Warfarin

About warfarin

What is warfarin?

Warfarin is a medicine that helps stop your blood from clotting. This lowers your risk of blood clots and a stroke.

You may be given warfarin if you have:

• atrial fibrillation
• a mechanical heart valve
• a blood clotting disorder
• had a mini stroke (transient ischaemic attack or TIA).

You’ll be given an alert card that you should always carry with you. It lets other healthcare professionals know that you’re taking warfarin.

Warfarin may not be suitable for everyone. Talk to your doctor about whether it’s suitable for you and other treatment options.

What type of medicine is warfarin?

Warfarin is a type of medicine called an anticoagulant. You may hear people calling them “blood thinners”. However, warfarin does not thin your blood.

Other anticoagulants you may hear about include direct oral anticoagulants (DOACs).

How does warfarin work?

Warfarin makes it harder for your blood to clot.

Blood clots need vitamin K to form. Vitamin K is produced by your liver. Warfarin stops your liver producing as much vitamin K so blood clots cannot form or they form slower.

This can mean that if you cut yourself, it will take longer to stop bleeding as your blood does not clot as easily.

Side effects

What are the side effects of warfarin?

All medicines have potential side effects, but most people will not experience side effects.

Side effects from this medicine may include:

• bleeding more than normal, for example if you cut yourself or have a nosebleed
• a rash on your skin that’s painful
• swollen throat or tongue, a raised itchy rash or difficulty breathing (allergic reaction).

Call NHS 111 or your GP if you’re worried about bleeding. If you injure yourself and you cannot stop the bleeding, call 999.

Different medicines will have different side effects. If you’ve started taking warfarin recently it’s worth bearing in mind that side effects with any new medicine often improve over time or even disappear.

When should I get help?

Talk to your doctor if you’re having side effects. They can help you manage them or look at other options with you.

It’s important not to stop taking your medicine without speaking to a healthcare professional first.

How and when to take warfarin

What dose of warfarin should I have?

There are different strengths of warfarin, and your doctor will give you the dose they think is right for you.

You’ll need regular blood tests to check you’re having the right dose. The blood test measures how long it takes your blood to clot. This is called an international normalised ratio (INR) test because it looks at your INR levels.

When you start taking warfarin, you’ll have your blood tested every day or every few days. Once your INR levels are stable, you’ll have a blood test usually every 12 weeks. Although for some people it may be more often than this.

Blood tests are done at your GP surgery or local clinic. Some people can check their blood levels at home. Speak to your doctor to find out if this is possible for you.

How do I take this medicine?

Warfarin comes as tablets or a liquid that you swallow. Most people will take warfarin once a day. Try and take it at the same time every day.

You should be given a way to write down your medicine information and the dose.

What happens if I forget to take my medicine?

If you forget to take your medicine and:

• It's only a short time after your usual dose, take it as soon as you remember. Then take your next dose at the usual time.  
• It’s nearly time for your next dose, do not take the one you missed. Wait and take your next dose at the usual time. 

How long do I take it for?

How long you need to take warfarin for depends on your condition. Many people taking warfarin will need to take it long term.

Your doctor can give you more information and answer any questions you have. 

It’s important not to stop taking your medicine without speaking to a healthcare professional first.

Things to avoid if you’re taking warfarin

There are foods, drinks and over-the-counter medicines you need to limit or avoid if you’re taking warfarin. This is because they can stop your medicine from working properly.

This includes:

• NSAID medicines like ibuprofen, naproxen, diclofenac and higher-dose aspirin
• cranberry and cranberry juice
• grapefruit or grapefruit juice
• vitamin K supplements
• St John’s wort
• large amounts of alcohol.

There are also some prescription medicines to avoid. Speak to your doctor, nurse or pharmacist for a full list of foods and medicines to avoid.

You should not take warfarin if you’re pregnant. If you’re currently taking warfarin and you’re planning to get pregnant or you become pregnant, speak to your doctor.

• Find more information on the NHS website.

Get help with warfarin

• Our cardiac nurses can help answer your questions on warfarin, Call, email or live chat with them on our Heart Helpline.
• Check if you can get free prescriptions or help paying for your prescriptions.
• Talk to other people with high blood pressure and get support in our online community.