How and when to take warfarin

It's important to take warfarin as your doctor advises.

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.

How to take it

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.

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.