What is warfarin and how does it work?
Warfarin is one of a group of drugs called anticoagulants which work by affecting your body’s blood clotting
mechanism. They extend the normal time it takes for blood to clot. You may have heard of it referred to as a “blood
thinner”. Warfarin helps prevent your body from forming blood clots and helps stop any existing clots from getting
bigger. It won’t however dissolve any clots which have already formed. Your body has it’s own system which will
do this naturally over a few weeks.
Warfarin may be given to treat an existing problem eg a clot in the leg (Deep Vein Thrombosis, DVT), or it may be given
to help reduce the risk of a stroke in some people with a condition called atrial fibrillation or with damaged or
replaced heart valves.
Your doctor should have explained to you the reason for your treatment. Warfarin comes in the form of tablets
which are a different colour for every strength. In Nottingham only 3mg warfarin tablets are used.
The length of time that you need to take warfarin for will depend upon why you are taking it and will be decided by the doctor who
first prescribed the warfarin. The expected duration of therapy will be written in the front of your warfarin booklet.
If you are only to take warfarin for a specific time the clinic will advise you when you are due to stop.
How and why your blood is monitored
Each person’s body responds differently to this medication and initially you will need to have quite frequent blood
tests to find the best dose for you. This means that you may be taking different doses over the first few weeks. As your
response to the drug stabilises, the length of time between your blood tests will increase. You may still need small
alterations to your dose now and then to keep your blood just right.
The blood test (either a finger prick or venous sample) used to monitor your warfarin treatment is called the INR. This is
a measure of how long it takes your blood to clot compared with someone who is not taking warfarin. You will have
been given a target range for your INR results.
What are the possible side effects of warfarin?
If you take your warfarin as recommended, it is unlikely that you will have any problems. However as warfarin
affects blood clotting, the most important side effect is bleeding (haemorrhage). It is extremely important that you
know the signs to look out for and what to do if you notice any signs of bleeding. You should seek medical advice
immediately if you experience any of the following problems. This can be from your own family doctor, NHS
Direct or your local Accident and Emergency Department.
● Suffer any injury, especially to your head, eyes or joints
● Bleed a lot from a cut or suffer excess nose bleeds or bleeding from your gums
● Notice any unexplained bruising, especially black or brown spots on your skin where you have not received an injury
● Vomit up any blood or material which looks like coffee grounds
● For women, increased bleeding during periods (or any other vaginal bleeding)
● Red or dark brown urine
● Red or black stools.
It is also wise to seek immediate medical advice if you
● Any major changes to your general state of health eg vomiting, diarrhoea or fever
● Suffer chest pains or shortness of breath
● Begin to suffer indigestion/heartburn.
It is also wise to avoid activities that could cause you to bruise yourself or cut yourself, for example, contact sports.
Take care while shaving, do not walk around bare foot and always wear strong gloves when gardening.
Other medicines and warfarin
As you may know many medicines can have an effect on your warfarin. It is therefore important to tell the clinic staff if:
● you are prescribed any new medicines
● any of your current medicines are stopped
● the dose of any of your medicines is changed.
The information that the clinic will need to know about your medicines is:
● the name of the medicine (write it down or take the medicine with you)
● what dose you are or were taking
● when you started or stopped taking it, or when the dose changed.
Always make sure that:
● your doctor or dentist knows you are on warfarin if they are prescribing something new for you.
Show your warfarin booklet to the pharmacist if you wish to buy any medicine over the counter. This includes any
cough/cold remedies or herbal remedies.
Do not take aspirin (acetylsalicylic acid) unless a doctor (who knows you are or will be taking warfarin) has
specifically prescribed it for you. Aspirin is also present in many painkillers and flu remedies. Always seek advice from
a pharmacist before buying any over the counter medicine. Paracetamol 500mg (up to 4 tablets daily) can be taken
without an effect on warfarin.
Many other types of medicines may also interfere with your
warfarin. These include antibiotics, amiodarone, cholesterol
lowering agents, Non-steroidal anti-inflammatory agents
(either prescribed or ‘over the counter’) and some medicines
for epilepsy to name just a few. They may either increase the
effect of your warfarin making your blood take longer to
clot and some can reduce the effect of your warfarin.
Always take advice from your doctor or pharmacist
If your doctor wishes you to take a new drug (which may or
may not affect your warfarin), then you must contact your
anticoagulant clinic as soon as possible as they may advise
you to have a blood test done within 4 – 7 days after starting
the new medicine. Follow this advice if a medicine you
normally take has been stopped
Diet and warfarin
Warfarin can be taken either before, after or during a meal.
No special diet is required when taking warfarin and usually
it is fine to continue your normal diet. Some changes in
your diet can affect your blood results. It is best therefore to
adhere to the following advice:
● Do not go on ‘crash’ diets or start binge eating
● Do not use dietary supplements without first checking
with your doctor or pharmacist that it will not interfere
with your warfarin
● Do not make large changes to the amounts of certain
foods that you eat. It is perfectly alright to eat these
foods but important not to make big changes to the
amount of them you usually eat. Common ones are:
● lettuce (dark green or red)
● soya bean products
● Brussel sprouts
● cabbage (fresh boiled or raw green)
● collards (white cabbage)
● endive (chicory)
Herbal remedies and other alternative
Many herbal and alternative medicines are known to affect
warfarin therapy but for some we simply don’t know
whether or not they are safe to take with your warfarin. The
best advice is to avoid these products while you are
If, however, you are going to take these products then you
must inform your anticoagulant clinic. It is advisable that
you have your blood checked within a week of starting the
drug. You will need regular (probably weekly) blood checks
until it is established whether they may be affecting your
Alcohol and warfarin
Alcohol is known to affect the action of your warfarin if
taken in larger quantities than recommended. It can cause
your blood to become too thin if you drink more than
recommended on a ‘one off’ basis and this increases your
chance of bleeding. If you drink more than recommended
on a regular basis then this can cause your warfarin to be
ineffective leading to a risk of clots forming.
If your doctor has said that it is alright to continue to drink
alcohol then we recommend a maximum of 2 units a day.
Two units of alcohol = 1 pint of normal strength beer
or 2 pub measures of spirits
or 2 small glasses of wine
It is not safe to save up units and then have