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

Created: ; Last Update: October 5, 2017; Next update: 2020.

Anti-clotting medications make sure that certain components of the blood don’t stick together so easily, in order to prevent blood clots from forming. They’re also known as blood thinners, but this name is misleading because they don’t actually make blood thinner.

Anti-clotting medication is used to lower the risk of certain cardiovascular diseases that are caused by blood clots, such as heart attacks, strokes, and venous thromboses. The active ingredients of anti-clotting medications can be divided into three groups.

  • Oral anticoagulants stop certain clotting factors from forming or working. This group of medications includes vitamin K antagonists (coumarins) such as the active ingredient phenprocoumon (which is commonly known under its trade name Marcumar) as well as direct oral anticoagulants (DOACs). Oral anticoagulants are highly effective. They are mainly used in the treatment of atrial fibrillation, after implanting artificial heart valves, or after a lung embolism.
  • Antiplatelets such as ASA stop the blood platelets (thrombocytes) from clotting so easily. They are weaker than anticoagulants and are mainly taken by people who have had a heart attack or a stroke.
  • Heparins also inhibit certain clotting factors. Because they take effect more quickly than other substances, they’re useful for the treatment of acute symptoms.

The type of anti-clotting medication that is prescribed will mainly depend on the person’s underlying medical condition. But age, other medical conditions and risk factors for bleeding also have to be considered. Intolerances of certain anti-clotting medications and possible interactions with other medications can also influence the choice.

Vitamin K antagonists

Vitamin K antagonists like the active ingredients phenprocoumon and warfarin are highly effective medications: When people use them, it takes a lot longer for their blood to clot. Phenprocoumon is the most frequently prescribed vitamin K antagonist in Germany. Many people know it under its trade name Marcumar, but it’s also available as Falithrom and Phenprogamma.

Vitamin K antagonists are usually taken by people who have a high risk of stroke. They include people with artificial heart valves, atrial fibrillation and people who have had pulmonary embolism. Vitamin K antagonists are particularly suitable for long-term use because they can be taken as tablets.

They stop certain clotting factors that are produced in the liver using vitamin K from forming in the body. They do that by removing some of the vitamin K from the liver, which is why they’re called vitamin K “antagonists.” They only start to take full effect after two to four days once the clotting factors already in the blood have been broken down.

Although the strength of the anti-clotting effect does depend on the dose, it can also vary greatly from person to person, and even over time in the same person. It’s therefore important for the blood's clotting ability to be regularly monitored during treatment.

Pregnant women are usually not allowed to take vitamin K antagonists because they can be harmful to unborn babies. 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 should also avoid taking vitamin K antagonists while they are still breastfeeding. The active ingredients can enter the child's bloodstream via breastmilk and then also stop their blood from clotting.

Interactions of vitamin K antagonists

Vitamin K antagonists have a high number of interactions with other substances, including with over-the-counter medications and herbal products. There are warnings of possible interactions with the following:

  • Antibiotics such as amoxicillin and ciprofloxacin
  • Psychotropic medication such as citalopram and sertraline
  • Medication taken against stomach ulcers such as cimetidine and omeprazol
  • Herbal products such as gingko, St. John's wort and ginseng
  • Painkillers such as ASA and paracetamol

Influence of diet and exercise on the effectiveness

Because these medications work by removing vitamin K from the liver, the vitamin K content of your diet has an influence on their effect. For example, there's a lot of vitamin K in foods like cauliflower, broccoli, beef and pork. But there's no need to change your diet if you’re taking vitamin K antagonists. Only considerably changing what you eat, perhaps because you’re on a diet, can have an effect on blood clotting.

Large amounts of alcohol can also influence the blood's ability to clot and intensify the effects of anti-clotting medication. But consuming moderate amounts of alcohol from time to time is not a problem. Acute and chronic gastrointestinal tract conditions can affect the blood's ability to clot and the effects of the medication.

Physical activity and sports do not directly affect blood clotting. But if you take vitamin K antagonists, it's a good idea to make sure that the risk of injury and therefore of bleeding is not too high with certain sports.

If you change your eating habits and lifestyle or develop other illnesses, it's important that you have your blood coagulation levels checked more regularly. That way you can detect any changes early on and adjust the dose of your medication if necessary.

Direct oral anticoagulants (DOACs)

Another four anticoagulants have been approved in recent years: apixaban, dabigatran, edoxaban and rivaroxaban. They are also taken orally. When using them, your blood clotting doesn’t need to be monitored.

So far the new drugs have only been approved for the following: Prevention of stroke in people with atrial fibrillation who are high-risk, and treatment or prevention of deep vein thrombosis (DVT) and pulmonary embolism. Apixaban, dabigatran and rivaroxaban are also approved for the prevention of thrombosis after fitting an artificial knee or hip.

DOACs also immediately inhibit a certain clotting factor. They start to take effect after just a few hours.

There isn’t enough research on their use in pregnant or breastfeeding women, so women who are pregnant or breastfeeding should not take them.


Antiplatelets (platelet aggregation inhibitors) are weaker than anticoagulants, but side effects such as bleeding are also less common. They are mainly taken by people who have previously had a heart attack or stroke, in order to lower the risk of another attack. The most commonly used antiplatelet is acetylsalicylic acid (ASA). Other antiplatelet drugs include clopidogrel, dipyridamole, prasugrel and ticagrelor.

How do antiplatelets work?

Antiplatelets inhibit the functions of blood platelets. They stop them from attaching to damaged blood vessels and sticking to each other, and therefore prevent blood clots from forming. That’s why antiplatelets are also called platelet function inhibitors or platelet aggregation inhibitors (where aggregation means to stick together).

Interactions of antiplatelets

ASA is not only used as an antiplatelet but also in a higher dose as a painkiller. It can increase the risk of bleeding in the stomach. That's also true of other painkillers like diclofenac and ibuprofen. These medications can also limit the anti-clotting effect of ASA. Paracetamol might be a better option for people who are already taking ASA as an anti-clotting medication because it only slightly raises the risk of bleeding.


There is another group of anticoagulants called heparins. These products have an immediate anti-clotting effect. That makes them especially suitable for acute treatment of venous thrombosis, pulmonary embolism and heart attacks as well as for prevention of venous thrombosis in people who have to have major surgery.

Heparins directly and immediately inhibit certain clotting factors in the blood. They have to be injected below the skin or into a vein to be effective. Because most people who regularly have to take long-term medication prefer to take tablets rather than inject medication, heparins are generally not used for long-term treatment.

What side effects do blood thinners have?

The medications are not supposed to stop blood from clotting completely. After all, if the blood didn’t clot at all, injuries would lead to major blood loss. But if you are on anti-clotting medication, it does take longer for your body to close wounds and stop any bleeding.

Bleeding is the most common side effect of oral anticoagulant and antiplatelet medications. 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

Strokes are a rare, but very severe side effect of blood thinners. A very strong and sudden headache is a sign of such bleeding, especially if it occurs together with other problems like trouble seeing, dizziness, paralysis or numbness. If these kinds of symptoms occur, it is important to call an ambulance right away (112 in Germany).

Some anticoagulants can cause hair loss or rashes. In very rare cases clopidogrel and heparins can reduce the number of blood platelets to a dangerously low level.

If you have a more serious injury while using vitamin K antagonists, you might need synthetic clotting factors or vitamin K so that your blood clots faster.

One common way to make the use of blood thinners safer is to have a medical ID card. They are available from doctors’ practices in Germany. 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 that information is then on hand immediately. This can help to prevent complications.

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

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

You don’t necessarily 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 that you make it known that you’re taking blood thinners before injections or vaccinations. This is because, for example, an injection into a muscle could cause more severe bleeding if you’re using anti-clotting medication.


© IQWiG (Institute for Quality and Efficiency in Health Care)
Bookshelf ID: NBK279433


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