflet provided by the manufacturer. However, please note that this information explains about the use of medicines in children and young people so may differ from the manufacturer’s information.

Each person reacts differently to medicines so your child will not necessarily suffer every side effect mentioned. This information does not constitute health or medical advice and will not necessarily reflect treatment at other hospitals. If you have any questions, please ask your doctor. No liability can be taken as a result of using this information.