Anticoagulant medicines


Anticoagulant medicines


Anticoagulant medicines reduce the ability of the blood to clot (coagulation means clotting). This is necessary if the blood clots too much, as blood clots can block blood vessels and lead to conditions such as a stroke or a heart attack.

You may have heard them called blood-thinning medicines, although this isn't technically correct.

The most commonly prescribed anticoagulant medicine is warfarin.

Rivaroxaban, dabigatran and apixaban are newer anticoagulants that may be used as an alternative to warfarin for certain conditions.

In some cases, heparin may also be used as an alternative to warfarin.

A full list can be found in our anticoagulant medicines information pages.

Why is anticoagulant medicine necessary?

When the body is wounded, either inside or on the skin, blood can leak into the internal organs or out of the body. To prevent this, the blood forms clots that create a seal over the wound.

When the blood needs to clot, a series of complex processes takes place that cause the blood to become sticky. The blood then starts to clot at the site of the bleeding, which prevents further bleeding.

If one or more parts of the process fail to work, the blood can clot too much or not enough. If the blood doesn't clot enough, there's a risk of excessive bleeding (haemorrhaging). If it clots too much, blood clots can form where they aren't needed and block blood vessels.

Anticoagulants can reduce the ability of the blood to clot so that unnecessary blood clots are not formed.

When are anticoagulant medicines used?

There are several uses for anticoagulant medicines, but they are most commonly prescribed for people who have had a condition caused by blood clots or who are at risk of developing one. These conditions include:

deep vein thrombosis (DVT)

pulmonary embolism

atrial fibrillation

high or moderate risk of stroke

You may be prescribed anticoagulant medicine if you have had surgery and are at risk of developing blood clots in a part of the body such as your heart or through inactivity.

Things to consider

There are several things to consider when taking anticoagulant medicines. If you are going to have surgery or an investigation such as an endoscopy, make sure your surgeon is aware that you are taking them, as you may have to stop.

Some anticoagulant medicines are not suitable for pregnant women. Speak to your GP if you become pregnant or you are planning to try for a baby while taking anticoagulants.

There are several side effects of anticoagulant medicines, including excessive bleeding (haemorrhage), which can lead to severe bruising, blood in your urine or coughing up blood. Contact your GP immediately if you notice any side effects.

Anticoagulant medicine can also interact with other types of medicine, which can cause it to stop working. Speak to your GP before taking new medicine or changing your dose of any kinds of medicine, including:

prescription medicines

over-the-counter (OTC) medicines

herbal remedies

food and drink supplements

How anticoagulants work 

Anticoagulant medicines work by interrupting part of the process involved in the formation of blood clots. This means that blood clots are less likely to form where they are not needed, but can still form when they are.


Warfarin is a commonly prescribed anticoagulant medicine that is taken orally (swallowed) as a tablet. It interferes with your body's natural chemical processes by targeting a substance called vitamin K.

Vitamin K has an essential role to play in the production of prothrombin, a protein found in the blood. Prothrombin plays an important part in the process of the formation of clots.

If the production of vitamin K is slowed down, the production of prothrombin is also slowed. This means that it will take longer for blood clots to form.


Heparin occurs naturally in the body, but it can also be extracted and purified to be used as an anticoagulant medicine. Heparin inhibits the blood clotting process.

It can be given as an injection or through a drip into a vein. Different types of heparin medication include:

enoxaparin (Clexane)

tinzaparin (Innohep)

dalteparin (Fragmin)

Monitoring your dose

How well your warfarin is working is measured using the international normalisation ratio (INR).

While you are taking warfarin, your INR will be regularly tested by your GP, pharmacist or by a nurse at your local hospital to make sure that the dose is correct.

If your INR is too high, blood clots will not form quickly enough and you may experience bruising or be at increased risk of bleeding. In this case, your dose may need to be reduced.

If your INR is too low, your medication is not working sufficiently, which means that clots could still form unnecessarily and block a blood vessel. In this case, your dose may need to be increased.

When you first start taking warfarin, your INR will be tested within the first two to four days of starting treatment. Depending on your reading, further tests will be carried out one to two times a week. If your INR stabilises within the correct range, testing may become less frequent.

International normalisation ratio (INR)

The INR is a way of measuring how fast your blood clots. As INR is an internationally recognised test, it can be used by healthcare professionals around the world.

During the test, a sample of your blood is taken and a chemical is added to it. The chemical starts a chain of chemical reactions that should make the blood in the sample clot (thicken).

During the clotting process, a protein in the blood (prothrombin) turns into an enzyme called thrombin. The time that it takes the prothrombin to turn into thrombin is called the prothrombin time ("pro-time", or PT). This is measured in seconds.

Your PT is compared with the PT of someone who is not taking warfarin. This gives your INR. Once the warfarin begins to work, your INR should start to increase. Normal INR ratings should be:

between 0.8 and 1.2 for people who are not taking warfarin

between 2.5 and 3.5 for people who are on warfarin, depending on the reason for taking it

When anticoagulants are needed 

Anticoagulant medicines are used if your blood is clotting too quickly. When this happens, blood clots can form in the wrong places. These clots can break off and block a blood vessel, disrupting the flow of blood around your body.

This can lead to several serious medical conditions, including:

If you are at risk of any of the above conditions, for example if you have had one before, you may be prescribed anticoagulant medicines to reduce your chance of the condition recurring.

Aspirin and clopidogrel are antiplatelet medicines that also reduce the ability of the blood to clot. In some cases, one of the above conditions may be treated with aspirin or clopidogrel instead. The healthcare professionals treating you will explain to you which medication is most suitable.

Increased risk of blood clots

A number of conditions lead to an increased risk of blood clotting in blood vessels. The term used to describe these conditions is thrombophilia.

The majority of cases of thrombophilia that can cause deep vein thrombosis or pulmonary embolism are passed on from your parents. The most common type of inherited thrombophilia is Factor V Leiden.

You may be prescribed anticoagulants to prevent a clot forming if you show a positive result in a screening test. These tests will only be done with your permission and only if a close relative develops a blood clot that cannot be explained.

Certain other conditions can increase the risk of a blood clot, particularly if they reduce the flow of blood in your legs or heart, or affect your immune system.

Reduced flow of blood

A reduced flow of blood in the veins of your leg is usually caused by long periods of not being able to move. This is more likely after certain types of surgery such as a hip or knee replacement. After this type of surgery you may be given a low dose of heparin to help prevent deep vein thrombosis or pulmonary embolism.

A reduced flow of blood in the heart is usually a result of palpitationscaused by certain irregular heartbeats (arrhythmias). The most common of these conditions is atrial fibrillation, which can lead to blood clots forming in the heart. People with atrial fibrillation are usually treated with warfarin to reduce the risk of a clot forming.

Immune system

Certain conditions can also affect the immune system, which leads to an increased risk of developing a clot or embolus. Examples of these conditions include:

If you are diagnosed with any of these conditions, you may be prescribed long-term anticoagulants to treat blood clots or to prevent them forming.

After surgery

You may be prescribed an anticoagulant or an antiplatelet agent if you have recently undergone some kinds of surgery, such as aortic valve replacement. Your aortic valve is a valve in your heart that controls the flow of blood out to the rest of your body. This valve can become damaged as you age and in some people it is replaced with a man-made valve.

Blood clots can form on the surface of the new valve, which could disrupt the flow of blood through your heart. Anticoagulant medicines can reduce the risk of this happening by making it harder for your blood to clot.

strokes – when a blood clot restricts the flow of blood to your brain, causing brain cells to die and possibly resulting in permanent brain damage or death

transient ischaemic attacks (TIAs) – also called "mini-strokes" with symptoms similar to a stroke, but the effects usually only last 24 hours

heart attacks – when a blood clot blocks part of your heart, starving it of oxygen and causing chest pain and sometimes death

deep vein thrombosis (DVT) – when a blood clot forms in one of the deep veins in your body, usually your legs, causing pain and swelling

pulmonary embolism – when a blood clot blocks one of the blood vessels around the lungs, stopping the supply of blood to your lungs

antiphospholid syndrome – a disorder of the immune system, where antibodies attack fats and proteins in the blood vessels instead of bacteria and viruses, causing the blood to clot

paroxysmal nocturnal haemoglobinuria (PNH) – a rare blood disorder that can affect the blood clotting process, leading to an increased risk of blood clots in the abdomen and liver


If you are prescribed anticoagulants, always follow the instructions of your GP or other healthcare professional. Taking too much of these medicines can result in severe bleeding, especially if you are bruised or injured.

If you are unsure of your instructions, check the patient information leaflet that should come with your medication or call your GP. You can also call 111 for further advice.

Having surgery

If you are taking anticoagulants and you need to have surgery or any kind of invasive procedure, make sure that the healthcare professionals treating you are aware of your medication. This includes procedures used to diagnose other conditions, such as endoscopy, cystoscopy or colonoscopy.

Anticoagulants reduce the ability of your blood to clot, which could be dangerous if any kind of cut (incision) is made during a surgical procedure. It may be necessary to stop taking anticoagulants for a while before and after surgery. This will prevent excessive bleeding during surgery and help the healing process afterwards.

If you are having a dental procedure, such as having a tooth removed, inform your dentist that you take anticoagulants. You do not usually need to stop taking your medication, but you may need to have your international normalisation ratio (INR) tested before the procedure to make sure that it's at a safe level.

Only stop taking your medication on the advice of your GP or another healthcare professional.


Warfarin is not suitable for pregnant women because it can cross the placenta (the organ that links the mother's blood supply to her unborn baby's) and affect the unborn baby. This can cause birth defects or excessive bleeding from the placenta or foetus.

Warfarin should be avoided in pregnancy if possible, and especially during the first trimester (up to week 13 of the pregnancy) and the third trimester (from week 27 until the birth of the baby).

Heparin may be taken during pregnancy to manage venous thromboembolism (when a blood clot forms in a blood vessel and breaks off, blocking the flow of blood) if the healthcare professional treating you thinks it's necessary.

If you are on anticoagulant medicines and find out you are pregnant or you plan to start trying for a baby, speak to your GP about stopping or changing your prescription.


You can usually take warfarin while you are breastfeeding. However, you should first discuss this with your GP or midwife.

Heparin is safe to take while you are breastfeeding.

If you are on anticoagulants and you are breastfeeding or are planning to breastfeed, speak to your GP or midwife to find out if you need to change your prescription.

Avoiding injury

Taking anticoagulant medicines can make you more prone to bleeding if you are injured. Try to avoid minor injuries and cuts and grazes by:

taking care when brushing your teeth and shaving

using protection when gardening, sewing or playing contact sports

using insect repellent to avoid insect bites or stings


Side effects 

A side effect of all anticoagulants is the risk of excessive bleeding (haemorrhages). This is because these medicines increase the time it takes blood clots to form. If they take too long, you can experience excessive bleeding.

Other side effects

As well as excessive bleeding, there are other symptoms to look out for. These are more common with warfarin. If you notice any of the following symptoms when taking anticoagulants, seek medical attention immediately:

You must also seek immediate medical attention if you:

While you are taking anticoagulant medicines, you will be monitored closely to check that you are on the correct dose and not at risk of excessive bleeding (haemorrhages). The most common test for this is the international normalisation ratio (INR).


Additional side effects caused by warfarin include:

See your GP immediately if you experience jaundice (yellowing of the skin and whites of the eyes) or any persistent side effects while taking warfarin.


Although heparin occurs naturally within the body, extra amounts of it can cause side effects, including:

Reporting side effects

The Yellow Card Scheme allows you to report suspected side effects from any type of medicine that you are taking.

It is run by a medicines safety watchdog called the Medicines and Healthcare products Regulatory Agency (MHRA).

Interactions with other medicines 

Anticoagulant medicines can interact with a wide range of other medication. In some cases, the interaction will stop the anticoagulant medicine working. In other cases, it will increase the anticoagulant effect.

If you are taking anticoagulants and you start, stop or change the dose of any other kind of medication, speak to your GP, pharmacist or another healthcare professional.

These types of medication include:

prescription medicines

medicines bought over the counter without prescription, such as aspirin

any herbal remedies

any food or drink supplements

Prescription medicines that can interact with anticoagulants include the following:


non-steroidal anti-inflammatory drugs (NSAIDs) 


oral contraceptives

selective serotonin reuptake inhibitors (SSRIs; a type of antidepressant medication)

For a full list of medicines that interact with the anticoagulant you are taking, see our anticoagulant medicines information pages.

Your prescription should also come with a booklet that may provide this information, or you can ask your pharmacist or GP. If you have any further questions, you can call 111.


Warfarin may not be effective if you take too much vitamin K, either through your diet or in supplements.

The effect of warfarin is increased by alcohol. If you are taking warfarin, do not drink more than one or two alcoholic drinks a day and never binge drink. 

The maximum recommended daily limits for alcohol consumption are three to four units a day for a man and two to three units a day for a woman. One unit of alcohol is approximately half a pint of weak lager or bitter.

passing blood in your urine or faeces (stools)

passing black faeces

severe bruising

prolonged nosebleeds (lasting longer than 10 minutes)

bleeding gums

blood in your vomit or coughing up blood

unusual headaches

sudden severe back pain

difficulty breathing or chest pain

in women, heavy or increased bleeding during your period, or any other bleeding from your vagina

are involved in a major accident

experience a significant blow to the head

are unable to stop any bleeding



nausea (feeling sick) or vomiting

hair loss

hair loss (alopecia)

thrombocytopenia – an abnormal drop in the number of platelets in your blood, which can cause bleeding into vital areas

Anticoagulant medicines