Anticoagulant medication is used to slow the blood clotting process. It is either used to prevent a clot forming in the blood or to treat clots that have already formed. The most common anticoagulant medication is warfarin.

Why do I need anticoagulant treatment?

There are many reasons why you may have been prescribed anticoagulant medication. The most common are:

  • Deep vein thrombosis 
    This occurs when a blood clot forms in one of the large veins in your leg or arm.
  • Pulmonary embolus 
    This is a blood clot that forms in, or travels to, your lungs.
  • Atrial fibrillation 
    This term refers to an irregular heart rhythm. This occurs when the atria (small pumping chambers of the heart) do not contract simultaneously. This means that blood does not flow through them as rapidly as it should and makes the blood more likely to clot. 
    If a clot becomes dislodged it can travel to the brain and result in a stroke. It may also travel to other parts of the body. The likelihood of stroke in patients with atrial fibrillation is approximately five times higher than that of the general population. 
    Warfarin can reduce the risk of stroke by 60 to 70 per cent.
  • Mechanical heart valves 
    If you have been fitted with a mechanical heart valve you will need long-term anticoagulation treatment to prevent blood clots forming on the valve. If blood clots were to form on a mechanical valve it would stop working properly.

Please ask your nurse if you would like more information about why you have been prescribed anticoagulant therapy.

How long will I need to take warfarin for?

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 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.

Why do I need regular blood tests?

You will need to have regular blood tests to ensure 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.

How much warfarin will I need to take?

The required daily dose of warfarin may be different for each patient. Your dose of warfarin will depend 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.

When should I take warfarin?

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.

Does warfarin have any side effects?

Side effects from warfarin treatment are rare but some patients may experience nausea, diarrhoea, rash or hair loss. If they do occur, please report them to the anticoagulant clinic.

Are there any risks associated with taking warfarin?

The main risk of taking warfarin is bleeding. Bleeding may be a sign that your INR is too high. You should contact the anticoagulant clinic, your GP or local accident and emergency department if you experience any unexpected bleeding. 

Signs of bleeding may include:

  • unexplained severe bruising
  • prolonged bleeding from small cuts
  • nose bleeds
  • bleeding gums
  • blood in urine
  • red or black stools
  • unusually heavy periods or bleeding between periods. 

Will other medication I take affect warfarin?

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.

What should I do if I am 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.

Can I drink alcohol?

Too much alcohol or changes in alcohol consumption can alter the effect of warfarin. It is safe to drink alcohol in moderate amounts (eg one to two units daily).

Do I need to alter my diet?

You should try to eat a well-balanced diet. Changes in diet can upset warfarin control and these should be discussed with your anticoagulant clinic.

You should not go on crash diets or eat foods that contain vitamin K in excess (such as spinach, asparagus, broccoli, brussel sprouts, lettuce, avocado, liver, eggs or oats). You do not need to cut them out completely, but you should eat them in moderation.

You should avoid drinking cranberry juice. 

Who should I tell that I am taking warfarin?

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

Will I be able to participate in sports?

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.

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.

Further information and advice

If you would like more information, please contact the clinical nurse specialist at: 

  • Royal Brompton Hospital on 020 7351 8498
  • Harefield Hospital on 01895 828 802