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Anticoagulant medicines

Page last reviewed: 13/07/2011

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.

The two most common anticoagulant medicines are:

  • heparin, and
  • warfarin.

Aspirin is an anti-platelet medicine. This is also an anticoagulant, as it reduces the ability of the blood to clot. However, aspirin does this in a slightly different way to the medicines above.

What makes up blood 

Blood is made up of:

  • plasma - a liquid containing proteins, nutrients, hormones and waste products, but mainly water (90%),
  • red blood cells - to transport oxygen around the body and remove carbon dioxide and other waste products,
  • white blood cells - to fight infection, and
  • platelets - to help the blood to clot.

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

How the blood clots

When the blood needs to clot, a series of complex chemical processes take place in the blood. These chemical processes change the surface of the platelets so that they become sticky (known as 'activated'). The platelets then stick to the site of the bleeding and start to form a clot.

The activated platelets also cause a number of other chemical reactions that activate clotting factors in the blood. Clotting factors are chemicals or proteins that are produced by the liver. There are 13 clotting factors that can be found in the plasma in the blood.

One of these clotting factors is a protein called prothrombin. During the clotting process, prothrombin turns into an enzyme called thrombin. An enzyme is a protein that speeds up and controls chemical reactions in your body. In this case, the thrombin acts upon a protein in the blood called fibrinogen, and turns it into a different protein, called fibrin.

Fibrin is like sticky strands of string. These strands stick to the site of the wound and form a web. Red blood cells stick to the web and start to form a clot. As the fibrin and the platelet clots bind together, a solid clot is created that stops further bleeding.   

Why is anticoagulant medicine necessary?

The process of forming clots is complicated and relies on a lot of processes happening correctly. If one or more parts of the process fail to work, then the blood can clot too much, or not enough.

If the blood does not clot enough, there is a risk of excessive bleeding (haemorrhaging). If it clots too much, then blood clots can form where they are not needed, and could block blood vessels. Anticoagulant medicines can reduce the ability of the blood to clot so that unnecessary blood clots are not formed.

Who takes anticoagulant medicines?

Anticoagulant medicines are most commonly prescribed for people who are at risk of developing conditions caused by blood clots, such as stokes. This includes:

  • people over 65 - as this age group is more at risk of strokes,
  • men - as men are more likely to have heart problems than women,
  • people of Afro-Caribbean origin - as they have a genetic predisposition (a natural tendency) to develop heart disease and high blood pressure (hypertension), which can cause strokes, and
  • people who are overweight or have a diet high in fat - as they are more likely to have other health problems, such as high blood pressure, which can lead to strokes or heart problems.

Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

Plasma
Plasma is the liquid part of blood, which holds other blood cells together.

Stroke
A stroke is a serious medical condition that occurs when the blood supply to the brain is disturbed or restricted. Brain cells begin to die and this can lead to brain damage and possibly death.

Heart attacks
A heart attack happens when there is a blockage in one of the arteries in the heart.

Anticoagulant
Anticoagulant is a substance that stops blood from clotting (prevents coagulation). For example, warfarin.

 

Page last reviewed: 13/07/2011

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) in tablet form. 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 is a clotting factor, which 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. It can be given to you by a healthcare professional, either as an injection or through a drip into a vein. Alternatively, you can be trained to administer the medication yourself.

Heparin interacts with the enzyme, thrombin. During the clotting process, thrombin acts upon a protein called fibrinogen to turn it into fibrin, which then forms the clot. Heparin can slow down the effect of the thrombin, so that it takes longer for fibrinogen to turn into fibrin. This means it takes longer for clots to form.

Different types of heparin medication include:

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

International normalisation ratio

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 that is found in the blood, called prothrombin, turns into the 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 to the PT of someone who is not taking anticoagulants. This then gives your INR.

For people who are on anticoagulant medicines, the aim is usually an INR of between 2.5-3.5, depending on what the anticoagulant medicines are being taken for (however, in some people, it can range from between 2-4.5).

This means that your PT will be between 2.5-3.5 times longer compared with the PT of someone who is not taking anticoagulant medicines. People who are not taking anticoagulant medicines usually have an INR of between 0.8-1.2.

Once your anticoagulant medicines begin to work, your INR should start to increase. 

Your dose

While you are taking anticoagulant medicines, your INR will be regularly tested by your GP, your pharmacist, or by a nurse at your local hospital. This is 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 2-4 days of starting treatment. Depending on your reading, further tests will be carried out 1-2 times a week. If your INR stabilises within the correct range, testing may become less frequent.

Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

Haemorrhage
To haemorrhage means to bleed or lose blood.

Anticoagulant
Anticoagulant is a substance that stops blood from clotting (prevents coagulation). For example, warfarin.

Page last reviewed: 13/07/2011

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, and
  • 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 of developing this condition again.

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 what 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 need to take anticoagulation medicines 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. 

Anticoagulant
Anticoagulant is a substance that stops blood from clotting (prevents coagulation). For example, warfarin.
Aortic valve
The aortic valve is the valve that controls the flow of blood out of the left ventricle (chamber) of the heart, to the aorta (the body's main artery).
Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Haemorrhage
To haemorrhage means to bleed or lose blood.
Heart attacks
A heart attack happens when there is a blockage in one of the arteries in the heart.
Stroke
A stroke is a serious medical condition that occurs when the blood supply to the brain is disturbed or restricted. Brain cells begin to die and this can lead to brain damage and possibly death.

Page last reviewed: 13/07/2011

If you are prescribed anticoagulant medicines, you should 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.

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:

  • an endoscopy - when a thin, flexible tube with a light source and a camera on one end, called an endoscope, is inserted into your body, either through a natural opening, like your throat, or through a small surgical incision (cut),
  • a cystoscopy - when a thin, fibre-optic tube with a light and a camera on one end, called a cystoscope, is used to examine the inside of your bladder, and
  • a colonoscopy - when a specially designed endoscope is used to examine your colon.

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 these medicines for a period of time before and after surgery. This will prevent excessive bleeding during surgery, and assist 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 ration (INR - see How it works) tested before the procedure, to make sure that it is 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 foetal abnormalities (birth defects) or placental or foetal haemorrahage (excessive bleeding from either the placenta or the 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 is necessary.

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

Breast feeding

Warfarin should not usually be taken while you are breastfeeding. It should only be taken on the advice of your GP or midwife.

Heparin is safe to take while you are breastfeeding.

If you are on anticoagulants and you are breast feeding, or planning to breast feed, 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 cuts and injuries by:

  • taking care when brushing your teeth and shaving,
  • using protection when gardening, sewing, or playing contact sports, and
  • using insect repellent to avoid insect bites or stings.
Anticoagulant
Anticoagulant is a substance that stops blood from clotting (prevents coagulation). For example, warfarin.
Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Foetus
A foetus is an unborn baby, from the eighth week of pregnancy until birth.
Haemorrhage
To haemorrhage means to bleed or lose blood.
Incision
An incision is a cut made in the body with a surgical instrument during an operation.
Placenta
The organ that links a pregnant woman's blood supply to her unborn baby's.

Page last reviewed: 13/07/2011

Haemorrhages

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

Whilst excessive bleeding may be obvious if you have a wound, 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, you should 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, and
  • in women, heavy, or increased bleeding during your period, or any other bleeding from your vagina.

Whilst you are taking anticoagulant medicines, you will be closely monitored to check that you are on the correct dose and not at risks of haemorrhages. The most common test for this is the international normalisation ratio (INR - see How it works for an explanation).

Warfarin

For a full list of the side effects caused by warfarin, see the warfarin Health A-Z topic. Some side effects include:

  • rashes,
  • diarrhoea,
  • nausea (feeling sick), and
  • vomiting.

Heparin

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

  • hair loss (alopecia),
  • osteoporosis (fragile bones), and
  • thrombocytopenia - an abnormal drop in the number of platelets in your blood, which can cause bleeding into vital areas.
Anticoagulant
Anticoagulant is a substance that stops blood from clotting (prevents coagulation). For example, warfarin.
Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Haemorrhage
To haemorrhage means to bleed or lose blood.

Page last reviewed: 13/07/2011

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, you should 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, and
  • any food or drink supplements. 

Prescription medicines that can interact with anticoagulants include:

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

For a more complete list of medicines that interact with the anticoagulant you are taking, check the leaflet that comes with your prescription or you can ask your pharmacist or GP.

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, you should not drink more than one or two alcoholic drinks a day, and you should never binge drink. 

The recommended daily limits for alcohol consumption are 21 standard drinks per week for a man, and 14 standard drinks for a woman. One standard drink of alcohol is approximately half a pint of beer, a pub measure of spirits or small glass of wine. Binge drinking is defined as six or more standard drinks in one day

Anticoagulant
Anticoagulant is a substance that stops blood from clotting (prevents coagulation). For example, warfarin.
Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Haemorrhage
To haemorrhage means to bleed or lose blood.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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