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 two most common anticoagulant medicines are: 

• heparin
• 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 which causes 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, then 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.

Read more about how anticoagulant medicines work.

What are anticoagulant medicines used for?

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 are at risk of developing one. These conditions include:

• deep vein thrombosis (DVT)
• pulmonary embolism
• atrial fibrillation 
• high or moderate risk of stroke

If you have had surgery and are at risk of developing blood clots due to inactivity, or in a part of the body such as your heart, you may also be prescribed anticoagulant medicine.

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 or breastfeeding women. Speak to your GP if you become pregnant or you are planning to try for a baby while you are 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

Read more about how anticoagulant medicines interact with other medicine.

Anticoagulant medicines work by interrupting part of the process that is 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

Warfarin is a commonly prescribed anticoagulant medicine that is taken orally (swallowed) as a tablet. Warfarin 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, which is 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

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, your pharmacist or by a nurse at your local hospital to make sure that your dose is correct.

• If your INR is too high, blood clots will not form quickly enough and you may experience bruising or you may 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 your warfarin begin to work, your INR should start to increase. Normal INR ratings should be:

• between 0.8-1.2 for people who are not taking warfarin
• between 2.5-3.5 for people who are on warfarin, depending on the reason for taking it

Anticoagulants 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:

• 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): or "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

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 the condition reoccurring.

Aspirin and clopidogrel are anti-platelet 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.

Some other conditions make you more prone to developing blood clots. For example, atrial fibrillation, a heart condition that causes episodes of irregular and abnormally fast heart rates, can lead to blood clots forming in the heart. If you have atrial fibrillation, you may be prescribed anticoagulants to prevent blood clots forming.

After surgery

You may be prescribed an anticoagulant or an antiplatelet agent if you have recently undergone some kinds of surgery, for example 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.

Some kinds of surgery have a long recovery period, which may mean you that you are immobile for a long period of time. Being immobile can increase your risk of developing DVT or pulmonary embolism. If you are at risk of either of these, you may be given a low dose of the anticoagulant medicine heparin, to prevent these conditions developing.

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. If you need further advice, you can contact NHS Direct on 0845 4647.

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 an endoscopy, cystoscopy or colonoscopy.

Anticoagulants reduce the ability of your blood to clot, which could be dangerous if any kind of 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.

Pregnancy

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.

Breastfeeding

Warfarin should not usually be taken while you are breastfeeding, unless you are advised to do so by your GP or midwife.

Heparin is safe to take while you are breastfeeding.

If you are on anticoagulants and you are breastfeeding, or 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

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:

• passing blood in your urine, or faeces (stools)
• passing black faeces
• severe bruising
• prolonged nosebleeds (lasting longer than 10 minutes)
• blood in your vomit
• 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

While you are taking anticoagulant medicines, you will be closely monitored 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).

Warfarin

Additional side effects caused by warfarin include:

• rashes
• diarrhoea
• nausea (feeling sick)
• vomiting

Heparin

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

• hair loss (alopecia)
• thrombocytopenia – an abnormal drop in the number of platelets in your blood, which can cause bleeding into vital areas

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. This includes:

• prescription medicines
• medicines bought over-the-counter (OTC) without prescription, such as aspirin
• any herbal remedies
• any food or drink supplements

Prescription medicines that can interact with anticoagulants include the following:

• antibiotics 
• non-steroidal anti-inflammatory drugs (NSAIDs) 
• corticosteroids
• oral contraceptives

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 an information booklet that may provide this information, or you can ask your pharmacist or GP. If you have any further questions, you can call NHS Direct on 0845 4647.

Warfarin

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.

Read more about which medicines interact with warfarin.