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Warfarin is a blood thinner.

It makes your blood flow through your veins more easily so you don’t get dangerous blood clots.

Warfarin is only available on prescription and comes as tablets. It's part of a class of medicines called anticoagulants.

The two main brands available in Ireland are Warfant and Warfarin Teva. Never change brands of warfarin without medical advice.

Urgent advice: Tell your GP straight away if you take warfarin and you:

  • test positive for COVID-19
  • have COVID-19 symptoms

You need to have regular blood tests when you take warfarin. Keep having these tests at the times you're advised.

Uses of warfarin

Warfarin is used to:

  • treat people who have had a blood clot in the leg (deep vein thrombosis, or DVT)
  • treat people who have had a blood clot in the lungs (pulmonary embolism)
  • prevent blood clots if you're at high risk of having them

This includes people with:

  • a replacement or mechanical heart valve
  • an abnormal heartbeat (atrial fibrillation)
  • a blood clotting disorder, such as thrombophilia
  • a higher chance of having a blood clot after an operation

Read about how to reduce the risk of blood clots

Get emergency help

You might have to get emergency help:

  • for serious bleeding
  • for a serious allergic reaction
  • if you take too much warfarin
  • for serious side effects

Symptoms of serious bleeding

Warfarin can make you bleed more than normal.

If this happens you may need to go tell your anticoagulant clinic or go to an emergency department (ED)

Emergency action required: Call your GP or anticoagulant clinic, or go to an ED immediately if you have:

  • red pee or black poo
  • large bruises or bruises that happen for no reason
  • nosebleeds without an obvious reason
  • blood in your vomit or you're coughing up blood
  • severe headaches, fits (seizures), changes to your eyesight, loss of consciousness or slurred speech
  • dizziness numbness or tingling in your arms or legs, or feel very tired, weak or sick
  • any bleeding from a cut or injury that will not stop or slow down
  • any other unexplained bleeding you are worried about

You should also stop taking warfarin.

Serious allergic reaction

You may need immediate treatment in hospital if you have a serious allergic reaction (anaphylaxis).

Emergency action required: Call 999 or 112 or go to an ED immediately if:

  • you get a skin rash that may include itchy, red, swollen, blistered or peeling skin
  • you're wheezing
  • you get tightness in the chest or throat
  • you have trouble breathing or talking
  • your mouth, face, lips, tongue or throat start swelling

You should also stop taking warfarin.

If you take too much warfarin

Urgent advice: Get immediate medical advice from a GP or anticoagulant clinic if you:

  • take more than 1 extra dose of warfarin

If you cannot contact either, go to an emergency department (ED).

Bring the warfarin packet or leaflet, plus any remaining medicine. If you have a yellow book, take that too.

You may need to change your next dose of warfarin or have a blood test.

Serious side effects

Emergency action required: Call a GP urgently if you develop any of these serious side effects:

  • yellowing of your skin or the whites of the eyes
  • dark pee
  • a painful red skin rash or skin blisters
  • painful swollen patches on your skin - particularly if you have chronic kidney disease

Stop taking warfarin if you get these side effects.

Check if you can take warfarin

Adults can take warfarin.

But it's not suitable for children and for some adults.

Talk to your GP or pharmacist before taking warfarin. They'll tell you if warfarin is OK for you.

Before taking Warfarin tell your GP or pharmacist if you:

  • have had an allergic reaction to any medicines in the past
  • are pregnant, breastfeeding, trying to get pregnant or have just had a baby
  • have an existing medical condition
  • have had any bleeding problems including a stroke
  • have had surgery within the past 3 days or are due to have an operation within the next 3 days
  • are taking other medicines or remedies, including the herbal remedy St John's wort for depression
  • are lactose intolerant

Warfarin and pregnancy

Warfarin should not be taken during pregnancy or if you have given birth within the last two days.

It can be harmful to your baby, particularly during the first 12 weeks of pregnancy.

Talk to your GP before taking warfarin if you think you might be pregnant or are trying to get pregnant.

Warfarin and breastfeeding

Talk to your GP or pharmacist before taking warfarin if you are breastfeeding.

When you start taking warfarin

When you start taking warfarin, you may be given a yellow book about anticoagulants.

This explains your treatment. There's also a section for you to write down and keep a record of your warfarin dose.

It's a good idea to take your yellow book with you to all your warfarin appointments.

How much warfarin you are prescribed depends on many things.

This includes:

  • what you eat and drink
  • what other medicines you're taking
  • if you become unwell

Expect your warfarin dose to go up or down. This is normal.

Your warfarin dose may change often, especially in the first few weeks. It can change until your GP finds the dose that's right for you.

How and when to take your warfarin

Take warfarin exactly as your GP tells you to.

Usually you'll take it once a day, at about the same time. Most people take it in the evening. This is so that if you need to change the dose after a routine blood test, you can do this that day. Then you will not have to wait until the next morning.

Warfarin does not usually upset your stomach. You can take it with or without food.

If you forget to take it

Try to take your warfarin on time.

If you forget to take a dose at the right time often, you'll be at risk of a blood clot

If you miss a dose of warfarin, write it down in your yellow book. Take the missed dose as soon as you remember.

If you do not remember until the next day, skip the missed dose and take your normal dose at the usual time.

Never take more than 1 dose a day.

If you often forget doses, it may help to set an alarm to remind you.

If you're worried, contact your anticoagulant clinic or GP.

How long warfarin takes to work

Warfarin takes about 3 days to build up its blood-thinning effect.

But it can take weeks or even months before you settle on the right dose.

Everyone reacts differently to warfarin, so it's usual for the dose to go up and down when you first start taking it.

You will not feel any different while you take warfarin.

The only way to check it is working, is to take a blood sample and see how long your blood takes to clot.

Get blood tests

You'll get regular blood tests at least every 12 weeks.

The dose of warfarin you need depends on your blood test result. If the blood test result has gone up or down, your warfarin dose will be increased or decreased.

You'll have the blood tests at your GP surgery or anticoagulant clinic.

You might only need a blood test once every 8 to 12 weeks if your blood test results are stable.

If it's unstable or you have just started on warfarin, you might need to have a blood test every week.

How long you need to take warfarin

You’ll usually take a short course of warfarin for 6 weeks to 6 months if you have had a blood clot in your leg or lungs.

Your treatment will be for longer than 6 months, maybe even for the rest of your life, if:

  • you take warfarin to reduce your risk of having a blood clot in future 
  • you keep getting blood clots 

If you want to stop taking warfarin

Do not stop taking warfarin without talking to your GP.

If you stop taking warfarin, your blood clots may quickly return.

This means you may be at an increased risk of serious problems like strokes, heart attacks, DVT or pulmonary embolism.

Prevent bleeding

When you're taking warfarin, be careful when you do activities that may cause an injury, cut or bruising.

It can help to:

  • stop playing contact sports or other activities than can cause a head injury, such as football, rugby, hockey and horse riding
  • wear gloves when you use sharp objects like scissors, knives and gardening tools
  • stop wet shaving or removing hair with wax - use an electric razor or hair-removing cream instead
  • take false teeth (dentures) or retainers out for a few hours a day, if you wear them, to give your gums a rest
  • tell your GP, dentist or nurse that you take warfarin ahead of any medical or dental appointments

Side effects of warfarin

Warfarin can cause side effects, although not everyone gets them.

The most common side effect of warfarin is bleeding more easily than normal, such as having nosebleeds and bruising.

You might also get the following side effects:

  • hair loss
  • feeling sick (nausea) or being sick (vomiting)
  • diarrhoea
  • purple toes
  • high temperature of 38 degrees Celsius or higher

Talk to your GP or pharmacist if you get these side effects or any others. This includes any possible side effects not listed on this page.

See the patient information leaflet that comes with your medicine for a full list of side effects.

Non-urgent advice: Find your patient information leaflet

Your patient information leaflet is the leaflet that comes with your medicine. You can find a digital version of the leaflet online.

Report side effects

You can report any suspected side effects to the the Health Products Regulatory Authority (HPRA): report an issue -

Carry your alert card

You'll get an anticoagulant alert card, usually from a pharmacist. Carry this with you all the time.

It tells healthcare professionals that you're taking an anticoagulant. This can be useful for them to know in case of a medical emergency.

Show it to your doctor or dentist before you have any medical or dental procedures. This includes vaccinations and routine appointments with the dental hygienist.

Your doctor may advise you to stop taking warfarin or reduce your dose for a short time before your treatment.

If you have lost your alert card or were not given one, ask your doctor or anticoagulant clinic.

Food and drink

It's very important to keep your diet stable. This means your dose of warfarin is more likely to stay the same.

Any big changes in what you eat or drink can change how your body responds to warfarin.

Speak to your GP or nurse before changing what you eat - for example, before you go on a diet to lose weight.

Foods containing a lot of vitamin K can interfere with how warfarin works. For example, leafy green vegetables, such as broccoli, cabbage, brussel sprouts and spinach.

Do not drink cranberry juice, grapefruit juice or pomegranate juice while you're taking warfarin. It can increase the blood-thinning effect of your medicine.


Do not drink large amounts of alcohol while taking warfarin.

Taking warfarin with other medicines

Many medicines and supplements can interfere with warfarin. This can make you more likely to have bleeding.

You might need a blood test to check the other medicine is not affecting how your blood clots.

Check with a GP or pharmacist before taking any other medicines with warfarin.

Taking warfarin with everyday painkillers

Tell your GP or pharmacist if you are taking medicines for pain, such as paracetamol.

Do not take aspirin or ibuprofen while you're taking warfarin unless a GP has said it's OK to. They increase the chance of bleeding.

Herbal remedies and supplements

Do not take St John's wort, the herbal remedy for depression, while you're taking warfarin.

It can increase your risk of side effects.

Tell your GP or pharmacist if you're taking any other medicines, including herbal medicines, vitamins or supplements.

Fact check

This content was fact checked by a pharmacist, a GP, the National Medication Safety Programme (Safermeds) and the Health Products Regulatory Authority (HPRA).

Slaintecare logo
This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.

Page last reviewed: 24 September 2021
Next review due: 24 September 2024