Warfarin is the main oral anticoagulant used in the UK (oral means it is taken by mouth). An anticoagulant is a medicine that stops blood from clotting.

Anticoagulant medicines are most commonly prescribed for people who have had a condition caused by a blood clot (thrombosis) such as:

deep vein thrombosis (blood clots in the veins of the legs)

pulmonary embolism (a blood clot in the lungs)

Warfarin may also be prescribed for people:

with a condition that increases the risk of a blood clot developing, such as atrial fibrillation

with a prosthetic (replacement or mechanical) heart valve

who have had a heart attack

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.

Taking warfarin

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

How it works

The blood needs vitamin K to be able to clot. Warfarin slows the production of vitamin K in the body, which increases the time it takes for your blood to clot.

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

Things to consider

While taking warfarin, your dose will be monitored once or twice a week using the international normalisation ratio (INR), which measures how long it takes your blood to clot. You may be monitored less frequently, depending on your readings.

Although there are now three new anticoagulants that don’t require regular monitoring – rivaroxaban, apixaban and dabigatran – most patients who need an anticoagulant will be prescribed warfarin.

It is important to avoid taking warfarin if you are pregnant or have certain health conditions, including:

uncontrolled severe hypertension (high blood pressure)

a risk of bleeding inside the body, for example if you have a peptic ulcer

a bleeding disorder, such as haemophilia



Warfarin can interact with many other medicines. The patient information leaflet that comes with your medicine should tell you if it’s safe to take with warfarin, but ask your GP or pharmacist if you’re unsure.

Some foods and drinks can interfere with the effect of warfarin if eaten in large amounts, including food high in vitamin K and cranberry juice.

It is also dangerous to binge drink or get drunk while taking warfarin as this increases the risk of excess bleeding.


Side effects

Warfarin can cause several side effects and you should see your GP if you experience any unusual and persistent symptoms.

It is also important to avoid injury while taking any anticoagulant, as they make you more prone to bleeding if you are injured. Try to avoid minor injuries and cuts and grazes by:

taking care when brushing your teeth and shaving

using protection when gardening, sewing or playing contact sports

using insect repellent to avoid insect bites or stings


Things to consider when taking warfarin 

People with certain health conditions should avoid warfarin. Anyone taking warfarin will also need to have their dose monitored.

Monitoring your dose

While you are taking warfarin, you will need regular blood tests at your GP surgery or local anticoagulant clinic. This is to make sure that your dose is correct.

How well your warfarin is working is measured using the international normalisation ratio (INR). This measures how long it takes your blood to clot. The results of your INR tests determine the dose of anticoagulant you need to take.

If your INR is too high – blood clots will not form quickly enough and you may experience bruising or you may be at increased risk of bleeding. In this case, your dose may need to be reduced.

If your INR is too low – your medication is not working sufficiently, which means that clots could still form unnecessarily and block a blood vessel. In this case, your dose may need to be increased.

When you first start taking warfarin, your INR will be tested within the first three to four days of starting treatment. Depending on your results, further tests will be carried out once or twice a week. If your INR stabilises within the correct range, testing will become much less frequent.

Once your INR levels are stable, you may be shown how to use a self-testing machine, which can be used to measure your own INR at home. This can also be useful if you are going on holiday, for example.

Tablet strengths

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.

In the UK, the strengths and colours of warfarin tablets are:

0.5mg – white tablets

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 with or without food.

When to avoid warfarin

Warfarin should be avoided if you have:

uncontrolled severe hypertension (high blood pressure)

had a recent stroke

a risk of bleeding inside the body – for example, if you have recently had major surgery or an organ biopsy, or if you have apeptic ulcer

a bleeding disorder, such as haemophilia (a blood clotting disorder), liver failure or kidney failure

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

Using warfarin with caution

Warfarin should be taken with caution if you have a high risk of falls – for example, if you are elderly or have recently had surgery.

You should avoid high-impact sports, such as rugby or martial arts, which carry a risk of serious injury, particularly head injuries. Any bleeding or bruising while taking warfarin will be worse than usual.

You should also be aware of how warfarin interacts with certain foods, alcohol and other medications.

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.


Side effects of warfarin 

It is important to check for bleeding (haemorrhages) when taking warfarin as anticoagulant medication can cause excessive 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

Other common side effects

nausea or vomiting (feeling sick)



hair loss

skin rashes

fever (a temperature of 38C (100.4F) or above)

red or purple patches of skin (purpura)

pancreatitis (inflammation of the pancreas) – which leads to a dull pain around the top of the stomach

kidney problems

See your GP if you experience any persistent side effects while taking warfarin.

Warfarin-induced skin necrosis

In rare cases, warfarin can lead to skin necrosis (tissue death). This condition is caused by blood clots blocking the blood vessels, leading to damaged areas of skin.

The first symptom of skin necrosis is red or purple patches of skin (purpura), followed by blood blisters and then painful skin lesions. There is often a red, net-shaped rash around the damaged skin.

Symptoms are more likely to appear on fatty areas, such as the breasts, abdomen, buttocks and thighs. Symptoms usually appear within the first two to five days after starting to take warfarin.

See your GP immediately if you suspect skin necrosis.

Reporting side effects

The Yellow Card Scheme allows you to report suspected side effects from any medicine that you are taking. It is run by a medicines safety watchdog called the Medicines and Healthcare products Regulatory Agency (MHRA).

Interactions with other medicines 

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 medicines or supplements 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 this could lead to bleeding 

do not take ibuprofen

you can take paracetamol but do not take more than the recommended dose


Interactions with food

Consult your GP before making any major changes to your diet while taking warfarin. They can also supply further information about foods to avoid or to limit.

Vitamin K

It is important to eat a consistent, healthy diet when taking warfarin.

Some foods, including liver and green leafy vegetables, such as spinach and broccoli, contain large amounts of vitamin K. When your first dose of warfarin is prescribed, it does not matter how much vitamin K you are eating as the dosage will be based around your current blood clotting levels.

However, if you make significant changes to your diet and begin to eat these foods in large amounts, or cut them out of your diet, this could interfere with how your warfarin works.

Cranberry and grapefruit juice

Cranberry juice can also increase the effects of warfarin, so you should avoid drinking it regularly if you are taking warfarin.

There is also some limited evidence to suggest that grapefruit juice can interfere with warfarin and cause a slight increase in your international normalisation ratio (INR, which is a measure of how long it takes your blood to clot). So again, it's best not to drink grapefruit juice regularly if you are taking warfarin. However, having the occasional small glass of cranberry or grapefruit juice is unlikely to cause any serious problems. 

Losing weight

Losing or putting on weight can also affect your INR. If you are taking warfarin you will need to be weighed regularly. See Considerations when taking warfarin for more information about INR.

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.

Do not drink more than three units of alcohol a day if you are a man or two units a day if you are a woman. It is not safe to save up units to have on one day.

One unit is roughly equivalent to half a pint of beer or lager or a single measure (25ml) of a spirit such as vodka. A small glass (125ml) of wine is around 1.5 units. 

Heavy drinkers or people with liver disease who are taking warfarin should not drink alcohol.


Missed or extra doses 

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

Missed doses


If you usually take your warfarin in the morning and have forgotten to take it, take the dose as soon as you remember and then continue as normal.

However, if it is time to take your next dose, do not take a double dose to catch up (unless your GP has specifically advised this).


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, leave that dose and take your normal dose the next day at the usual time.

If you are not sure what to do if you have missed a dose, ask your GP or anticoagulant clinic. Alternatively, call 111 for advice.

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.

If you still need advice, either speak with a pharmacist or call 111.

Frequently asked questions 

Below are the answers to some common questions about taking warfarin.

What do I do if I have a nosebleed?

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 GP surgery or anticoagulant clinic to get your international normalisation ratio (INR) checked. INR is a measure of how long it takes your blood to clot.

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

Because of 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.

However, if your INR is under four you probably won’t need to have your warfarin dose adjusted.

Can I have normal vaccinations?

Yes, but all injections and vaccinations must:

be given under the skin

have firm pressure applied to the site for 10 minutes afterwards

not be given into the muscle as this may cause bruising

Can I play sports?

Yes, but because of the risk of bleeding:

contact sports that could lead to a head injury, 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?

Because of 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.