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In brief: What are anticoagulants?

Last Update: October 25, 2022; Next update: 2025.

Anticoagulants are medicines that prevent blood clots from forming in the bloodstream. This reduces the risk of medical problems that are caused by blood clots – such as heart attacks, strokes, thrombosis and embolism.

The commonly used name "blood thinners" may be misleading because these medications don’t actually make blood thinner.

Anticoagulants don't completely stop blood from clotting. Otherwise people would bleed to death if they cut themselves. Instead, it takes longer for wounds to stop bleeding. For this reason, bleeding is one of the main side effects of anticoagulants. But there are many things you can do yourself to lower the risk of bleeding.

What types of anticoagulants are there?

Anticoagulants are divided into different groups. The most suitable type of anticoagulant will depend on the medical condition that needs to be treated:

  • Antiplatelets are mainly suitable for people who have had a heart attack or a stroke due to arteriosclerosis (narrowing of blood vessels). The most commonly used antiplatelet is acetylsalicylic acid (the drug in medicines like Aspirin). Antiplatelets are taken as tablets.
  • Oral anticoagulants are much more effective than antiplatelets. Examples include vitamin K antagonists and direct oral anticoagulants (DOACs). These medications are mainly used in the treatment of atrial fibrillation, after implanting artificial heart valves, or after a pulmonary (lung) embolism. Some of them can be used after major surgery, in order to reduce the risk of deep vein thrombosis – or after a pulmonary embolism, to prevent further embolisms from developing. Oral anticoagulants are also taken as tablets.
  • Heparin is particularly suitable for acute treatment, for instance in venous thrombosis or pulmonary embolism. It is injected under the skin or into a vein, and starts working very soon.
  • The medication fondaparinux is also injected under the skin – mainly for the acute treatment of thrombosis and pulmonary embolism, or for the prevention of thrombosis.

The choice of anticoagulant will also depend on the person's age, other medical problems they may have, interactions with other drugs and – in rare cases – allergies.

How do antiplatelets work?

Antiplatelets stop blood platelets from working properly. They make it harder for blood platelets to attach to damaged blood vessels, stick to each other, and form a blood clot. That’s why antiplatelets are also called platelet function inhibitors or platelet aggregation inhibitors (where “aggregation” means to stick together).

As mentioned above, one well-known antiplatelet drug is acetylsalicylic acid (e.g. in Aspirin). Other examples of antiplatelets include clopidogrel, dipyridamole, prasugrel and ticagrelor. A dose of 50 to 100 milligrams of acetylsalicylic acid is already enough to reduce the blood's clotting ability. At higher doses of about 500 milligrams, this medication is also used as a painkiller.

What oral anticoagulants are there?

Oral anticoagulants are very effective. When people use them, it takes a lot longer for their blood to clot. The medication works by affecting something known as clotting factors. These are proteins in the blood that the body can activate if needed. They make blood platelets stick together and form clots. Some clotting factors are made in the liver, and others are made in blood vessels.

There are two groups of oral anticoagulants:

  • Vitamin K antagonists (coumarins): These include the drugs phenprocoumon (which is commonly known under its trade name Marcumar) and warfarin.
  • Direct oral anticoagulants (DOACs): These include apixaban ("Eliquis"), dabigatran ("Pradaxa"), edoxaban ("Lixiana") and rivaroxaban ("Xarelto").

DOACs are sometimes still called "new (or novel) oral anticoagulants" (NOACs) because they have only been approved since the year 2008.

What should you know about vitamin K antagonists?

The liver needs vitamin K to make certain clotting factors. Vitamin K antagonists prevent blood clots from forming by reducing the action of vitamin K in the liver. They only start working after a few days. That's how long it takes for the body to break down the clotting factors that are already in the blood.

The effectiveness of vitamin K antagonists will depend on various factors, including the dose, interactions with other drugs, the person's metabolism and whether they have other illnesses too. Because of this, the clotting ability of their blood has to be measured regularly. If it’s too high or too low, the dose of medication is adjusted. You can attend a patient education class to learn how to monitor your blood yourself and adjust the dose if necessary.

Vitamin K antagonists can be harmful to unborn babies if pregnant women use them. Because of this, women of childbearing age are advised to use effective contraception when taking these medications.

How do direct oral anticoagulants (DOACs) work?

DOACs inhibit certain clotting factors directly, so they start working after just a few hours. Another advantage of these drugs is that you don't have to check the clotting ability of your blood while taking them.

Only little is known about the effects of using DOACs in pregnancy. Because of this, women who take DOACs and could become pregnant are advised to use contraception and seek advice from their doctor if they would like to have children.

What role do heparin and fondaparinux play?

Heparin and fondaparinux have a similar structure, and both drugs are very effective anticoagulants. Unlike vitamin K antagonists and DOACs, they are injected. They are particularly suitable in acute situations and at the start of treatment.

There are two main types of heparin, known as low-molecular-weight heparin (LMWH) and high-molecular-weight or “unfractionated” heparin (UFH). Unfractionated heparin starts working particularly fast, but it’s harder to get the dose just right so you have to monitor your blood very closely.

What are the side effects of anticoagulants?

The main side effect of anticoagulants is bleeding. Although heavy bleeding is rare, it is a serious complication and needs to be treated in a hospital very quickly.

The higher the dose of anticoagulant medication, the greater the risk of severe bleeding. Further risk factors include older age and the use of certain other medications, such as other anticoagulants, non-steroidal anti-inflammatory drugs (NSAIDs) or painkillers. Medical conditions like chronic kidney or liver disease, blood clotting disorders and cancer can increase the risk of bleeding too, and so can drinking a lot of alcohol over a long period of time.

The risk of bleeding in the stomach when using antiplatelets is particularly high if this medication is taken together with anti-inflammatory painkillers such as (high doses of) acetylsalicylic acid (e.g. in Aspirin), diclofenac, ibuprofen and naproxen, or if it is used together with other anticoagulant medications. But there are sometimes good medical reasons to use more than one anticoagulant. For instance, it’s quite common to use a combination of two antiplatelet drugs in the first year after having a heart stent put in. It can also make sense to use a combination of anticoagulants if you have several different heart problems at the same time.

If you take antiplatelets over a long period of time, stomach-protecting medicines like omeprazole and pantoprazole can reduce the risk of stomach ulcers and bleeding in the stomach.

Which anticoagulants can be used in pregnancy and while breastfeeding?

Pregnant women and breastfeeding women can use low doses of the antiplatelet drug acetylsalicylic acid (e.g. found in Aspirin). Very little is known about the effects of other antiplatelets in pregnancy.

Heparin doesn’t reach the unborn baby through the placenta and can be used for the treatment of pulmonary embolism, for instance. It can be injected in the usual way (into skin in the belly) without any risk to the baby. This drug is also suitable for breastfeeding mothers.

Fondaparinux and direct oral anticoagulants reach the unborn baby through the placenta. Very little is known about the effects of these medications in pregnancy. Wherever possible, fondaparinux and DOACs should not be used while breastfeeding either.

Vitamin K antagonists are known to be potentially harmful to unborn babies. But they might still be used in pregnant women in rare cases – for instance, if heparin isn’t an option or if the woman has an artificial heart valve. Vitamin K antagonists don’t pass into breast milk (or only in an inactive form), so breastfeeding women can use them. It is a good idea for doctors to give newborn babies vitamin K in the first few weeks of life. This ensures that the babies’ blood can clot enough.


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