Warfarin
Warfarin (brand names Coumadin and Jantoven) is a water-soluble compound and medicine. It is an anticoagulant, which means that it prevents blood from clotting[1].
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| Pronunciation | /ˈwɔːrfərɪn/ |
| Trade names | Coumadin, others[3][4][5] |
| AHFS/Drugs.com | Monograph |
| MedlinePlus | a682277 |
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| Routes of administration | By mouth, intravenous |
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| Bioavailability | 79–100% (by mouth)[8] |
| Protein binding | 99%[7] |
| Metabolism | Liver: CYP2C9, 2C19, 2C8, 2C18, 1A2 and 3A4[7] |
| Elimination half-life | 1 week (active half-life is 20-60 hours)[7] |
| Excretion | Kidney (92%)[7] |
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| ECHA InfoCard | 100.001.253 |
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This medicine can cause serious allergic reactions which require emergency treatment. Make sure the signs of these reactions are known: read allergic reaction symptoms of drugs.
It works by reducing the production of factors by the liver that promote clotting. It is called an "anticoagulant agent". It is a powerful rat poison but it affects the rodent in different ways to humans. There are newer rodenticides now that some rats are immune to warfarin. In concentrated form, it causes bleeding of internal organs which leads to death.[9]
In medicine[change | change source]
It is also used, in very low doses, to prevent thrombosis in humans.[1][10] This is the local coagulation or clotting of the blood in the circulatory system. In such cases, it makes heart attacks, strokes, and pulmonary embolism less likely.[10]
It does this by making the blood flow through veins more easily. This means that 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 one has happened previously, such as:[1]
- a blood clot in the leg (deep vein thrombosis, or DVT)
- a blood clot in the lungs (pulmonary embolism)
- it is also used to prevent blood clots in people at a higher risk of having them in the future.
This also means people with:[1]
- 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
Who may not be able to take warfarin[change | change source]
Most adults and children can take warfarin. But warfarin is not suitable for some people. To make sure it is safe, problems or conditions that should be reported:[1]
- have ever had an allergic reaction to warfarin or any other medicine
- trying to get pregnant or are already pregnant
- have liver or kidney problems
- have had an infection of the lining of the heart, known as endocarditis
- have a health problem that causes bleeding (such as a stomach ulcer) or makes bruises appear more easily
- high blood pressure (hypertension)
How and when to take warfarin[change | change source]
It is important to take warfarin as directed by a doctor.[1]
Warfarin is only available on prescription. It comes as tablets and as a liquid that are swallowed[1]
Dosage and strength[change | change source]
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.[1]
Warfarin tablets come in 4 different strengths. The tablets and the boxes they come in are different colours to make it easier to take the right dose[1]. The strengths and colours are:[1]
- 0.5mg – white tablet
- 1mg – brown tablet
- 3mg – blue tablet
- 5mg – pink tablet
But the dose may be made up of a combination of different coloured tablets.[1]
Warfarin also comes as a liquid, where 1ml is equal to a 1mg (brown) tablet[1]. Warfarin liquid comes with a plastic syringe to help people measure out the right amount.[1]
The warfarin dose may change often, especially in the first few weeks of treatment, until the doctor finds the dose that is right for the specific patient.[1]
How to take it[change | change source]
Usually, warfarin is taken once a day in the evening. It should be taken at around the same time each day. This is so that if the dose needs to change after a routine blood test, this can be done in the same day rather than waiting until the following morning.[1]
Warfarin does not usually cause an upset stomach, so it can be taken with or without food.[1]
How long to take it for[change | change source]
If there was a blood clot in the leg or lungs, then probably, it will be a short course of warfarin for 6 weeks to 6 months.[1]
If warfarin is taken to reduce the risk of having a blood clot in the future or because a person keeps getting blood clots, it is likely that treatment will last for longer than 6 months, maybe even for the rest of the persons life.[1]
Regular blood tests[change | change source]
The aim of warfarin treatment is to make the blood clot more slowly, not to stop it from clotting completely. Getting this balance right means that the dose of warfarin must be carefully monitored.[1]
There will be a regular blood test called the international normalised ratio (INR). It measures how long it takes blood to clot. The longer the 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.[1]
The dose of warfarin needed depends on the blood test result. If the blood test result has gone up or down, then the warfarin dose will be increased or decreased.[1] Blood tests are done at the GP surgery or local hospital's anticoagulant clinic.[1]
There will be a test every 1 or 2 days while first starting taking warfarin, then once or twice a week, until the INR ratio is stable at the target level.[1]
Once blood test results are stable, blood test might be needed only up to once every 12 weeks. Blood tests may be needed more often than normal while starting other new medicines, or while ill.[1]
Forgetting to take warfarin[1][change | change source]
It is important to take warfarin on time.
It is not a problem to occasionally forget to take a dose at the correct time. But if forgetting happens often, the blood could be affected – it might become thicker and put the patient at risk of having a blood clot.
If dose of warfarin is missed, write it down in the yellow book if one was given, or in a diary or journal.
Take the missed dose as soon as it is remembered. If remembering does not happen until the next day, then skip the missed dose and take the 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 often forgetting doses, it may help to set an alarm to remember. A pharmacist will also have advice on other ways to remember to take medicines.
Taking too much[1][change | change source]
If taking too much warfarin, it may be advised to change the amount of the next dose of warfarin or have a blood test.
Taking more than the prescribed dose of warfarin means there is risk of serious bleeding. Call non-emergency medical services or the anticoagulant clinic if too much has been taken.
Call an ambulance or go to A&E, and take the warfarin packet and leaflet inside it, plus any remaining medicine, and the yellow book if there is one, too.
The yellow book and alert card[1][change | change source]
While starting warfarin, in some countries people may be given a yellow book about anticoagulants. This explains things about treatment. There will also be a section where a record of the warfarin dose can be written and recorded. It is a good idea to take the yellow book to all warfarin appointments if it was needed.
A lot of people will also be given an anticoagulant alert card. It should be carried with the person at all times. It tells healthcare professionals that anticoagulants are being taken. This can be useful for them to know in case of a medical emergency.
If any medical or dental treatment is needed, show the anticoagulant alert card to the nurse, doctor or dentist beforehand. This includes before vaccinations and routine sessions with the dental hygienist.
The doctor may advise stopping taking warfarin or to reduce dose for a short time before treatment.
If the alert card was lost or one was not given, ask the doctor or anticoagulant clinic for one.
Diet changes[change | change source]
It is very important to keep a stable diet while taking warfarin. This means that the dose of warfarin is more likely to stay the same. Any big changes in eating or drinking can change how the body responds to warfarin.[1]
Speak to the doctor or nurse before changing diet – for example, before going on a diet to lose weight.[1]
Foods that affect warfarin[change | change source]
Foods containing a lot of vitamin K can affect how warfarin works[1]. These include:[1]
- green leafy vegetables, including broccoli, spinach and lettuce
- chickpeas
- liver
- egg yolks
- mature cheese and blue cheese
- avocado
- olive oil
While taking warfarin, it is important to eat foods containing vitamin K, so rather than leaving them out of a diet, eating similar amounts of them regularly is good. This will mean the level of vitamin K in the blood stays fairly constant and makes it more likely that the INR level stays stable.[1]
Cranberry or grapefruit juice drinks with warfarin is not good and not recommended. It can increase the effect of the medicine and put the person taking at higher risk of bleeding.[1]
Cautions with other medicines[change | change source]
Cranberry or grapefruit juice drinks with warfarin is not good and not recommended. It can increase the effect of the medicine and put the person taking at higher risk of bleeding.
Other things including many medicines and supplements can affect warfarin. This can make someone more likely to bleed. They might need a blood test to check the other medicine is not affecting how the blood clots.[1] Check with the doctor before taking any of these medicines with warfarin:[1]
- medicines for heart problems, such as amiodarone, quinidine or propafenone
- non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or aspirin
- tramadol, a strong painkiller
- 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
Taking warfarin with everyday painkillers[change | change source]
It is safe to take paracetamol while on warfarin[1]. But take the lowest dose that controls pain. Taking more than four, 500mg tablets over 24 hours for longer than a few days may make the blood clot more slowly. This puts the person taking the drugs at a higher risk of bleeding.[1]
If pain still lasts after taking paracetamol for 3 or 4 days, speak to a pharmacist or doctor.[1]
Do not take aspirin or ibuprofen while taking warfarin unless a doctor has said it is OK. They increase the chance of bleeding.[1]
Mixing warfarin with herbal remedies and supplements[change | change source]
Do not take St John's wort, the herbal remedy for depression, while taking warfarin. It can increase the risk of side effects.[1]
Tell a doctor if vitamin K supplements are already being taken before starting to take warfarin. Do not start taking vitamin K supplements while taking warfarin without checking with a doctor first.[1]
There is not enough information to say that other herbal remedies and supplements are safe to take with warfarin. They are not tested in the same way as pharmacy and prescription medicines.[1]
Side effects[change | change source]
Bleeding and what to do about it[change | change source]
While warfarin has enormous benefits, the downside is that it can make bleeding happen more than normal. This is because while taking warfarin, the blood will not clot as easily.[1]
Apart from the risk of bleeding, warfarin is a very safe medicine. It is safe to take for a long time, even many years.[1]
Less serious bleeding[change | change source]
It is usual to bleed more easily than normal while taking warfarin. The kind of bleeding may include:[1]
- periods that are heavier and last longer than usual
- bleeding for a little longer than usual if a cut happens
- occasional nosebleeds (that last for less than 10 minutes)
- bleeding from gums while brushing teeth[1], and bleeding from the mucous membranes
- 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, to keep taking warfarin is recommended, but tell a doctor if the bleeding is a bother or does not stop.[1]
Things the patient could do to cope with or ease symptoms[change | change source]
- Cuts – pressing on the cut for 10 minutes with a clean cloth.[1]
- Nosebleeds – sit or stand upright (do not lie down), pinch the nose just abov nostrils for 10 to 15 minutes, lean forward and breathe through the mouth, and place an icepack (or a bag of frozen peas wrapped in a teatowel) at the top of the nose.[1]
- Bleeding gums – try using a soft toothbrush and waxed dental floss for brushing teeth.[1]
- Bruises – these are harmless, but can be unsightly. If they are, it might help make them fade quicker by putting an ice pack wrapped in a towel over the bruise for 10 minutes at a time several times a day.[1]
Things to do to prevent bleeding[change | change source]
While taking warfarin, be careful whilst doing activities that might cause an injury or a cut or bruising[1]. It can help to:[1]
- avoid playing contact sports or other activities that can cause an injury, such as football, rugby, hockey and horse riding
- wear gloves while using 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 they are worn, to give the gums a rest – do not wear dentures or retainers that do not fit properly
- tell doctors, dentists or nurses that warfarin is being taken before having any medical or dental procedures or surgery – this includes vaccinations and routine appointments with the dental hygienist
Other common side effects[change | change source]
There are things that can be done do to help cope with these. Other common side effects are usually mild and include:[1]
- a mild rash
- hair loss
Talk to a doctor or pharmacist if these side effects are upsetting or bothering or do not go away.[1]
Serious bleeding[change | change source]
Occasionally, serious bleeding can occur from taking warfarin. This can be dangerous and needs urgent medical attention. Get medical attention if there is:[1]
- red urine or black poo
- bruising or bruises which happen for no reason, or bruises that are larger than what would be expected, or that keep growing in size.
Call an ambulance or go to A&E (the person not driving themselves), if there are:
- nosebleeds will not or cannot stop and that last longer than 10 minutes
- blood in the vomit or coughing up blood
- severe headaches or getting severe headaches
- any bleeding from a cut or injury that will not stop or slow down
These are symptoms of serious bleeding. If there are any serious bleeding, stop taking warfarin.
Bleeding in the brain[change | change source]
Very rarely, warfarin can lead to bleeding in the brain. Call emergency services or go to A&E now if:[1] There are any of these symptoms:[1]
- a very severe headache
- seizures or fits
- sudden loss of vision or blurred vision
- numbness or tingling in the arms or legs
The patient may also feel tired, weak or sick.
Serious allergic reaction[change | change source]
In rare cases, warfarin can cause a serious allergic reaction (anaphylaxis). Call an ambulance.[1]
These are not all of the sise effects of warfarin. Read the leaflet in the packet or see a doctor for more information.[1]
Related pages[change | change source]
References[change | change source]
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 1.27 1.28 1.29 1.30 1.31 1.32 1.33 1.34 1.35 1.36 1.37 1.38 1.39 1.40 1.41 1.42 1.43 1.44 1.45 1.46 1.47 1.48 1.49 1.50 1.51 1.52 1.53 1.54 1.55 1.56 1.57 1.58 1.59 "How and when to take warfarin". nhs.uk. 2022-04-28. Retrieved 2024-06-02.
- ↑ "Warfarin Use During Pregnancy". Drugs.com. 4 September 2019. Archived from the original on 9 February 2018. Retrieved 7 February 2020.
- ↑ Cite error: The named reference
brandswas used but no text was provided for refs named (see the help page). - ↑ Cite error: The named reference
brands2was used but no text was provided for refs named (see the help page). - ↑ Cite error: The named reference
brands3was used but no text was provided for refs named (see the help page). - ↑ "Coumadin- warfarin sodium tablet". DailyMed. Archived from the original on 26 December 2021. Retrieved 25 December 2021.
- ↑ 7.0 7.1 7.2 7.3 Cite error: The named reference
TGAwas used but no text was provided for refs named (see the help page). - ↑ Holford NH (December 1986). "Clinical pharmacokinetics and pharmacodynamics of warfarin. Understanding the dose-effect relationship". Clinical Pharmacokinetics. 11 (6): 483–504. doi:10.2165/00003088-198611060-00005. PMID 3542339. S2CID 92210077.
- ↑ Tornkvist, Max; Smith, J. Gustav; Labaf, Ashkan (2018-02-01). "Current evidence of oral anticoagulant reversal: A systematic review". Thrombosis Research. 162: 22–31. doi:10.1016/j.thromres.2017.12.003. ISSN 0049-3848. PMID 29258056.
- ↑ 10.0 10.1 "Warfarin Monograph for Professionals". Drugs.com. Retrieved 2022-11-14.