What are blood thinners (anti-clotting medications) and how are they used?

Blood thinners (anti-clotting medications) prevent blood clots from forming in the bloodstream. This lowers the risk of medical problems that are caused by blood clots, such as heart attacks, strokes, and venous thrombosis.

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

Blood thinners 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 blood thinners. But there are many things you can do yourself to lower the risk of bleeding.

What types of blood thinners are there?

Blood thinners are divided into different groups:

  • Antiplatelets are mainly suitable for people who have had a 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 in the form of tablets.
  • (Direct) oral anticoagulants are much more effective than antiplatelets. They are mainly used in the treatment of atrial fibrillation, after implanting artificial heart valves, or after a pulmonary (lung) embolism. Some anticoagulants can also be used after major surgery, in order to reduce the risk of deep vein thrombosis. They are also taken as tablets.
  • Heparin is particularly suitable for acute treatment, for instance in venous thrombosis. It is injected under the skin or into a vein, and works very quickly. This medication is only used temporarily over a few days or weeks.
  • Like heparin, the medication fondaparinux is injected under the skin – mainly for the treatment of acute problems or the prevention of thrombosis.

The type of anti-clotting medication that is prescribed will mainly depend on the medical condition that needs to be treated. Other aspects that play a role include the person's age, other medical problems they may have, interactions with other drugs and – in rare cases – allergies.

How do antiplatelets work and when are they used?

Antiplatelets stop blood platelets from working properly. They make it harder for blood platelets to attach to damaged blood vessels and stick to each other. This prevents blood clots from forming. Another name for antiplatelets is platelet aggregation inhibitors. Aggregation means "to come together."

Antiplatelets are mainly taken by people who have previously had a or stroke, in order to lower their risk of having another or stroke.

Apart from acetylsalicylic acid (e.g. in Aspirin), examples of antiplatelet drugs include clopidogrel, dipyridamole, prasugrel and ticagrelor. 50 to 100 milligrams of acetylsalicylic acid are enough to reduce the blood's clotting ability. At higher doses of about 500 milligrams, this medication is also used as a painkiller.

What are the side effects of antiplatelets?

Antiplatelets can increase the risk of bleeding – especially in the stomach. This risk is bigger if they are used together with anti-inflammatory painkillers such as (high doses of) acetylsalicylic acid, diclofenac, ibuprofen and naproxen, or together with other blood thinners.

But there are sometimes good medical reasons for taking more than one blood thinner. For instance, it's normal to take a combination of two antiplatelet drugs in the first year after receiving a cardiac (heart) stent. People who have several heart problems at the same time are also often advised to take a combination of different blood thinners.

Stomach-protecting medication such as omeprazole and pantoprazole can lower the risk of stomach ulcers and bleeding in the stomach related to the long-term use of antiplatelets.

What oral anticoagulants are there?

Oral anticoagulants are very effective. When people use them, it takes a lot longer for their blood to clot. Oral anticoagulants (anticoagulants that are taken by mouth) work 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 ("Padaxa"), edoxaban ("Lixiana") and rivaroxaban ("Xarelto").

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

How do vitamin K antagonists work?

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 two to four days. That's how long it takes for the clotting factors that are already in the blood to be broken down.

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 is 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.

It's important to check the clotting ability of your blood more regularly if

  • you make big changes to your eating habits or lifestyle,
  • you develop other medical problems, or
  • you start or stop taking other medication.

How does your diet influence the effect of vitamin K antagonists?

Foods that have a lot of vitamin K in them can somewhat reduce the effect of vitamin K antagonists. There's a lot of vitamin K in foods like cauliflower, broccoli, sauerkraut, beef and pork, for example. There's no need to change your diet if you’re taking vitamin K antagonists. If you make big changes to your diet – for instance, to lose weight – you may have to check your blood more often and adjust the dose of your medication if necessary.

How do direct oral anticoagulants (DOACs) work?

DOACs inhibit certain clotting factors immediately, 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.

How can you lower the risk of bleeding?

The most common side effect of blood thinners is bleeding, particularly in the digestive tract.

People who take blood thinners are often advised to carry a medical ID card on them. The card includes information about which illness the medication is taken for, what dosage needs to be taken and which doctor ought to be contacted. In an emergency, helpers will then know that you are on blood thinners and can react quickly if you have any injuries. Medical ID cards are available from doctors’ practices in Germany.

Alcohol can affect your blood's ability to clot, increasing the risk of bleeding. So it's a good idea to drink only a little alcohol, or none at all. Because high blood pressure increases the risk of bleeding too, it's important to get good treatment for high blood pressure if you have it.

Physical activity and sports do not directly affect blood clotting. But if you take oral anticoagulants, it's a good idea to carefully consider whether the risk of injury and bleeding is worth it when doing certain sports.

As a general rule, the more medications you take, the higher the risk of side effects and interactions between them. So it's important to have a good overview of which medications you are taking, and regularly talk to your doctor about your medication schedule.

What are the signs of bleeding?

Depending on where bleeding occurs, the signs can vary greatly: Light nose bleeds, gum bleeding or bruises on the skin are generally not a problem. But heavier bleeding has to be treated quickly.

You should consult a doctor if you notice any of the following:

  • Heavy, persistent bleeding from the nose or gums
  • Large bruises
  • Red-colored urine
  • Blood in stool (dark red or black stool
  • Blood in vomit

In very rare cases, blood thinners can lead to bleeding in the brain (a brain hemorrhage). Similar to when someone is having a stroke, it's then important to react quickly and call the emergency services straight away. The possible signs of bleeding in the brain include the following:

  • A very bad and sudden headache
  • Problems like trouble seeing, dizziness, paralysis or numbness
  • Impaired consciousness or slower reactions
  • Seizures

But bleeding in the brain is such a rare side effect that the advantages of this medication clearly outweigh the disadvantages: Blood thinners prevent far more heart attacks and strokes than they cause.

What happens if bleeding occurs in the digestive tract?

Considerable bleeding in the digestive tract usually isn't life-threatening, but it does have to be treated quickly in a hospital. Depending on how heavy the bleeding is, there are different treatment options:

  • Infusions to compensate for the loss of blood
  • Blood transfusions
  • Medication to make the blood clot faster
  • Medication to reduce the production of stomach acid
  • If necessary, a procedure to look at the inside of your food pipe and stomach (gastroscopy) or bowel (colonoscopy) and stop the bleeding using endoscopic instruments.

What should be kept in mind before surgery and other procedures?

If you’re due to have major surgery or another procedure such as a gastroscopy, it can be necessary to stop taking or adjust your blood thinners a few days beforehand. For this reason, you should make sure that the doctor knows in advance that you are taking blood thinners. It can also be helpful to show them your medical ID card.

You don’t always have to stop taking blood thinners before smaller procedures and dental treatment. But even then it’s better to let your doctor know beforehand. That gives the doctor the chance to prepare in case any problems do occur during the treatment. It’s also important to say that you’re taking blood thinners before having injections or vaccinations. This is because, for example, an injection into a muscle can cause more severe bleeding in people who are on blood thinners.

Can pregnant women take blood thinners?

Pregnant women are usually not allowed to take oral anticoagulants. Vitamin K antagonists are known to be potentially harmful to unborn babies. There has been very little research on treatment with DOACs in pregnant women. So it's important that women who are taking these medications make sure that they use effective contraception and talk to their doctor if they are planning to get pregnant.

Women shouldn't take vitamin K antagonists while they're still breastfeeding either. The drug can enter the child's bloodstream through breast milk and then also stop the child's blood from clotting.

Low doses of the antiplatelet drug acetylsalicylic acid (the drug in medicines like Aspirin) can be used in pregnancy if there are good reasons to do so. There is a lack of research on the use of other antiplatelet drugs in pregnant women.

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IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

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Updated on January 5, 2021
Next planned update: 2024

Authors/Publishers:

Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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