Overview

Vitamin K antagonists (VKAs), such as warfarin or acenocoumarol, are anticoagulant medicines that prevent and treat blood clots. You might have heard them called blood thinners, but they don’t actually thin the blood.

There are different types of VKAs. Warfarin and acenocoumarol (also called Sinthrome^®) are the most commonly used ones in the UK, but others are used in different parts of the world. They have been used to treat and prevent blood clots for many decades.

Why you might be prescribed VKAs

You might be prescribed them if you have:

• blood clots, including deep vein thrombosis (DVT)
• a pulmonary embolism (PE)
• blood clots in arteries or your heart
• an irregular heart rhythm (atrial fibrillation, AF)
• a mechanical heart valve

If you have a DVT or PE, anticoagulants stop the clot from getting larger, and new clots from forming.

If you have atrial fibrillation (AF), you are at a higher risk of getting blood clots in the brain which can lead to a stroke. Anticoagulants reduce this risk by:

• making the blood take longer to clot
• reducing the risk of blood clots forming

If you have a mechanical heart valve, anticoagulants reduce the risk of blood clots forming on the valve, and breaking off to other parts of your body. Blood clots that travel around your body can cause complications, such as a stroke.

Blood test monitoring

If you are taking VKAs, you will need regular, long-term blood test monitoring. This is done in our anticoagulation clinic.

Other anticoagulants, called direct oral anticoagulants (DOACs) can be used for most people with AF, DVT or PE. You do not need regular blood tests if you take DOACs. If you want to know if this is an option for you, please contact your anticoagulation clinic.

DOACs cannot be used for patients with mechanical heart valves.

VKAs are usually suitable for patients that have had blood clots and have:

• poor kidney function
• antiphospholipid syndrome

Taking VKAs

VKAs are very good at treating and preventing blood blots and have been used effectively for many decades.

For people who cannot take DOACs, VKAs are the only anticoagulants available that can be swallowed rather than injected.

Although VKAs are very effective, people can respond to them in different ways. There is no standard dose (amount prescribed), so your clotting time needs to be measured regularly to guide the amount we give you.

The INR blood test

The clotting time is measured using a blood test called the international normalised ratio (INR). You will be given a target INR range, such as 2.0 to 3.0 or 2.5 to 3.5, depending on your risk of forming a clot.

We usually do an INR test by taking a blood sample from a vein. Some people can get their blood using the finger-prick process.

When you are settled on VKAs therapy, you might be able to check your INR at home using an INR home-testing device. This is not available at all clinics, so please ask your local clinic for more information.

When you start taking VKAs, you usually need 2 blood tests in the first week, and then weekly blood tests.

If the INR test results are in the target range, the gap between tests can be gradually extended up to every 3 months.

If the INR is out of your target range, you will be asked to come back for a blood test sooner, usually after a week.

Dose (amount) of VKAs to take

As there are no standard doses of VKAs, and doses are guided by your INR results, your dose might need to change often. Your clinic will always tell you how much you should be taking.

The dose you take might be fixed (such as, 3mg every day), or can vary depending on the day of the week (for example, 5mg Monday to Thursday and 4mg Friday to Sunday).

Warfarin

Warfarin comes as 0.5mg, 1mg, 3mg and 5mg tablets. We recommend that you always have a supply of each of the 1mg, 3mg and 5mg so that, if your dose changes, you can make sure you can take the right amount.

The tablets are colour coded, so:

• 1mg will always be brown
• 3mg will always be blue
• 5mg will always be pink 

An example of this is if your dose is 8mg it can be made up in any of these ways:

• 1 x 5mg (pink tablet) and 1 x 3mg (blue tablet)
• 1 x 5mg (pink tablet) and 3 x 1mg (brown tablet)
• 2 x 1mg (brown tablet) and 2 x 3mg (blue tablet)
• 8 x 1mg (brown tablet)

Acenocoumarol

Acenocoumarol only comes as a 1mg tablet.

As doses can change frequently, the dose will not be on the label of the box of medicine.

The clinic will write your dose either on a special dose slip, or in your yellow anticoagulation record book. This will specify the dose, your INR result, and the date of your next test.

When to take VKAs

Both warfarin and acenocoumarol are tablets that you can swallow. You can take them on a full or empty stomach.

It is best to take warfarin or acenocoumarol tablets in the evenings (after 6pm). This allows the anticoagulation clinic to contact you with your new dose before you have taken it, if it needs to be changed.

If taking it in the evening does not suit your routine, you can take it at any other time. The most important thing is to take it regularly and not miss doses. If you take warfarin at an earlier time, you should tell your clinic so that they can contact you earlier in the day if they need to change your dose.

If you forget to take your medicine

If you miss a dose, take it as soon as you remember provided it is before midnight.

Never take 2 doses to make up for a missed dose.

If you miss more than 1 dose you can contact your anticoagulation clinic for advice.

Taking VKAs long term

How long you take the medicine depends on your condition, and your long-term risk of forming a blood clot.

Most people take VKAs for a long time. This includes people with AF and mechanical heart valves.

If the VKA is treating a blood clot that was provoked (for example, by surgery or a long-haul flight), you might take VKAs for a shorter time, such as 3 to 6 months, but your specialist will decide this with you.