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Anticoagulant medicines

Anticoagulant medicines work by interrupting part of the process involved in the formation of blood clots.

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:

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

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Anticoagulant medicines

If you are prescribed anticoagulant medicines, always follow the instructions of your GP or other healthcare professionals.

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 NHS 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 endoscopycystoscopy 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

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Anticoagulant medicines

A side effect common to all anticoagulants is the risk of excessive bleeding (haemorrhages) due to clots take too long to form.

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)
  • 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

You must also seek immediate medical attention if you:

  • are involved in a major accident
  • experience a significant blow to the head
  • are unable to stop any bleeding

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:

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

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