Warfarin is a type of medicine known as an anticoagulant.
It makes your blood flow through your veins more easily. This means your blood will be less likely to make a dangerous blood clot.
Warfarin is used to treat blood clots and help prevent future blood clots if you've had one previously, such as:
- a blood clot in the leg (deep vein thrombosis, or DVT)
- a blood clot in the lungs (pulmonary embolism)
It's also used to prevent blood clots if you're at high risk of having them in the future.
This includes people with:
- an irregular heartbeat (atrial fibrillation)
- a replacement or mechanical heart valve
- a blood clotting disorder, such as thrombophilia
- a higher chance of having a blood clot after an operation
Warfarin is only available on prescription. It comes as tablets and as a liquid that you swallow.
When taking warfarin, you need to have regular blood tests. It's important to keep having these tests at the times you're advised.
It's also important to tell your doctor straight away if you take warfarin and have tested positive for COVID-19 or have COVID-19 symptoms.
Updated: 15 October 2020
Who can take warfarin
Most adults and children can take warfarin.
Warfarin is not suitable for some people. To make sure it's safe for you, tell your doctor if you:
- have ever had an allergic reaction to warfarin or any other medicine
- are trying to get pregnant or you're already pregnant
- have liver or kidney problems
- have had an infection of the lining of your heart, known as endocarditis
- have a health problem that causes bleeding (such as a stomach ulcer) or makes you bruise easily
- have high blood pressure (hypertension)
Dosage and strength
The usual warfarin dose for adults is 10mg a day for the first 2 days, then between 3mg and 9mg a day after that.
The usual warfarin dose for children depends on how much they weigh.
Warfarin tablets come in 4 different strengths. The tablets and the boxes they come in are different colours to make it easier for you to take the right dose.
The strengths and colours are:
- 0.5mg – white tablet
- 1mg – brown tablet
- 3mg – blue tablet
- 5mg – pink tablet
Your dose may be made up of a combination of different coloured tablets.
Warfarin also comes as a liquid, where 1ml is equal to a 1mg (brown) tablet.
Warfarin liquid comes with a plastic syringe to help you measure the right amount.
Your warfarin dose may change often, especially in the first few weeks of treatment, until your doctor finds the dose that's right for you.
You'll usually take warfarin once a day in the evening. Take it at around the same time each day. This is so that if you need to change the dose after a routine blood test, you can do this the same day rather than waiting until the following morning.
Warfarin does not usually upset your stomach, so you can take it with or without food.
How long to take it for
If you have had a blood clot in your leg or lungs, you'll probably take a short course of warfarin for 6 weeks to 6 months.
If you take warfarin to reduce your risk of having a blood clot in future or because you keep getting blood clots, it's likely your treatment will be for longer than 6 months, maybe even for the rest of your life.
Regular blood tests
The aim of warfarin treatment is to make your blood clot more slowly, not to stop it from clotting completely. Getting this balance right means your dose of warfarin must be carefully monitored.
You'll have a regular blood test called the international normalised ratio (INR). It measures how long it takes your blood to clot. The longer your blood takes to clot, the higher the INR.
Most people taking anticoagulants have a ratio of between 2 and 3.5. This means their blood takes 2 to 3.5 times longer to clot than usual.
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 local hospital's anticoagulant clinic.
You'll have a test every 1 or 2 days when you first start taking warfarin, then once or twice a week, until your ratio is stable at the target level.
Once your blood test results are stable, you might only need a blood test up to once every 12 weeks. You might need blood tests more often than normal if you start other new medicines, or when you are ill.
The yellow book and alert card
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.
You'll also be given an anticoagulant alert card. 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.
If you need any medical or dental treatment, show your anticoagulant alert card to the nurse, doctor or dentist beforehand. This includes before you have vaccinations and routine sessions 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 for one.
If you forget to take warfarin
It's important to take warfarin on time.
It's not a problem if you occasionally forget to take a dose at the correct time. But if you forget often, your blood could be affected – it might become thicker and put you at risk of having 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 next dose at the usual time.
Never take 2 doses at the same time. Never take an extra dose to make up for a forgotten one.
If you often forget doses, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways to remember your medicines.
If you take too much
If you take too much warfarin you may be advised to change your next dose of warfarin or have a blood test.
If you take more than your prescribed dose of warfarin, you're at risk of serious bleeding.
- you take more than your prescribed dose of warfarin
If you need to go to A&E, take the warfarin packet or leaflet inside it, plus any remaining medicine, with you. If you have a yellow book, take that too.
Bleeding and what to do about it
While warfarin has enormous benefits, the downside is that it can make you bleed more than normal.
This is because while you're taking warfarin, your blood will not clot as easily.
Apart from the risk of bleeding, warfarin is a very safe medicine. It's safe to take for a long time, even many years.
It's usual to bleed more easily than normal while you're taking warfarin.
The kind of bleeding you might have includes:
- periods that are heavier and last longer than usual
- bleeding for a little longer than usual if you cut yourself
- occasional nosebleeds (that last for less than 10 minutes)
- bleeding from your gums when you brush your teeth
- bruises that come up more easily and take longer to fade than usual
This type of bleeding is not dangerous and should stop by itself. If it happens, keep taking warfarin, but tell your doctor if the bleeding bothers you or does not stop.
Things you can do to help yourself:
- Cuts – press on the cut for 10 minutes with a clean cloth.
- Nosebleeds – sit or stand upright (do not lie down), pinch your nose just above your nostrils for 10 to 15 minutes, lean forward and breathe through your mouth, and place an icepack (or a bag of frozen peas wrapped in a teatowel) at the top of your nose.
- Bleeding gums – try using a soft toothbrush and waxed dental floss to clean your teeth.
- Bruises – these are harmless, but can be unsightly. It might help to make them fade more quickly if you put an ice pack wrapped in a towel over the bruise for 10 minutes at a time several times a day.
What you can do to prevent bleeding
While you're taking warfarin, be careful when you do activities that might cause an injury or a cut or bruising.
It can help to:
- avoid playing contact sports or other activities that can cause an 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 – do not wear dentures or retainers that do not fit properly
- tell your doctor, dentist or nurse that you take warfarin before you have any medical or dental procedures or surgery – this includes vaccinations and routine appointments with the dental hygienist
Other common side effects
These side effects are usually mild. There are things you can do to help cope with them:
Talk to your doctor or pharmacist if these side effects bother you or do not go away.
Occasionally, you can have serious bleeding from taking warfarin. This can be dangerous and needs urgent medical attention.
- you have red pee or black poo
- you get bruises that happen for no reason, or bruises that are larger than you'd expect or that keep growing in size
- you get nosebleeds that you cannot stop and that last longer than 10 minutes
- you have blood in your vomit or you're coughing up blood
- you get severe headaches
- you have any bleeding from a cut or injury that will not stop or slow down
These are symptoms of serious bleeding. If you have any serious bleeding, stop taking warfarin.
Very rarely, warfarin can lead to bleeding in the brain.
you get any of these symptoms:
- a very severe headache
- seizures or fits
- sudden loss of vision or blurred vision
- numbness or tingling in your arms or legs
You may also feel tired, weak or sick.
In rare cases, warfarin can cause a serious allergic reaction (anaphylaxis).
These are not all the side effects of warfarin. For a full list, see the leaflet inside your medicine's packet.
Changes to your diet
It's very important to keep your diet stable while taking warfarin. 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 doctor 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 affect how warfarin works.
- green leafy vegetables, including broccoli, spinach and lettuce
- egg yolks
- mature cheese and blue cheese
- olive oil
It's important that you eat foods containing vitamin K, so rather than leaving them out of your diet, make sure you eat similar amounts of them regularly.
This will mean the level of vitamin K in your blood stays fairly constant and makes it more likely that your INR level stays stable.
Do not drink cranberry juice, grapefruit juice or pomegranate juice while you're taking warfarin. It can increase the effect of your medicine and put you at higher risk of bleeding.
Warfarin and pregnancy
Warfarin is not recommended during pregnancy as it can sometimes cause birth defects and bleeding problems for the baby.
However, if you have a metal heart valve you may be advised to continue taking warfarin in pregnancy because the risk of the valve clotting is greater than the risk to the baby. This would always be under the supervision of a specialist doctor.
If you are taking warfarin, it's important that you speak to your doctor before you try for a baby so that your medicine can be reviewed. They will help you weigh up the risks and benefits of staying on warfarin. It may be possible to switch to another medicine (heparin injections) before and during your pregnancy.
It's important to use reliable contraception while you're taking warfarin.
Talk to your doctor or midwife immediately if you become pregnant and you're taking warfarin. They should review you within the next working day. Do not stop taking your warfarin unless you are told to do so.
If your doctor of midwife says your baby is healthy, it's OK to take warfarin while breastfeeding.
Warfarin gets into breast milk in tiny amounts, and it is unlikely to cause side effects in your baby.
Talk to your health visitor, midwife, pharmacist or doctor as soon as possible if:
- your baby is not feeding as well as usual
- your baby seems to bleed or bruise easily
- you have any other concerns about your baby
Warfarin and fertility
There's no evidence to suggest that taking warfarin reduces fertility in either men or women.
Speak to your doctor if you're trying to get pregnant. Because warfarin is not recommend in pregnancy they will want to review your medicine.
Cautions with other medicines
Many medicines and supplements can affect warfarin. This can make you more likely to bleed.
You might need a blood test to check the other medicine is not affecting how your blood clots.
Tell your doctor if you're taking any of these medicines before you start taking warfarin:
- medicines for heart problems, such as amiodarone, quinidine or propafenone
- non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or aspirin
- cholesterol-lowering medicines, such as bezafibrate, gemfibrozil, clofibrate or cholestyramine
- antibiotics, such as erythromycin, co-trimoxazole or norfloxacin
- miconazole gel for fungal infections like thrush
It's safe to take paracetamol while you're on warfarin. But take the lowest dose that controls your pain. Taking more than four, 500mg tablets over 24 hours for longer than a few days may make your blood clot more slowly. This puts you at risk of bleeding.
If you're still in pain after taking paracetamol for 3 or 4 days, speak to your pharmacist or doctor.
Mixing warfarin with 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 doctor 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 doctor first.
There's not enough information to say that other herbal remedies and supplements are safe to take with warfarin. They're not tested in the same way as pharmacy and prescription medicines.
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