Drugs that Prevent Blood Clotting

Drugs used to prevent unwanted blood clots developing or to stabilize existing blood clots

Common drugs

    Antiplatelet drugs

  • Aspirin

  • Clopidogrel

  • Dipyridamole

    Oral anticoagulants

  • Warfarin

    Injected anticoagulants

  • Dalteparin

  • Enoxaparin

  • Heparin

  • Tinzaparin

Drugs may be prescribed to prevent unwanted blood clots (thrombi) from developing in blood vessels. They may also be used to prevent existing blood clots from enlarging and to reduce the risk of an embolism, in which a piece of an existing clot in a vein breaks off and travels to a vital organ. Thrombolytic drugs are used when a blood clot needs to be dissolved rapidly.

What are the types?

Antiplatelet drugs and anticoagulants are used to prevent blood clots from forming. In general, antiplatelet drugs are used to prevent clots from forming in arteries and anticoagulants are prescribed to prevent clots from forming or enlarging in veins or within the heart.

Antiplatelet drugs

These drugs are used to help prevent blood clots from forming in the arteries. The drugs work by reducing the tendency of platelets, a type of blood cell that plays a vital role in the clotting process, to stick together. If you have symptoms of coronary artery disease, such as angina, or have had a heart attack (see Myocardial infarction), a stroke, or a transient ischaemic attack, you may need to take an antiplatelet drug for the rest of your life.

Aspirin is the most commonly prescribed antiplatelet drug. However, you should not take aspirin if you are pregnant, breast-feeding, or have a peptic ulcer. Clopidogrel works by reducing the “stickiness” of platelets in a similar way to aspirin and is often recommended as an alternative for people who cannot take aspirin. Combination treatment with clopidogrel and aspirin may be recommended for people who have had a heart attack, a severe attack of angina, or who have undergone a coronary angioplasty and stenting. When used together with aspirin in this way, clopidogrel further reduces the likelihood of another heart attack or a stroke during such periods of high risk. The drug should not be taken if you are breast-feeding.

Oral anticoagulants

These drugs are given as a long-term treatment to prevent deep vein thrombosis and pulmonary embolism. Oral anticoagulants may also be given to people with the heart rhythm disorder atrial fibrillation. Warfarin is the most frequently used oral anticoagulant.

Oral anticoagulants act by preventing the formation of clotting factors, the proteins that are essential for normal blood clotting. The drugs usually take 48–72 hours to become effective.

While you are taking oral anticoagulants, you will need frequent blood tests for the first few days and then regular tests during your treatment so that the dose can be adjusted to your needs. If the dose is too high, these drugs may cause abnormal bleeding. For this reason, you should consult your doctor immediately if you develop symptoms such as nosebleeds or blood in your urine. To treat abnormal bleeding, your doctor may prescribe drugs that reverse the effect of anticoagulants (see Drugs that promote blood clotting).

Oral anticoagulants may cause other side effects, including rashes, diarrhoea, easy bruising, hair loss, and impaired liver function. You should avoid drinking alcohol and making any sudden changes to your diet, both of which may affect the action of anticoagulants. Since many drugs interact with anticoagulants, you should not take other medications, particularly aspirin, without first consulting your doctor. It is very important that you inform your doctor if you plan to become pregnant because certain anticoagulants can cause abnormalities in a developing fetus. You should not discontinue anti-coagulant drug treatment suddenly. You will be given an anticoagulant treatment card or a medical alert bracelet or pendant, which you should always carry with you to inform health professionals in case of an emergency.

Injected anticoagulants

If blood clotting must be controlled quickly, an anticoagulant, such as heparin, may be given by injection or infusion so that it has immediate effect. These fast-acting injected anticoagulants are used to treat disorders such as pulmonary embolism before oral anticoagulants take effect. Injected anti-coagulants are given as a preventive measure before orthopaedic surgery. They may also be given before surgery to people who are at particular risk of blood clotting and are often prescribed for people who are immobilized during treatment in hospital.

Some newer, longer-acting drugs, such as tinzaparin, which need be given only once a day, can be self-administered for the treatment of deep vein thrombosis. Injected anticoagulants may cause side effects such as skin rashes.


Contact your doctor immediately if you are taking an oral anticoagulant and you have nosebleeds or you notice blood in your urine.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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