Anticoagulants are a family of medications that stop your blood from clotting too easily. They can break down existing clots or prevent clots from forming in the first place. These medications can help stop life-threatening conditions like strokes, heart attacks and pulmonary embolisms, all of which can happen because of blood clots.


What are anticoagulants?

Anticoagulants are a group of medications that decrease your blood’s ability to clot. They do that by letting your body break down existing clots or by preventing new clots from forming.

Anticoagulants come in many different forms, including injections, intravenous (IV) drugs, and medications you take by mouth. They often treat and prevent life-threatening conditions that can happen because of blood clots, like strokes, heart attacks and pulmonary embolisms.


Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Why would I need to take these medications?

When blood clots work like they’re supposed to, they form at the site of an injury that needs repair and they stay put. However, when clots don't stay in one place or form in your bloodstream, they can be extremely dangerous. If a clot is too large, it can get stuck in a smaller blood vessel. If that smaller blood vessel is in a critical location, it can block blood flow that one of your organs needs to survive.

Blockages from blood clots can cause the following deadly events:

  • Stroke. Blood clots are particularly dangerous if they travel up to your brain, where they can easily get stuck in the smaller blood vessels.
  • Pulmonary embolism (PE). This occurs when a blood clot gets stuck and blocks an artery in your lungs. If the blockage is severe enough, a pulmonary embolism can be deadly.
  • Heart attack (myocardial infarction). These occur when arteries that supply blood to your heart become blocked. These can also be deadly.

Anticoagulants can protect individuals who have a condition or disease that could cause them to have any of the above clot-related events.

Some of those conditions include:

  • Atrial fibrillation. This is an irregular heart rhythm in the upper chambers of your heart. Fibrillation means blood can pool because the upper chambers of your heart are beating too fast to pump effectively. Pooling blood can clot, and clots can easily travel from your heart to your brain, causing a stroke.
  • Heart valve surgery or replacement. Certain heart valve replacements have an increased risk of causing clots to form at the site of the new valve. Anticoagulants keep clots from doing so.
  • Hip or knee replacement. Joint replacements can increase the risk of clots forming in veins in your leg. This condition, deep vein thrombosis, is one of the main causes of pulmonary embolism.
  • Blood clotting disorders. This includes conditions and diseases that affect how your blood clots. Some of these disorders are genetic, meaning you can inherit them from your parents or pass them to your children.

How commonly are anticoagulants prescribed?

Anticoagulants are commonly prescribed medications. This is especially true thanks to the approval of several newer drugs, which you take by mouth, within the past 10 years. In the United States, more than 5 million individuals covered by Medicare Part D received a prescription for an anticoagulant medication in 2019. In addition, about one-third of all hospital patients receive some form of anticoagulant medication.


Procedure Details

How do they work?

Your body is constantly balancing clotting and anti-clotting processes. If your blood doesn't clot enough, an injury can cause severe blood loss or even death. If it clots too much, it can cause the dangerous medical events mentioned above. Certain blood components keep your clotting processes in an inactive state. That way, your body can activate them quickly when you have an injury that needs repair.

Thanks to that balancing act, clotting is usually a helpful process. It stops bleeding, creates a protective covering to keep germs and debris out of a wound, and then rebuilds the skin so it’s good as new (or nearly so, if the wound leaves a scar).

Anticoagulants work by interfering with the normal clotting processes. Just like their name suggests, they prevent or undo coagulation, the process where your blood solidifies to form a clot. Depending on the type of anticoagulant, the clotting process disruption happens in different ways.

IV and injectable medications

Heparin and its derivatives

Heparin is a medication that inhibits clotting by activating your body's anti-clotting processes. One of the anticlotting processes uses a type of blood protein called antithrombin. Heparin works by activating antithrombin, and then antithrombin keeps other parts of the clotting process from working normally.

Heparin comes in two different types, and there is a third medication that is closely related:

  • Unfractionated heparin (UFH). Unfractionated heparin is stronger and fast-acting. This is because UFH has a longer molecule, which means it’s long enough to help wrap around both antithrombin and thrombin, a protein that promotes clotting, holding them together. This neutralizes both proteins, further preventing clotting. UFH also needs constant monitoring with lab tests. That’s because its effectiveness depends on its dosage, and the needed dosage can be very different from person to person. Too little won't do enough to prevent clotting, and too much will create a risk of bleeding.
  • Low-molecular-weight heparin (LMWH). Low-molecular-weight heparin has shorter molecules, which means it can only attach to antithrombin. This also means the effects are longer-lasting, more predictable and LMWH doesn’t need the close monitoring required with UFH.
  • Fondaparinux. Fondaparinux is a synthetic medication that works similarly to heparin. Like heparin, fondaparinux activates antithrombin but acts over a much longer period. However, it’s not as strong as UFH or LMWH, so it’s most often used to prevent clots rather than treat clotting problems that are already happening (unless given along with other medications).

Direct thrombin inhibitors

Thrombin inhibitors work by attaching to thrombin, keeping it from assisting clotting processes. They are often used as alternatives to heparin and its variants, especially to prevent the formation of clots after certain medical procedures. These include argatroban, desirudin and bivalirudin.

Oral medications

Warfarin (vitamin K antagonist)

Warfarin is a vitamin K antagonist, meaning it blocks the use of vitamin K — a key ingredient in the clotting process. However, a major drawback of warfarin is that it needs careful dosing and regular lab testing to prevent complications. When the dosage isn’t precise enough, it can lead to severe bleeding.

In some cases, certain conditions mean that warfarin is the only anticoagulant that you can take. These include:

  • Diseases affecting the mitral valve of your heart.
  • Having a mechanical heart valve.
  • End-stage kidney disease.

Direct oral anticoagulants

These medications can all be taken regularly without regular lab testing and are often used when warfarin isn’t an option. One medication, dabigatran, is a thrombin inhibitor similar to the IV thrombin inhibitors listed earlier. Other medications, apixaban, edoxaban and betrixaban, are all inhibitors of factor Xa (10-A), a key clotting component.

Risks / Benefits

What are the advantages of anticoagulants?

Anticoagulants are extremely effective in preventing life-threatening conditions like stroke, pulmonary embolism and heart attack. There are also several different ways that these medications work. That means people who can’t take one medication may still be able to take a similar medication.


What are the side effects or complications of these medications?

The most common side effect risk with any anticoagulant is bleeding. Depending on the medication used, other potential risks exist.


Warfarin has a much higher risk of causing bleeding because the dosage must be precise. Other risks and side effects include:

  • Skin necrosis. This is a rare complication where warfarin causes clots to form in the blood vessels in your arms and legs, or in surface fatty tissue like breasts, buttocks or thighs. Those clots block blood flow, causing the affected areas of skin to die. This is most often seen in people who have deficiencies in certain blood proteins (these deficiencies are often inherited). It’s usually seen within five days after you start warfarin, but is possible as late as 10 days after you start warfarin.
  • Blue or purple toe. This is a color change in your toes and feet, especially on the soles of your feet or the sides of your toes. The condition is sometimes painful and usually happens anywhere from a few weeks to two months after you start taking warfarin.
  • Congenital disabilities or miscarriages. Warfarin can damage an embryo or fetus, so it shouldn't be taken during pregnancy. However, warfarin is safe when breastfeeding because it can’t be passed through breast milk.
  • Complications in patients with lupus. Warfarin dosing is usually higher in people with lupus or similar conditions. People who have lupus will often need to stop taking it before a medical procedure and switch to other medications to avoid bleeding problems.

Heparin and its derivatives

Heparin can affect other components of your blood and your bones, with side effects ranging from mild to severe.

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (throm-bo-sigh-toe-pee-knee-uh), often abbreviated as HIT, has two forms:

  • Type I: This type of HIT causes a slight decrease in platelets, a type of blood cell that is instrumental in blood’s ability to clot. This type of HIT is more common and happens to about 10% of the people on heparin or its variants. It's not considered dangerous, and it usually goes away within a week after you're no longer receiving heparin.
  • Type II: This type of HIT is when heparin triggers a response in your immune system that causes platelets — blood cells that form clots — to activate, triggering a chain reaction of clotting. This type of HIT is extremely dangerous because it can cause widespread clotting in your body. It's also very rare, occurring in 1% of people who receive heparin (it is more common with unfractionated heparin than with low-molecular-weight heparin).

HIT has a very high rate of causing severe or permanent complications. Because of that, platelet level monitoring is very important for anyone who’s receiving unfractionated or low-molecular-weight heparin. If a healthcare provider suspects HIT, they will immediately stop giving you heparin (in either of those two forms) and switch to another medication.


This complication is usually only seen with long-term use of heparin (more than one month). It happens because heparin decreases the formation of new bone cells and increases the rate at which old bone cells are naturally broken down by your body. It’s less likely to happen with LMWH heparin.

Direct oral anticoagulants

Direct oral anticoagulants can sometimes cause indigestion or bleeding in your gastrointestinal tract.

Are there any conditions that should prevent me from taking an anticoagulant?

In general, your healthcare provider is the person who can best explain any possible reasons you shouldn’t take anticoagulant medications. You should ask your healthcare provider if you shouldn’t take anticoagulants because of any other medications you take or conditions you have.

There are several conditions that mean you shouldn’t take any kind of anticoagulant (these conditions are called absolute contraindications). Some of these include:

  • Current or recent trauma or bleeding.
  • Recent major surgery.
  • History of bleeding in the brain, including stroke or aneurysm.
  • End-stage liver disease.
  • Certain conditions that affect blood clotting.

Relative contraindications are conditions where anticoagulant use should be considered on a case-by-case basis. These include:

  • Ulcers or other types of bleeding in the gastrointestinal tract.
  • Recent surgery that’s low-risk for bleeding.
  • High blood pressure that medication isn’t controlling.
  • Conditions where your aorta (the largest artery in your body) is prone to tear or rupture.
  • Taking other medications that affect your blood’s clotting ability.

Several conditions might keep you from taking certain anticoagulants. These conditions, listed by medication, include:


  • History of heparin-induced thrombocytopenia.
  • Low platelet levels.
  • Kidney problems (LWMH and fondaparinux only).

Direct thrombin inhibitors

  • Argatroban: Severe liver problems.
  • Desirudin: Moderate to severe kidney problems.
  • Bivalirudin: Severe kidney problems.


  • Pregnancy.
  • Conditions involving other clotting factors, especially deficiencies of Protein C and Protein S.
  • Certain genetic mutations that affect how your body uses warfarin.

Direct oral anticoagulants

  • Apixaban: Pregnancy.
  • Betrixaban: Pregnancy.
  • Dabigatran: Severe kidney or liver problems.
  • Edoxaban: Severe kidney problems.
  • Rivaroxaban: Severe kidney problems.

Recovery and Outlook

How long can I stay on these medications?

How long you can take an anticoagulant depends on which medication you take and how you take it. Most IV and injectable anticoagulants aren’t meant for long-term use.

However, you can take many oral anticoagulants for longer periods. Depending on which one your healthcare provider prescribes, you may be able to take it indefinitely.

When To Call the Doctor

When should I see my healthcare provider?

You should call your doctor if you have any symptoms of moderate to severe bleeding. These include:

  • Bleeding that won’t stop, such as from your gums, nose, or cuts and scrapes.
  • Bruising more easily, or suddenly finding bruises and you can’t remember how they happened.
  • Feeling unusually dizzy, weak or tired.

People who take blood thinners are also at risk for severe bleeding — especially internal bleeding —when they’re injured. You should get immediate medical care if you have any of the following symptoms:

  • Any kind of fall where you hit the floor or an object, even if you don’t have a cut or wound that is bleeding. This is especially true if you hit your head. People who take anticoagulants have a high risk of internal bleeding, especially in their brain, from falls and injuries. This also includes if something strikes you in the head, even if it doesn’t cause you to fall.
  • Being in a car crash or being struck by any kind of object that leaves you with a severe bruise.
  • Headache or stomach pain, especially when sudden, severe or both.
  • Vomiting or coughing up blood (especially vomit that looks like it has coffee grounds in it).
  • Blood in your urine (pee that is orange, red or brown) or stool (poop that is red or looks like tar).

A note from Cleveland Clinic

Anticoagulants are a group of medications that see widespread use for a variety of reasons. They help prevent and treat clot-based health conditions like stroke and pulmonary embolism. These medications have life-saving potential. While they're extremely helpful, they also increase your risk of bleeding, so it’s important to talk with your healthcare provider about how to avoid severe bleeding if you’re injured.

Medically Reviewed

Last reviewed on 01/10/2022.

Learn more about our editorial process.

Appointments 800.659.7822