Common questions about warfarin

Warfarin is a type of medicine known as an anticoagulant. It makes your blood clot more slowly.

Blood clotting (thickening) is a complicated process involving substances called clotting factors.

Clotting factors are made by the liver and help stop bleeding. They work with blood cells called platelets that trigger the clotting process to make sure your blood clots properly.

To make some of the clotting factors, the liver needs a good supply of vitamin K.

Warfarin blocks the liver from using vitamin K to make clotting factors. This thins your blood, so it takes longer to clot.

Warfarin takes about 3 days to work fully.

But it can take weeks or even months before you settle on the right dose.

This is because everyone reacts differently to warfarin, so it's normal for your dose to go up and down when you first start taking it.

You will not feel any different while you take warfarin. The only way to check it's working is to have a blood test to see how long your blood takes to clot.

Do not stop taking warfarin without talking to your doctor.

If you stop taking warfarin, your blood will start clotting at the same rate as before you started taking it, usually within a day or two of stopping.

This means you may be at an increased risk of serious problems like strokes, heart attacks, deep vein thrombosis (DVT) or pulmonary embolism.

Warfarin is one of the oldest and most commonly used anticoagulants.

There are newer anticoagulants, such as rivaroxaban, dabigatran, apixaban and edoxaban.

Warfarin does not seem to be any better or worse than newer anticoagulants at treating and preventing blood clots.

But the newer anticoagulants may be more convenient because you do not need to have regular blood tests or be quite as careful about what you eat.

Because of the risk of bleeding, your dose of warfarin may need to be lowered or stopped a few days before you have an operation.

Tell the surgeon or nurse that you're taking warfarin.

Also tell anyone else involved with your care, such as your anticoagulant specialist, so they can arrange any changes to your medicine for you.

You can still go to your dentist as usual. But tell your dentist that you're taking warfarin before you have any treatment, especially if you need to have a tooth out.

You may need to have blood tests before starting treatment to make sure your blood will clot enough to stop any bleeding.

Before taking out a tooth, your dentist will need to know how well your blood is clotting. This will be checked using the international normalised ratio (INR) test.

Most people do not need to change their warfarin dose.

It's safe for you to have vaccinations, but tell the nurse or doctor that you're taking warfarin.

If the vaccine is normally given by injection into the muscle, provided you're up to date with your blood tests and the results are within the target range, you can have the injection as normal into muscle.

Or sometimes the injection can be given into the layer of fat underneath your skin. This is called a subcutaneous injection.

It might help prevent bruising if you press on the injection site for 10 minutes after the vaccination.

Having a body piercing or tattoo is not recommended while you're taking warfarin. This is because of the increased risk of bleeding and infection.

If you still want to go ahead, talk to your doctor about your specific risks in case you need antibiotics or changes to your warfarin dose.

And let the tattooist or piercer know beforehand that you're taking an anticoagulant medicine.

There's no evidence that taking warfarin affects your sex life.

Warfarin does not affect any type of contraception, including the combined pill and emergency contraception.

But you'll need to have an international normalised ratio (INR) blood test when you start or stop taking contraception.

This is to check whether your warfarin dose is still at the right level or needs to be changed.

You can drink alcohol while taking warfarin.

But drinking heavily, especially binge drinking lots of alcohol in one go, is dangerous while taking warfarin. The alcohol can increase the risk of bleeding.

It's thought that cannabis can affect your blood tests and increase your risk of serious bleeding.

It's not clear how other recreational drugs like cocaine and heroin affect warfarin. They could be potentially dangerous.

Injecting while taking warfarin will increase your risk of bleeding.

Talk to your doctor if you think you might use recreational drugs while taking warfarin.

You can find out more about the side effects of some recreational drugs on the Frank website.

You can reduce your risk of having a blood clot by making changes to your lifestyle, such as:

• quitting smoking
• eating a healthy, balanced diet – but speak to your doctor before making changes to your diet
• getting regular exercise
• keeping to a healthy weight, or losing weight if you need to