Warfarin - A Patient Information Guide
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What is Warfarin?
Warfarin is a drug used to ‘thin’ the blood. Warfarin is described as a
blood thinner because it prevents blood from clotting in places where
blood should not clot and it prevents existing clots from getting bigger.
Why would I need to take Warfarin?
Your doctor may prescribe Warfarin to prevent blood clots from forming.
Blood clots can form in our bodies for a number of reasons. If a blood
clot becomes lodged in a smaller vessel, this can lead to a stroke, a
heart attack or a pulmonary embolus (a clot on the lung). All of these can
be life-threatening and need urgent treatment.
You may be prescribed Warfarin if you have a condition known as atrial
fibrillation (an abnormal heart rhythm), or if you have an artificial heart
valve.
Blood clots can form if you have recently had a hip replacement or
abdominal surgery. You could also develop a clot if you are sitting in one
position on a long journey such as a flight, car, coach or train journey.
Will I need any special tests?
All patients taking Warfarin must have regular blood testing. The test is
called INR (International Normalised Ratio). These monitor if the blood is
too thick and sticky or too thin.
The INR of a person who has not been on Warfarin is 1.0, therefore the
higher the INR the, thinner the blood. It is important therefore to have
close monitoring to measure the INR to see whether more, less or the
same dose of Warfarin is required. This will probably be weekly initially
and will gradually extend up to 8-12 weeks when stable.
If you have not had your INR checked within the last 12 weeks then it is
strongly advised that you get your INR checked within the next 7 days
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What happens when I have my INR done?
If you have the test at a clinic, a finger prick blood test is done to
measure your clotting time (INR). Your GP may do a different kind of
blood test where blood is taken from your arm.
Identification
You should be given Yellow Card, [anticoagulant treatment booklet]
when you start treatment with Warfarin. This will contain details specific
to your treatment including:




The name of the condition requiring treatment
The name of your anticoagulant
Record of dosage (historic and current)
INR (historic, current and target). Your target INR will vary
depending on the reason for Warfarin being prescribed and you
should be aware of your target INR
 Advice on your anticoagulant treatment.
You should carry the yellow card with you at all times.
What will my dosage be?
The dose of anticoagulant that you need to take will depend on your INR
test result. If your result is out of the range appropriate for your
condition, your dose of anticoagulant will be increased or decreased
accordingly. The anticoagulant dose required to achieve the target INR
varies for each person.
Does Warfarin come in different strengths?
Warfarin tablets come in four different strengths and are colour coded to
help you take the correct dose.
Colours are as follows:




White - 0.5mg
Pink – 5 mg
Blue – 3mg
Brown – 1mg
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When should it be taken?
The dose should be taken regularly at the same time each day.
What if I miss a dose?
If a dose is missed do not take a double dose the next day. Just take
your normal dose the next day. If more than one dose is missed; you
should contact the surgery or clinic where you have your blood tests.
Will other medicines affect the blood thinning effect of Warfarin?
Yes, many medicines interfere with Warfarin. Always tell any doctor or
dentist who is about to prescribe other medicine that you are taking
Warfarin and do not take any non-prescription medicines without first
checking with your doctor or pharmacist.
If you are started on any new medication, including over the counter or
herbal / alternative medication you must have your INR checked within
5 days. Speak to the Nurse or Doctor who takes your Warfarin blood
tests. If you require pain relief, Paracetamol is advisable. If you require
anything other than this, check with your pharmacy to obtain a suitable
alternative.
What should I do if I take too much Warfarin?
Signs of having taken too much Warfarin could include bruising, nose
bleeds and blood in the urine. Contact the clinic that monitors your blood
for advice if you have any symptoms. If this happens outside of normal
working hours e.g. in the evening or on a weekend, contact your GP out
of hours’ service or NHS Direct Wales.
Diet and Alcohol
As Warfarin works by inhibiting the action of vitamin K, changes to your
dietary intake of vitamin K can alter the effect of your Warfarin. For this
reason, avoid making sudden major changes to your diet, particularly
your consumption of green tea, salad and green vegetables (e.g.,
broccoli, brussel sprouts, or spinach), which contain large amounts of
vitamin K. Large amounts of green vegetables (more than 500g daily)
can reduce the effect of Warfarin and should be avoided.
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Changes to your consumption of fats and oils can also alter the effect of
Warfarin, as vitamin K is a fat soluble vitamin.
The effects of Warfarin can also be altered by other foods, for example
soya bean products can reduce the effect of Warfarin, and avocados
and large amounts of ice cream (over a litre a day) have also been
reported to have this effect.
Cranberry juice should be avoided, as should large amounts of alcohol,
as these may increase the effect of Warfarin. The effect of Warfarin can
also be altered by sudden increases or decreases in your body weight.
For these reasons you should avoid making sudden major changes to
your diet during treatment with Warfarin. For more information, please
speak to your pharmacist.
If you drink alcohol, you should only drink small amounts and avoid
binge-drinking, so that the alcohol does not have an adverse effect on
your INR.
Are there any risks if I take Warfarin?
There are risks associated with taking all medicines. The main risk with
Warfarin is bleeding. Usually this is a minor problem. For example, you
may bleed for slightly longer if you cut yourself. However, sometimes the
risk is high. Even if the risk is high, the risk of not taking the Warfarin
and of a clot developing may be greater than the risks from taking
Warfarin. It is important that you discuss the risks and benefits of taking
Warfarin with your doctor. There is no increase in risk if you take
Warfarin over a long period of time. In fact, the most difficult time is at
the beginning when the doctors are trying to find the right dose for you.
About 600,000 people in the UK take Warfarin. That’s about 1 in every
100 people. Out of these 600,000, each year 2 in every 100 will have a
major bleed requiring hospital admission. About 1 in 100 may die from
the bleed. However, the risk of having a major bleed is less than the
chances of a fatal clot forming.
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Is there anything else that could cause a problem?
You should avoid contact sports if you take Warfarin, as they increase
the risk of severe internal bleeding.
If you are planning to become pregnant and you take Warfarin, you
should discuss this carefully with your doctor before you become
pregnant, as Warfarin can affect an unborn baby
If you have any concerns you should not stop taking your Warfarin
medication unless told to do so by your GP/Hospital Doctor
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Useful organisations
Anticoagulation Europe
PO Box 405
Bromley
Kent BR2 9WP
Phone: 020 8289 6875
www.anticoagulationeurope.org
British Heart Foundation
Heart Help Line (For information and advice)
0300 330 3311 (Monday to Friday 9am to 6pm)
www.bhf.org.uk
Email: internet@bhf.org.uk
NHS Direct Wales:
Telephone: 0845 46 47
Minicom: 0845 606 46 47
Type talk: 1 8001 0845 46 47
Website: www.nhsdirect.wales.nhs.uk
There is a Health Information Online enquiry and information
prescription facility available on the website
References:
Net Doctor 2006. Marevan (Warfarin) [WWW]
http://www.netdoctor.co.uk/medicines/100004290.html
Hughes Jones S & Griffiths R (2008) Warfarin Therapy Information
Sheet, North West Wales NHS Trust
British Heart Foundation. 2007. Warfarin [WWW]
http://www.bhf.org.uk/publications/view_publication.aspx?ps=1000763
National Patient Safety Association. 2008. Oral Anticoagulation Therapy.
Important Information for Patients [WWW]
http://www.npsa.nhs.uk
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