Warfarin is the main oral
anticoagulant used in the UK. Oral means it's taken by mouth. An anticoagulant
is a medicine that stops blood clotting. Clotting (thickening) is a complex process involving a number of
substances called clotting factors.
Clotting factors are produced by the liver and help control bleeding.
They work with cells that trigger the clotting process (platelets) to ensure
blood clots effectively. To produce some of the clotting factors, the liver needs a good supply
of vitamin K.
Warfarin blocks one of the enzymes (proteins) that uses vitamin K to
produce clotting factors. This disrupts the clotting process, making it take
longer for the blood to clot.
When warfarin is
prescribed
Anticoagulant medicines, such as
warfarin, are often prescribed for people who've had a condition
caused by a blood clot, such as:
- a stroke
- a heart attack
- deep vein thrombosis – a blood clot within a deep vein in the body, usually in a leg
- pulmonary embolism – a blood clot in the lungs.
Warfarin may also be prescribed for people at an increased risk of
developing harmful blood clots, such as those with:
- a replacement or mechanical (prosthetic) heart valve
- an irregular heart rhythm, known as atrial fibrillation
- a blood clotting disorder, such as thrombophilia
- an increased risk of blood clots following surgery.
Taking warfarin
It is very important that you take warfarin exactly as directed. Do not
increase your prescribed dose unless the doctor in charge of your care advises
you to.
Warfarin is taken once a day, usually in the evening. It is important to
take your dose at the same time each day, before, during or after a meal.
The aim of warfarin therapy is to decrease the blood's tendency to clot,
but not stop it clotting completely. This means the dose of warfarin
you are taking must be carefully monitored and, if necessary, adjusted.
You will have regular blood tests at your GP surgery or
local anticoagulant clinic to make sure your dose is correct. The international normalised ratio (INR) is a measure of how long it
takes your blood to clot. The longer it takes your blood to clot, the higher
your INR. Your INR will be used to determine the dose of warfarin you need
to take.
Although there are now three new anticoagulants that don't require
regular monitoring – rivaroxaban, apixaban and dabigatran – most people who
need an anticoagulant will be prescribed warfarin.
When you start taking warfarin, you may be given a yellow booklet about
anticoagulants, which explains your treatment. How long you will need to take warfarin for will depend on the condition
for which it has been prescribed. You must ask the healthcare professional responsible for
your care if you are not sure.
Schematic diagram showing the intrinsic, extrinsic and common pathways of the coagulation cascade, and the mechanism of action of Warfarin.
Missed doses
If you usually take warfarin in the morning and forget to take it at
your normal time, take it as soon as you remember and continue as normal. However, if it is time to take your next dose, do not take a
double dose to catch up, unless your GP has specifically advised you to.
If you forget to take your dose of warfarin in the evening but remember
before midnight on the same day, take the missed dose. If midnight has passed, leave that dose and take your normal dose the
next day at the usual time.
You should ask your GP or staff at your local anticoagulant clinic if you are not
sure what to do about a missed dose of warfarin. You can also call NHS 111 for advice.
Who should not take
warfarin
The following people should not take warfarin:
- pregnant women – it can affect the development of the baby
- people with uncontrolled high blood pressure (severe hypertension)
- people with a high risk of internal bleeding – for example, those with a stomach ulcer
- people with a bleeding disorder – such as haemophilia.
Side effects of warfarin
Bleeding is the main side effect associated with warfarin, as it slows
down the blood's normal clotting ability. You are at greatest risk of bleeding in the first few weeks of starting
treatment with warfarin and when you are unwell.
You should therefore seek medical attention if you:
- pass blood in your urine or faeces
- pass black faeces
- have severe bruising
- have long nosebleeds – lasting more than 10 minutes
- have bleeding gums
- cough up blood or have blood in your vomit
- experience unusual headaches
- have heavy or increased bleeding during your period, or any other bleeding from your vagina (in women).
Take extra care to avoid cutting yourself while taking anticoagulant
medication because of the risk of excessive bleeding. For example, you should:
- take care when shaving and brushing your teeth
- wear protective clothing when gardening, sewing or playing contact sports
- use insect repellent to avoid insect bites or stings
You must seek urgent medical attention if you are taking warfarin and you:
- have a fall or accident
- experience a significant blow to your head
- are unable to stop any bleeding
- have signs of bleeding, such as bruising.
Skin rashes and hair loss are also common side effects
of warfarin. You should contact your doctor/ GP or the healthcare specialist responsible for your care
if you experience any persistent side effects while taking warfarin.
Interactions
Medicines
Warfarin can interact with many other medicines. The patient information
leaflet that comes with a medicine should tell you if it's safe to take with
warfarin. You should ask your GP or pharmacist if you are unsure. When taking warfarin:
- do not take aspirin, or treatments containing aspirin, unless it is prescribed by a healthcare professional as it could cause bleeding
- do not take ibuprofen unless it is prescribed by a healthcare professional
- you can take paracetamol, but do not take more than the recommended dose.
Herbal medicines and supplements can also interact with warfarin. You
should therefore avoid taking them without first checking with your GP,
pharmacist, or staff at your local anticoagulant clinic.
Food and drink
Some food and drink can interfere with the effect of warfarin if
consumed in large amounts, including foods that are rich in vitamin K. Foods containing large amounts of vitamin K include:
- green leafy vegetables, such as broccoli and spinach
- vegetable oils
- cereal grains.
Small amounts of vitamin K can also be found in meat and dairy foods. When your first dose of warfarin is prescribed, it does not matter how
much vitamin K you are eating because the dosage will be based on your current
blood clotting levels.
However, if you make significant changes to your diet, such as
increasing your vitamin K intake or cutting out foods that contain
vitamin K, it could interfere with how warfarin works.
Consult the healthcare professional responsible for your care before
making any significant changes to your diet while taking warfarin. They aill also
be able to give you more information about foods to avoid or limit.
Alcohol
Getting drunk or binge drinking is dangerous while
taking warfarin. It may increase the effect of the drug, increasing the risk of
bleeding. The latest guidelines on drinking alcohol state
that regularly drinking more than 14 units of alcohol a week (for both men and
women) risks damaging your health.
Fourteen units is equivalent to six pints of average-strength beer or 10
small glasses of low-strength wine. People with liver disease who are taking warfarin should not drink
alcohol at all.
Surgery and dental work
Because of the risk of bleeding, your dose of warfarin may
need to be lowered or stopped a few days before having an operation or dental
work.
Tell the surgeon or dentist that you are taking warfarin. You should also
tell anyone else involved with your care, such as an anticoagulant nurse, if
you need an operation so they can make arrangements.
Having vaccinations
You can have vaccinations while taking warfarin. If the vaccine is normally given by injection into the muscle, then
providing you are up to date with your INR testing (see above), and that the
results are within the correct range, you can have the injection as normal into
muscle. This is known as an intramuscular (IM) injection.
Alternatively, the injection can be given into the layer of fat
underneath your skin. This is known as a subcutaneous injection. Firm pressure
applied to the site for 10 minutes after the vaccination may reduce potential
bruising.
Playing sports
You can play sports while taking warfarin, but because of the risk
of bleeding:
- contact sports that could lead to a head injury, such as football, rugby, cricket and hockey, are best avoided if played competitively
- martial arts and kickboxing must be avoided.
You can continue to take part in non-contact sports, such as running,
athletics, cycling and racket sports. However, you should make sure you wear protective
clothing, such as a cycle helmet.
Going on holiday
You must tell your GP or anticoagulant nurse if you are going on holiday, in this
country or abroad, and arrange to have your INR checked before you go.
If you're going to be away longer than a month, you may need to arrange
to have your INR checked while you're away. Make sure you have enough warfarin
tablets to last for the duration of your trip.
Body piercings
Body piercings are not recommended
while taking warfarin because of the increased risk of bleeding and risk of
infection.
Source: NHS Choices