Anticoagulants are a blood-thinning medication used to reduce the risk of having a stroke, or when you have other conditions such as:

Although warfarin is the most widely-used anticoagulant, there are others, including the new or novel oral anticoagulants (NOACs):

  • Dabigatran
  • Rivaroxaban
  • Apixaban


Warfarin is the most widely-used anticoagulant and works by increasing the time it takes for your blood to clot, which reduces the risk of a clot forming in the heart. Warfarin also blocks vitamin K, which is involved in the clotting process.

When you start taking warfarin, its effect will be monitored by blood tests. You will be referred to an anticoagulation clinic or your GP will monitor your treatment. You will also be given a yellow Oral Anticoagulation Therapy booklet with more information.  
Find out more about warfarin, your diet and vitamin K on the information page

How warfarin works

We monitor how well warfarin is working on you by measuring its effect on your international normalised ratio levels (INR).  INR levels tell us how your blood is clotting. The higher the INR level, the longer it takes for your blood to clot.

To reduce the chances of a stroke, we aim to get your INR target level between two  and three. When levels fall below two, we may be concerned that you are not fully protected from a stroke. If levels are too high (above three), the concern would be that you are at a higher risk of bleeding. 

Blood tests

You will need to have regular blood tests to make sure that your blood clotting is kept at a safe and effective level. The blood test you will have to monitor your clotting is called an INR (International Normalised Ratio).

If you are not taking warfarin your normal INR will usually be between 0.9 and 1.3. When you take warfarin your INR will be kept within a range of 2.0 to 3.0, 2.5 to 3.5, or 3.0 to 4.0 depending on your reason for anticoagulation.

Depending on the INR result, your warfarin dose will be adjusted to keep your INR within the desired range.

The blood test is normally taken by a finger prick test which is simple and quick to perform.  

Warfarin tablets

Warfarin comes in four different strengths, these are:

  • 0.5mg white tablets
  • 1mg brown tablets
  • 3mg blue tablets
  • 5mg pink tablets

Please keep a supply of different strengths, as the dosage of warfarin you need depends on the results of regular blood tests and may change from time to time. Your anticoagulant clinic will confirm with you which strength of warfarin tablets you will need to keep a supply of.

How much warfarin you need to take differs for each patient and depends on the result of your blood test.

When you first start taking warfarin your INR will be checked every few days and the dose of warfarin will be prescribed according to the results. The aim is to find the dose of warfarin that you can take regularly to keep your INR within the desired range.

As your INR and dose stabilises you will need blood tests less frequently. However, your INR can vary from time to time, therefore you will need to attend the anticoagulant clinic at regular intervals so that your blood can be monitored and your dose adjusted to keep it within the desired range.

Warfarin should be taken at the same time every day, preferably in the evening between 6pm and 8pm.

If you forget to take your dose at the usual time, please take it as soon as you remember, provided that you remember on the same evening. Please do not take the missed dose the next morning or double your dose the next day.

If you do not remember the missed dose until the next day then you should simply continue with your usual dose in the evening. Taking too much warfarin can cause bleeding and can be dangerous.
Find out more about who to tell about your warfarin prescription on the information page.

Length of prescription

You may need to take warfarin for a relatively short period of time (eg three to six months) if you have had a clot in your leg, for example. You are likely to be on warfarin for the rest of your life if you have a mechanical heart valve.

The appropriate length of treatment will be written in your yellow anticoagulant booklet and will be discussed with you.

Warfarin side effects

When you are on warfarin you are likely to bruise easily and it will take longer for a simple cut to stop bleeding.   

The most serious side effect of anticoagulants is excessive bleeding. If you experience any of the following, you must seek medical attention and have an urgent INR test: 

  • unexplained severe bruising
  • prolonged bleeding from small cuts, or nose bleeds (more than 10 minutes)
  • blood in vomit
  • blood in sputum
  • passing blood in your urine or faeces
  • passing black faeces
  • severe or spontaneous bruising
  • unusual headaches
  • for women, heavy or increased bleeding during your period or any other vaginal bleeding

You should seek immediate medical attention if you: 

  • are involved in a major trauma
  • suffer a significant blow to the head
  • are unable to stop the bleeding

Find out more about becoming unwell, taking other medications, and pregnancy while taking warfarin on the information page

New or novel oral anticoagulants (NOACS) 

New or novel oral anticoagulants (or NOACs) are now available as an alternative treatment to warfarin.  

NOACs include dabigatran, rivaroxaban and apixaban.

They have recently been approved by NICE (National Institute for Heath and Care Excellence) for the prevention of strokes in patients with AF. These new drugs work in a similar way to warfarin by increasing the time it takes for your blood to clot. They have been shown in clinical trials to be at least as effective as warfarin.   

NOACs advantages

The main advantage over warfarin is:

  • There is no need for regular blood tests
  • They are less affected by different foods (providing you take the tablet as instructed, the levels will remain stable)

NOACs disadvantages   

The main disadvantage compared with warfarin is that they are not as easy to reverse as warfarin if you experience bleeding problems. Drug manufacturers are currently looking at developing antidotes for the new medications, and these are estimated to be available within the next 2-3 years. 

NOACs side effects

The side effects of these medications are similar to warfarin. When you are on a NOAC, you are likely to bruise easily and it will take longer for a simple cut to stop bleeding. The most serious side effect of NOACs is prolonged or heavy bleeding, although the risk of serious bleeding is less than warfarin.  

You should contact your doctor immediately if you experience any of the following: 

  • prolonged nose bleeds (more than 10 minutes)
  • blood in vomit
  • blood in sputum
  • passing blood in your urine or faeces
  • passing black faeces
  • severe or spontaneous bruising
  • unusual headaches
  • for women, heavy or increased bleeding during your period or any other vaginal bleeding.

You should seek immediate medical attention if you:

  • are involved in major trauma
  • suffer a significant blow to the head
  • are unable to stop the bleeding.

Not all atrial fibrillation (AF) patients will be a suitable to receive these new medications.

If you are waiting for a catheter ablation for the treatment of your AF, because of the lack of antidote for bleeding you may need to be switched to warfarin, but this will be discussed with you in detail. 

If you have a metal (or mechanical) heart valve you will not be suitable for a NOAC and will be prescribed warfarin instead. 


Dabigatran is available in two different strengths (either 150mg or 110mg capsules) and should be taken twice a day with meals.  

The 150mg dose is the only NOAC that has been shown to be better than warfarin at preventing strokes in patients with AF. The 110mg dose has been shown to be equivalent. 

Depending on your age, weight, kidney function, other medical conditions you have and the tablets you are taking, your doctor or healthcare professional will decide with you which dose is the most appropriate.

Dabigatran side effects

The most common side effect people experience with dabigatran, other than bleeding and bruising easily, is indigestion or heartburn. These gastrointestinal side effects occur in around 10 per cent of patients. This can be minimised by taking your capsules with food and making sure that you swallow them whole.   

If you experience indigestion, speak to your doctor as they may want to give you a medication to reduce the acid in the stomach. These medications are sometimes known as PPIs (proton pump inhibitors) and include omeprazole, lansoprazole and pantoprazole.   

It is important that if you vomit blood, see blood in your urine (making it red or brown) or have blood in your stools (making it look red or black), you contact your doctor immediately.


Rivaroxaban is available in two different strengths (20mg and 15mg tablets) and should be taken once a day.  It is best to take the medication with your biggest meal of the day, at the same time each day. We will normally recommend that you take it with your evening meal. This helps to ensure that the medication is absorbed properly into the bloodstream.

Rivaroxaban has been shown to be equivalent to warfarin in preventing strokes in patients with AF. 

Depending on your kidney function and other medical conditions, your doctor or healthcare professional will decide with you which dose is the most appropriate. Rivaroxaban is sometimes tolerated better than dabigatran. However, the same bleeding risks still apply with this medicine.  


Apixaban is available in two different strengths (5mg and 2.5mg tablets) and should be taken twice a day.  

Depending on your age, kidney function and body weight, your doctor or healthcare professional will decide with you which dose is the most appropriate.

If you have had a bleed or ulcer in your stomach, your doctor may prefer you to be prescribed apixaban rather than warfarin or one of the other NOACs. This is because apixaban is the only NOAC that has been shown to have lower rates of gastrointestinal (or stomach) bleeding in comparison to warfarin.  

Go to the information section to find out taking warfarin with other medication, food which might affect your treatment, and more. 

If you need to attend one of our anticoagulant clinics, find out more. 

Deep vein thrombosis (DVT) is where a blood clot forms in a vein, usually the leg, and if part of it breaks off into the bloodstream and blocks a blood vessel in the lungs, is a pulmonary embolism

Atrial tachycardia is an abnormal heart rhythm which is usually seen in patients that have undergone heart surgery, have congenital heart defects or have undergone previous ablation procedures.

Atrial fibrillation is an abnormal heart rhythm. It is the most common heart rhythm disorder in the UK.

Things which may affect your medication

When taking warfarin you need to be aware of certain things that can alter your warfarin levels including alcohol, some foods and medicines. Warfarin may have other side effects, and it could affect operations and dental surgery as it increases the risk of bleeding during these procedures. 


Alcohol can increase your INR levels and, therefore, increase your risk of bleeding. It is recommended that you do not exceed the national guidelines of three units a day for men and up to two units a day for women. You should inform your GP or clinic if you have started consuming more alcohol. 

It is dangerous to 'binge drink' while taking anticoagulants as your INR levels can increase dramatically and lead to a higher risk of bleeding. 


Certain foods, particularly those rich in vitamin K, can affect your INR result, and reduce warfarin's effectiveness. Such foods include:

  • spinach
  • asparagus
  • broccoli
  • brussel sprouts
  • lettuce
  • avocado
  • olive oil
  • cereals containing wheat bran and oats
  • mature cheese (and blue cheese)
  • liver
  • egg yolks
  • oats
  • chickpeas

You don't need to avoid vitamin K-rich foods, but you should aim for balance and have similar amounts of these foods on a regular basis. This helps to reduce fluctuations in your INR readings.   

You should inform your GP or clinic if you have recently changed your diet, including if you are eating more green vegetables. 

Operations/dental surgery

If you are due to have an operation or dental procedure, you may be asked to temporarily stop your warfarin to reduce your risk of bleeding. 

You must consult your doctor, healthcare specialist or anticoagulant nurse before you do this. They will advise you if it is safe to temporarily stop your warfarin and how long before the procedure you should do it. 

If you are at a moderate/high risk of stroke, we recommend that your warfarin is only stopped for the shortest time possible (usually a maximum of three days). Depending on the type of surgery and your level of risk, you may require anticoagulant injections during the period you are not on warfarin. 

If you are waiting for an ablation procedure as a treatment for your AF, or you have recently had an ablation (within the last three months), we strongly recommend that you do not stop your warfarin before discussion with your arrhythmia specialist unless your condition is life-threatening.   

Warfarin and other medication

Some medication may increase or decrease the effect of warfarin, including some antibiotics and some medication prescribed for heart disease. You should tell the anticoagulant clinic if you have been prescribed new medication or if you have stopped taking any medication.

It is important to inform the clinic at the time these changes occur. Please do not wait until your next regular appointment if this is more than one week away. You may need to have your INR checked soon after starting or stopping your medication.

Please check with your pharmacist when you buy over the counter medication as some of these may interact with warfarin. You should avoid medication which contains aspirin or ibuprofen.

Paracetamol is the preferred option for pain relief, but it may interact with warfarin if taken regularly for a few days or more.

Some vitamin supplements and herbal remedies interact with warfarin. Please check with your pharmacist and the clinical nurse specialist in anticoagulation.

Feeling unwell

Prolonged bouts of nausea, vomiting and diarrhoea should be reported to the anticoagulant clinic or GP. Gastrointestinal upsets may affect your ability to absorb warfarin and vitamin K from your diet. You may need to bring your next clinic appointment forward for an INR as your dose of warfarin may need to be changed. 

If you are ill with a fever (eg influenza or a chest infection) you should also inform your anticoagulant clinic and GP as this can also affect warfarin therapy.

If you are admitted to hospital please check with a member of the medical staff when your next appointment for the anticoagulant clinic should be, before you go home. You will probably need an appointment a few days after you are discharged. 

If an appointment has not been arranged for you please contact the anticoagulant clinic when you arrive home.


Female patients of childbearing age will need to take precautions to avoid pregnancy while they are taking warfarin. Warfarin may harm an unborn child during the early weeks of pregnancy.

If you are planning to become pregnant it is important that you discuss this with your doctor so that special arrangements can be made to ensure safe anticoagulation treatment throughout your pregnancy.

Informing others

Your dentist 
Please tell your dentist before any treatment as there may be a risk of bleeding during some dental procedures.

Your doctor 
Please tell any doctor who treats you. If you have to undergo a surgical procedure there may be a risk of bleeding and your warfarin treatment may need to be adjusted or stopped.

Sport and exercise

Moderate exercise (eg walking, jogging, or swimming) is fine. However, you should avoid contact sports and those activities in which physical injury is more likely to occur. 

Injury will increase the risk of bruising and bleeding while taking warfarin. Safety equipment should be used when participating in certain activities, eg crash helmet when cycling.

Meet the team

The arrhythmia team includes: 

  • EP consultants
  • clinical nurse specialists
  • an arrhythmia pharmacist 
  • catheter laboratory technicians. 


Dr Jonathan Clague
Dr Sabine Ernst 
Dr John Foran
Dr Wajid Hussain 
Dr Julian Jarman
Dr David Jones
Dr Vias Markides
Dr Tushar Salukhe
Dr Jan Till 
Dr Tom Wong

Clinical nurse specialists

Natalie Crump 
Sue King
Sarah Plowright 
Alex Wise 

Arrhythmia pharmacist

Sally Manning 
Zainab Khanbhai

Contact the team

Harefield Hospital: 01895 828979
Royal Brompton Hospital: 020 7351 8364