What is warfarin?

Warfarin is an anticoagulant medicine. Anticoagulants help reduce the blood’s ability to clot.

Warfarin is taken by mouth and is used to help prevent harmful blood clots from forming or getting bigger.

Your dose must be carefully monitored because warfarin reduces clotting, but does not stop the blood clotting completely.

Read NHS 111 Wales information about anticoagulant medicines

When warfarin may be prescribed

Warfarin may be prescribed for people who have had a condition caused by a blood clot, or for people who have a higher risk of developing harmful blood clots.

This can include people who have had:

  • a stroke
  • a heart attack
  • deep vein thrombosis, also known as DVT
  • a pulmonary embolism, which is a blood clot in the lungs

It may also be prescribed for people with:

  • a replacement or mechanical heart valve
  • an irregular heart rhythm, such as atrial fibrillation
  • a blood clotting disorder
  • an increased risk of blood clots after surgery
NHS 111 Wales: stroke NHS 111 Wales: DVT NHS 111 Wales: atrial fibrillation

Taking warfarin

It is very important to take warfarin exactly as directed. Do not increase or change your dose unless the healthcare professional responsible for your care tells you to.

Warfarin is usually taken once a day, often in the evening. Try to take it at the same time each day.

You will need regular blood tests to check how quickly your blood clots. This is measured using the international normalised ratio, also known as INR.

Your INR result helps your healthcare team decide whether your warfarin dose needs to be changed.

Important safety advice

Bleeding is the main side effect linked with warfarin because it slows down the blood’s normal clotting ability.

You should seek medical advice if you:

  • pass blood in your urine or poo
  • pass black poo
  • have severe bruising
  • have a nosebleed lasting more than 10 minutes
  • have bleeding gums
  • cough up blood or have blood in your vomit
  • have unusual or severe headaches
  • have heavy or increased bleeding during your period, or any other vaginal bleeding
  • have a fall, accident or significant blow to the head
  • are unable to stop any bleeding

Call 111 for urgent medical advice. Call 999 in an emergency.

Missed doses

If you miss a dose of warfarin, follow the advice given by your GP, anticoagulant clinic or pharmacist.

Do not take a double dose to make up for a missed dose unless you have been specifically advised to do so by a healthcare professional.

If you are unsure what to do, contact your GP, pharmacist, anticoagulant clinic or NHS 111 Wales for advice.

Who should not take warfarin?

Warfarin is not suitable for everyone. It may not be suitable for:

  • pregnant women, as it can affect the development of the baby
  • people with uncontrolled high blood pressure
  • people with a high risk of internal bleeding, such as people with a stomach ulcer
  • people with certain bleeding disorders, such as haemophilia

Your healthcare professional will discuss the benefits and risks with you before prescribing warfarin.

Medicines, supplements and interactions

Warfarin can interact with many other medicines. Always check with your GP, pharmacist or anticoagulant clinic before starting any new medicine, herbal remedy or supplement.

When taking warfarin:

  • do not take aspirin, or medicines containing aspirin, unless prescribed by a healthcare professional
  • do not take ibuprofen unless prescribed by a healthcare professional
  • you can usually take paracetamol, but do not take more than the recommended dose
  • check before taking herbal medicines or supplements, as these can affect warfarin
Food, vitamin K and alcohol

Some food and drink can affect how warfarin works, especially if consumed in large amounts.

Foods rich in vitamin K include green leafy vegetables, vegetable oils and some cereal grains.

You do not usually need to avoid these foods, but it is important to keep your diet consistent and avoid making major changes without speaking to the healthcare professional responsible for your care.

Getting drunk or binge drinking is dangerous while taking warfarin, as it may increase the risk of bleeding.

Heavy drinkers or people with liver disease who are taking warfarin should not drink alcohol unless advised by their healthcare professional.

Surgery, dental treatment and procedures

Tell your surgeon, dentist or any healthcare professional involved in your care that you are taking warfarin.

Your warfarin dose may need to be changed or stopped before some operations, dental treatment or procedures because of the risk of bleeding.

Do not stop taking warfarin unless a healthcare professional tells you to.

Vaccinations, sport and travel

You can usually have vaccinations while taking warfarin, but tell the person giving the vaccine that you take warfarin.

Because of the risk of bleeding, contact sports or activities with a high risk of head injury may not be suitable. Ask your healthcare professional for advice if you are unsure.

If you are going on holiday, tell your GP or anticoagulant clinic. You may need to arrange an INR check before you travel, and you should make sure you have enough warfarin tablets for the full trip.

Further information

For more information about warfarin and anticoagulant medicines, visit NHS 111 Wales.

NHS 111 Wales: anticoagulant medicines NHS 111 Wales: stroke NHS 111 Wales: DVT