Warfarin is the main oral anticoagulant used in the UK. Oral means it's taken by mouth. An anticoagulant is a medicine that stops blood clotting.
Clotting (thickening) is a complex process involving a number of substances called clotting factors.
Clotting factors are produced by the liver and help control bleeding. They work with cells that trigger the clotting process (platelets) to ensure blood clots effectively.
To produce some of the clotting factors, the liver needs a good supply of vitamin K.
Warfarin blocks one of the enzymes (proteins) that uses vitamin K to produce clotting factors. This disrupts the clotting process, making it take longer for the blood to clot.
When warfarin is prescribed
- a stroke
- a heart attack
- deep vein thrombosis – a blood clot within a deep vein in the body, usually in a leg
- pulmonary embolism – a blood clot in the lungs
Warfarin may also be prescribed for people at an increased risk of developing harmful blood clots, such as those with:
- a replacement or mechanical (prosthetic) heart valve
- an irregular heart rhythm, known as atrial fibrillation
- a blood clotting disorder, such as thrombophilia
- an increased risk of blood clots following surgery
It's very important that you take warfarin exactly as directed. Don't increase your prescribed dose unless the doctor in charge of your care advises you to.
Warfarin is taken once a day, usually in the evening. It's important to take your dose at the same time each day, before, during or after a meal.
The aim of warfarin therapy is to decrease the blood's tendency to clot, but not stop it clotting completely. This means the dose of warfarin you're taking must be carefully monitored and, if necessary, adjusted.
You'll have regular blood tests at your GP surgery or local anticoagulant clinic to make sure your dose is correct.
The international normalised ratio (INR) is a measure of how long it takes your blood to clot. The longer it takes your blood to clot, the higher your INR. Your INR will be used to determine the dose of warfarin you need to take.
Although there are now three new anticoagulants that don't require regular monitoring – rivaroxaban, apixaban and dabigatran – most people who need an anticoagulant will be prescribed warfarin.
When you start taking warfarin, you may be given a yellow booklet about anticoagulants, which explains your treatment.
How long you'll need to take warfarin for will depend on the condition for which it's been prescribed. Ask the healthcare professional responsible for your care if you're not sure.
If you usually take warfarin in the morning and forget to take it at your normal time, take it as soon as you remember and continue as normal.
However, if it's time to take your next dose, don't take a double dose to catch up, unless your GP has specifically advised you to.
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.
Ask your GP or staff at your local anticoagulant clinic if you're not sure what to do about a missed dose of warfarin. You can also call NHS 111 for advice.
Who shouldn't take warfarin
The following people shouldn't take warfarin:
- pregnant women – it can affect the development of the baby
- people with uncontrolled high blood pressure (severe hypertension)
- people with a high risk of internal bleeding – for example, those with a stomach ulcer
- people with a bleeding disorder – such as haemophilia
Side effects of warfarin
Bleeding is the main side effect associated with warfarin, as it slows down the blood's normal clotting ability.
You're at greatest risk of bleeding in the first few weeks of starting treatment with warfarin and when you're unwell.
You should therefore seek medical attention if you:
- pass blood in your urine or faeces
- pass black faeces
- have severe bruising
- have long nosebleeds – lasting more than 10 minutes
- have bleeding gums
- cough up blood or have blood in your vomit
- experience unusual headaches
- have heavy or increased bleeding during your period, or any other bleeding from your vagina (in women)
Take extra care to avoid cutting yourself while taking anticoagulant medication because of the risk of excessive bleeding.
For example, you should:
- take care when shaving and brushing your teeth
- wear protective clothing when gardening, sewing or playing contact sports
- use insect repellent to avoid insect bites or stings
Seek urgent medical attention if you're taking warfarin and you:
- have a fall or accident
- experience a significant blow to your head
- are unable to stop any bleeding
- have signs of bleeding, such as bruising
Skin rashes and hair loss are also common side effects of warfarin.
Contact your GP or the healthcare specialist responsible for your care if you experience any persistent side effects while taking warfarin.
Warfarin can interact with many other medicines. The patient information leaflet that comes with a medicine should tell you if it's safe to take with warfarin.
Ask your GP or pharmacist if you're unsure.
When taking warfarin:
- don't take aspirin, or treatments containing aspirin, unless it's prescribed by a healthcare professional as it could cause bleeding
- don't take ibuprofen unless it's prescribed by a healthcare professional
- you can take paracetamol, but don't take more than the recommended dose
Herbal medicines and supplements can also interact with warfarin. You should therefore avoid taking them without first checking with your GP, pharmacist, or staff at your local anticoagulant clinic.
Food and drink
Some food and drink can interfere with the effect of warfarin if consumed in large amounts, including foods that are rich in vitamin K.
Foods containing large amounts of vitamin K include:
- green leafy vegetables, such as broccoli and spinach
- vegetable oils
- cereal grains
Small amounts of vitamin K can also be found in meat and dairy foods.
When your first dose of warfarin is prescribed, it doesn't matter how much vitamin K you're eating because the dosage will be based on your current blood clotting levels.
However, if you make significant changes to your diet, such as increasing your vitamin K intake or cutting out foods that contain vitamin K, it could interfere with how warfarin works.
Consult the healthcare professional responsible for your care before making any significant changes to your diet while taking warfarin. They'll also be able to give you more information about foods to avoid or limit.
Getting drunk or binge drinking is dangerous while taking warfarin. It may increase the effect of the drug, increasing the risk of bleeding.
The latest guidelines on drinking alcohol state that regularly drinking more than 14 units of alcohol a week (for both men and women) risks damaging your health.
Fourteen units is equivalent to six pints of average-strength beer or 10 small glasses of low-strength wine.
Read more about units of alcohol.
People with liver disease who are taking warfarin shouldn't drink alcohol at all.
Other useful information
Surgery and dental work
Because of the risk of bleeding, your dose of warfarin may need to be lowered or stopped a few days before having an operation or dental work.
Tell the surgeon or dentist that you're taking warfarin. You should also tell anyone else involved with your care, such as an anticoagulant nurse, if you need an operation so they can make arrangements.
You can have vaccinations while taking warfarin.
If the vaccine is normally given by injection into the muscle, then providing you're up to date with your INR testing (see above), and that the results are within the correct range, you can have the injection as normal into muscle. This is known as an intramuscular (IM) injection.
Alternatively, the injection can be given into the layer of fat underneath your skin. This is known as a subcutaneous injection. Firm pressure applied to the site for 10 minutes after the vaccination may reduce potential bruising.
You can play sports while taking warfarin, 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
You can continue to take part in non-contact sports, such as running, athletics, cycling and racquet sports. However, make sure you wear protective clothing, such as a cycle helmet.
Going on holiday
Tell your GP or anticoagulant nurse if you're going on holiday, in this country or abroad, and arrange to have your INR checked before you go.
If you're going to be away longer than a month, you may need to arrange to have your INR checked while you're away. Make sure you have enough warfarin tablets to last for the duration of your trip.
Body piercings aren't recommended while taking warfarin because of the increased risk of bleeding and risk of infection.
Page last reviewed: 15/05/2016
Next review due: 30/04/2019