Watch: What is warfarin and what does it do in your body?

Warfarin tablets

Warfarin is the most widely prescribed anticoagulant (a drug which reduces the risk of blood clots forming) in the UK. Find out why you might take it, possible side effects, and what you need to know if you take warfarin.

Available since the 1950s, warfarin is the most widely prescribed anticoagulant in the UK. It decreases the clotting ability of the blood, and so reduces the risk of blood clots forming. Blood clots can be dangerous because they can lead to serious life-threatening conditions such as stroke. Cardiac nurse June Davison puts common questions about warfarin to Dr Trevor Baglin, a consultant haematologist at Addenbrooke’s Hospital.

Why might I be given warfarin?

It is very effective at significantly reducing the risk of stroke in people with atrial fibrillation (AF), an irregular abnormal heart rhythm. About 60 per cent of people taking warfarin have AF. The condition can lead to pooling of blood in the heart chambers, which increases the risk of clots forming. If these clots are ejected by the heart, they can lead to a stroke. Warfarin helps to prevent the clots from forming.

People who are prone to blood clots in the circulation outside the heart are also often prescribed warfarin: for example, people who have had deep vein thrombosis (DVT) and those who have had clots travel to the lungs, causing a pulmonary embolism. People who have had a mechanical heart valve replacement are often prescribed warfarin to prevent clots forming on the valve. Warfarin can also help stop the mechanical valve from failing; if the valve becomes encased in blood clots, it can stop working properly.

How long do I need to take it for?

"Someone may be advised to take it for the rest of their life – as long as the benefit continues to outweigh the risk"

For as long as you’re considered to be at high risk of developing a blood clot. For many people who have AF or a mechanical valve, this risk is considered to be lifelong, so someone may be advised to take it for the rest of their life – as long as the benefit continues to outweigh the risk.

For patients who’ve had blood clots elsewhere, it depends on the cause. For example, people who have developed a blood clot as a result of having a hip replacement operation may never have developed one if they hadn’t had the surgery. In these situations, you’ll only need to take warfarin for a fixed period of time, until the problem is resolved. Patients who’ve had blood clots occur spontaneously are at high risk of it happening again, so it’s likely they’ll need to take warfarin for the rest of their life.

What monitoring do I need?

People taking warfarin need to have a regular blood test called an international normalised ratio (INR). INR measures the time it takes your blood to clot. This is increased by taking warfarin, which, in turn, increases the INR. The higher the INR, the longer it takes your blood to clot. So this helps to prevent blood clots from forming. However, if your INR is too high, there is an increased risk of bleeding, but a low INR may not provide enough protection against clotting.

The dose of warfarin can be adjusted to maintain the INR at a predetermined target level and requires careful monitoring. For the majority of patients on warfarin, their target INR is 2.5 (if you’re not on warfarin your INR is around 1). Your dose is then adjusted to keep the INR readings as close to your target as possible.

If your INR is stable, you might only need to go for an INR test every eight weeks; if it’s unstable or you’ve just started on warfarin, you may have to go weekly. On average, patients have their INR checked every four to five weeks. Some people have to go to hospital for their blood test. Others can go to their local GP practice to have a blood sample taken, which is sent to the lab at the hospital.

I’ve heard there are some nasty side effects. What are they and what should I do if I experience any of these?

While warfarin has enormous benefits, the downside is that it can increase the risk of bleeding. Your doctor will want to ensure that the benefits of you taking it outweigh the risks. Millions of patients have used warfarin for more than 60 years – and, apart from the risk of bleeding, warfarin has a fantastic safety profile.

"It’s important to follow advice from your healthcare professional to minimise the risk of bleeding"

The risk of a serious bleed is about 1–2 per cent a year. Unfortunately, there’s often no warning for a serious bleed, but keeping your INR in a target range minimises the risk of bleeding. It’s important to follow advice from your healthcare professional; if they tell you that you need a test in three weeks, don’t wait for eight weeks. If they tell you to change the dose, do so, or if you’re not happy with the recommendations, speak to them about it. Don’t make your own decisions, as that’s when anticoagulation can get out of control.

An everyday bleed, for example where you cut your finger, isn’t usually a cause for concern. If you do cut yourself, it might bleed for a longer period of time, but you just need to keep the pressure on it for a bit longer.

Are there any foods that I should avoid?

The most important thing is to keep your diet stable, as this means that your warfarin requirement is also likely to be stable. Don’t make any radical changes without discussing them with your GP.

"It’s also important to follow the recommendations for safe alcohol intake and don’t drink a large amount in one sitting"

Food that is high in vitamin K, such as liver or vegetables such as Brussels sprouts and broccoli, can inhibit the action of warfarin. However, rather than leaving these out of your diet, make sure you enjoy a constant amount regularly. This will mean the level of vitamin K in your bloodstream will remain fairly constant, increasing the likelihood that your INR level will remain consistent and stable.

There are reports that cranberry juice can increase the effect of warfarin, so it’s best to avoid drinking this. It’s also important to follow the recommendations for safe alcohol intake and don’t drink a large amount in one sitting.

Do I need to avoid any sporting activities?

Most sports are fine to participate in, but it is best to avoid contact sports such as boxing, karate, kickboxing and competitive rugby. This is to reduce the risk of sustaining an injury that may cause you to bleed, because if you’re on warfarin, you will bleed a bit more. The risk is relatively small though.

If you suffer a head injury, such as falling and hitting your head, or experience a blow to the head, seek medical help without delay to rule out internal bleeding which may not be immediately obvious

What are the alternatives to warfarin? Wouldn’t I be better off taking one of those?

There are some newer anticoagulation drugs called dabigatran, rivaroxaban and apixaban. These don’t require monitoring or dose adjustment and they are as effective as warfarin at preventing blood clots. The advantage of them is convenience rather than a major improvement in safety – they are still anticoagulants, which can increase the risk of bleeding.

With these newer anticoagulation drugs, it’s essential that people take them every day. They act for a much shorter period of time compared with warfarin, so if you miss a dose, you’re not protected.

Whichever drug you take, you’ll need to have a review at least once a year to check you’ve had no problems and to assess whether the benefit of the treatment continues to outweigh the risk.

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