Warfarin is a blood thinner.
It makes your blood flow through your veins more easily so you do not 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. It can affect your blood test results.
Uses of warfarin
Warfarin is a treatment:
- for blood clots in the leg - called a deep vein thrombosis (DVT)
- for blood clots in the lungs (pulmonary embolism)
- to prevent blood clots in people at high risk of getting a blood clot
You are more at risk of blood clots if you have surgery or if you have:
- a replacement or mechanical heart valve
- an irregular heartbeat (atrial fibrillation)
- a blood-clotting disorder, such as thrombophilia
Reduce your risk of blood clots
Check if you can take warfarin
Most adults can take warfarin but it is not suitable for some people.
Before taking warfarin tell your GP if you:
- had an allergic reaction to any medicines in the past
- are pregnant, breastfeeding, trying to get pregnant or recently had a baby
- have other health problems
- had any bleeding problems in the past, including a stroke
- had surgery within the past 3 days or are due to have surgery within the next 3 days
- are taking other medicines or remedies, including the herbal remedy St John's wort
- are lactose intolerant
Warfarin and pregnancy
You should not take warfarin during pregnancy or just after giving birth unless your doctor tells you to.
It can be harmful to your baby. This is particularly the case 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, your GP or clinic may give you a book about anticoagulants (yellow book).
This explains your treatment. There's also a section where you keep a record of your warfarin dose.
Your dose of warfarin can depend on:
- your diet (what you eat and drink)
- other medicines you're taking
- if you become unwell
Your GP or clinic may change your dose of warfarin after a blood test.
Take your yellow book with you to all your warfarin appointments.
If you need any medical or dental treatment, show your anticoagulant alert card to the nurse, doctor or dentist beforehand. Show your card even before vaccinations and routine sessions with the dental hygienist.
How and when to take warfarin
Take warfarin as your GP tells you to.
This is usually:
- once a day at about the same time
- in the evening - in case your dose changes after a blood test during the day
- with or without food
If you take too much warfarin
If you take more than your prescribed dose of warfarin, you're at risk of serious bleeding.
You may need to change your next dose of warfarin or have a blood test.
Emergency action required: Contact your GP or anticoagulant clinic immediately if:
- you take more than your prescribed dose of warfarin
If you cannot contact either, go to an emergency department (ED).
Bring the warfarin packet or leaflet and any remaining medicine with you. If you have a yellow warfarin record book, take that too.
If you forget to take it
Try to take your warfarin on time.
If you often forget to take a dose at the right time, 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, 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.
You cannot feel if warfarin is working or not. You need blood tests to check how it is working.
Your GP or clinic may change your dose after these tests. It can take weeks or months to find the dose that works for you.
Regular blood tests
You'll have a regular blood test called the international normalised ratio (INR). This is to check how long it takes your blood to clot.
The dose of warfarin you need depends on your blood test result. If the blood test result goes up or down, your warfarin dose will be increased or decreased.
You'll have the blood tests at your GP surgery or anticoagulant clinic.
When your blood tests are stable you may be able to check your blood with a home monitor, if your GP thinks it's OK.
You might only need a blood test every 8 to 12 weeks if your blood test results are stable.
You might need blood tests more often than normal if you start other new medicines, or when you are ill.
How long you need to take warfarin
You’ll usually take warfarin for 6 weeks to 6 months if you have had a blood clot in your leg or lungs.
Your treatment can last longer than 6 months if you:
- take warfarin to reduce your risk of having a blood clot in future
- keep getting blood clots
Some people need to take warfarin for the rest of their life.
Stopping warfarin
Do not stop taking warfarin without talking to your GP.
If you stop taking warfarin, your blood clot problem may quickly return.
This means you may be at an increased risk of a stroke, heart attack, DVT or pulmonary embolism.
Prevent bleeding
When you're taking warfarin, be careful to avoid getting injuries, including cuts or bruises.
There are things you can do to help avoid injury.
Do
-
wear gloves when you use sharp objects like scissors, knives and gardening tools
-
take false teeth (dentures) or retainers out for a few hours a day to give your gums a rest
-
avoid wearing dentures or retainers that do not fit properly
-
tell doctors, dentists or nurses that you take warfarin before medical or dental appointments - this includes vaccinations and routine appointments with the dental hygienist
Don't
-
do not play sports or take part in activities where injuries are common, such as football, rugby, hockey and horse riding
-
do not wet shave or remove hair with wax - use an electric razor or hair-removing cream instead
Side effects
The most common side effect from warfarin is bleeding. For example, you may have nosebleeds or bruising more often than usual.
Common side effects
Apart from bleeding, there are other common side effects.
There are things you can do to help manage common side effects:
Hair loss
Speak to a pharmacist or your GP if this is bothering you.
Feeling or getting sick (vomiting)
Stick to simple meals if you feel sick (nausea) or you have been getting sick (vomiting). Do not eat rich or spicy food. If you're getting sick, try small frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.
If you take contraceptive pills and you're getting sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Diarrhoea
Drink lots of fluids, such as water or squash, to avoid dehydration. Speak to a pharmacist if you have signs of dehydration such as peeing less than usual or having dark, strong-smelling pee. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or GP.
If you take contraceptive pills and you have severe diarrhoea for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet for advice.
A high temperature of 38 degrees Celsius or higher
Drink plenty of fluids, especially water (little and often is best). Try to drink 2 to 3 litres in 24 hours while you have a fever. This will reduce the risk of dehydration.
Talk to your GP or pharmacist if you get these side effects or any other possible side effects.
Check 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 Health Products Regulatory Authority (HPRA): report an issue - hpra.ie
Serious side effects
Warfarin can make you bleed more than normal.
This is because while you're taking warfarin, your blood does not clot as easily.
Emergency action required: Call your GP, anticoagulant clinic or go to an emergency department (ED) immediately if you have:
- blood in your pee or poo (may be dark red or black)
- bruises that happen for no reason, or are larger than you'd expect or that keep getting bigger
- nosebleeds that last longer than 10 minutes
- 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 even when you put firm pressure on it for at least 5 to 10 minutes
- any other unexplained bleeding you are worried about
Do not take any more warfarin until you get medical advice.
Other serious side effects
Urgent advice: Call a GP urgently if you develop any of these serious side effects:
- yellowing of your skin or the whites of the eyes (jaundice) - this can be a sign of liver problems
- a painful rash or blisters on your skin
- painful swollen patches on your skin - particularly if you have chronic kidney disease
Do not take any more warfarin until you get medical advice.
Serious allergic reaction
In rare cases, warfarin can cause a serious allergic reaction (anaphylaxis).
Emergency action required: Call 112 or 999 or go to an emergency department (ED) immediately if:
- you get a rash that is itchy, red, swollen, blistered or peeling
- you're wheezing
- you get tightness in your chest or throat
- you have trouble breathing or talking
- your mouth, face, lips, tongue or throat start swelling
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
Do not take any more warfarin until you get medical advice.
Anticoagulant 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 doctors or dentists before you have any medical or dental procedures. This includes vaccinations and routine appointments with the dental hygienist.
They may tell you to stop taking warfarin or reduce your dose for a short time before any procedures.
If you lose your alert card or do not have one, tell your GP or anticoagulant clinic. They can get 1 for you.
Food and drink
Do not make any big changes to your diet. This way your warfarin dose is more likely to stay the same.
Speak to your GP, nurse or anticoagulation clinic if you are going to change what you eat. For example, before going on a diet.
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.
Vitamin K can make warfarin less effective.
Foods rich in vitamin K include:
- green leafy vegetables - such as Brussels sprouts, kale, cabbage, spinach, lettuce
- beetroot
- broccoli
- cauliflower
- avocado
- peas
- asparagus
- beans
- lentils
- liver
Talk to your GP or anticoagulation clinic if you have any questions about your diet and warfarin.
Alcohol
Alcohol can increase the risk of bleeding. 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 bleed.
You might need a blood test to check other medicines do not affect how your blood clots.
Check with your 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 says it's OK. They increase the chance of bleeding.
Herbal remedies and supplements
Tell your GP or pharmacist if you're taking any other medicines, including herbal medicines, vitamins or supplements.
Do not take St John's wort while you're taking warfarin. St John's wort is a herbal remedy for depression, anxiety and other conditions.
It can increase your risk of side effects.
Tell your GP or anticoagulation clinic if you're taking vitamin K supplements before you start taking warfarin.
Do not start taking vitamin K supplements while you are taking warfarin without checking with your GP or anticoagulation clinic first.
Fact check
This content was fact checked by a pharmacist, a GP, and the National Medication Safety Programme (Safermeds).
