Warfarin is the most widely-used anticoagulant (or blood-thinning medication) to reduce the risk of having a stroke.

Warfarin works by increasing the time that it takes for your blood to clot and will reduce the risk of a clot forming within the heart. Warfarin works by blocking 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 about warfarin.  

The effect of warfarin is measured by your INR (international normalised ratio) levels. The INR level shows how your blood is clotting. The higher your INR level is, the longer it takes your blood to clot. 

For AF stroke reduction, your INR target level is usually between 2 and 3. If your levels are below 2 then you are not fully protected from a stroke. If your levels are too high (above 3), then you are at a higher risk of bleeding. 

When you first start this treatment you will need regular blood tests; usually once or sometimes twice per week until you reach the target INR level. Once this has been achieved you will need less frequent blood tests and visits to the clinic may be once every four to eight weeks. The blood test is normally taken by a finger prick test which is simple and quick to perform.  

You should make sure that you take your dose of warfarin once a day, at the same time each day; usually, the evening is best. 

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 can reduce the effectiveness of warfarin. The amount of vitamin K in your diet may affect your INR result. You do not need to avoid foods that are rich in vitamin K. You should try to have similar amounts of foods containing vitamin K on a regular basis. This will help to reduce fluctuations in your INR readings.   

Examples of foods rich in vitamin K include: leafy green vegetables, chickpeas, liver, egg yolks, cereals containing wheat bran and oats, mature cheese, blue cheese, avocado and olive oil. 

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

Other medications

You should always check with your anticoagulation clinic, GP or pharmacist if you have started any new medications or are taking any over-the-counter medications. Some medications can affect the INR levels so you may require closer monitoring. 

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: 

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

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.   

NOACs are now available as an alternative to warfarin.

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.

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