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How Anticoagulants Prevent Blood Clots From Forming

Physician consulting with patient

Blood clotting to stop bleeding, or coagulation, is a normal part of the body’s healing process after a cut or other type of injury. However, excessive blood clotting, or hypercoagulation, may occur when clots form too easily while blood moves normally through the body. Hypercoagulation can be serious: when a blood clot develops in the arteries, it may lead to a stroke, heart attack, leg pain, or other serious health complications.

Some people are born with a tendency to form blood clots, while in other cases the condition may develop as a result of surgery, trauma, a medical condition, or prescription medication usage.

Medical conditions that may lead to hypercoagulation include:

  • Atrial fibrillation, an irregular heart rate condition 
  • Obesity
  • Prolonged bed rest or immobility
  • Heart attack, congestive heart failure, stroke, and other illnesses that lead to less physical activity
  • Antiphospholipid syndrome, a condition in which the immune system mistakenly creates antibodies that make your blood more likely to clot
  • Previous history of deep vein thrombosis — typically a blood clot in the deep veins in your legs — or pulmonary embolism — a blood clot in the arteries in your lungs
  • Bone marrow and blood diseases including polycythemia vera, a blood cancer that thickens your blood, and essential thrombocytosis, a rare disorder in which your body produces too many platelets
  • Paroxysmal nocturnal hemoglobinuria, a rare blood condition in which your immune system attacks red blood cells
  • Inflammatory bowel syndrome
  • HIV/AIDS
  • Nephrotic syndrome, a kidney disorder in which the urine contains too much protein

Medications that may lead to hypercoagulation include:

  • Supplemental estrogen, including birth control pills and hormone replacement therapy
  • Certain cancer medications such as tamoxifen, a breast cancer medication that activates estrogen in bone and liver cells; bevacizumab, a medication used to treat a variety of cancers and which blocks a protein responsible for blood vessel production; thalidomide, a medication used to treat a variety of cancers of which blood clots are a known side effect, particularly for patients 65 and older; and lenalidomide, a medication used to treat certain blood cancers of which blood clots are a known side effect

With many of these medications, the risk of developing blood clots is low, but individuals with a previous history of blood clots or an underlying condition linked to hypercoagulation face a greater risk. 

If you are prescribed one of these medications, be sure to inform your doctor of your medical history, including any other medications you currently take, and ask about the potential benefits and risks of using them. And if you experience any of the primary signs of a blood clot — shortness of breath, chest pain, or leg pain or swelling — contact your doctor immediately.

Treating blood clots with anticoagulants

Hypercoagulation can be treated with anticoagulants, a class of medications that reduce the blood’s ability to form new clots and prevent existing clots from growing. These medications are primarily prescribed when a blood clot develops in a vein or artery or as a preventative measure for people who face a higher risk for blood clots due to an underlying medical condition. They work by interrupting the body’s natural blood clotting process.

There are three main types of anticoagulant medications that act on different parts of the blood clotting process to prevent irregular blood clots: vitamin K antagonists, direct oral anticoagulants (DOACs), and low-molecular weight heparins.

Vitamin K antagonists

Vitamin K, commonly found in green leafy vegetables such as broccoli and spinach, helps your blood clot. Warfarin (brand name options include Coumadin® and Jantoven®) — one of the most common treatments for blood clotting disorders — is a vitamin K antagonist that prevents your liver from processing vitamin K into “factors,” or proteins that control your bleeding and help form blood clots. It is taken orally as tablets.

Warfarin is so commonly prescribed because of its proven effectiveness: it has been in use longer than other anticoagulants and reduces the rate of stroke in people with atrial fibrillation by 60%. It is also easier than other anticoagulants to reverse if you experience sudden bleeding from an injury or during surgery.

Because warfarin is so effective at preventing blood clotting, one of the main side effects is bleeding. The risk of serious bleeding is low, but you may bleed or bruise more easily after even small injuries. Difficulty stopping a nosebleed is also common.

Contact your healthcare provider if you experience heavy bleeding or bruising, including heavier than normal menstrual bleeding, and seek immediate medical attention if you experience any of the following symptoms:

  • Red or brown urine
  • Black or bloody stool
  • Severe headache or stomach pain
  • Joint pain, discomfort or swelling, especially after an injury
  • Vomiting blood or coughing up blood
  • Bruising that develops without an injury you remember
  • Dizziness or weakness
  • Vision changes
  • Head injury

You will need to get frequent blood tests when taking warfarin to evaluate blood clotting and the medication’s effectiveness. Your doctor may choose to adjust your dosage based on your test results, which is why it's important to stay up to date with lab appointments.

Here are additional tips to avoid side effects and complications while taking warfarin:

  • Wear a medical ID bracelet to ensure that you receive proper medical care in an emergency situation.
  • Anticoagulants can interact with other medications. Speak to your doctor before taking any other medication, including over-the-counter medications or supplements.
  • Warfarin is not safe to take during pregnancy. If you are planning to become pregnant, speak with your doctor about switching to an alternate medication.
  • Consult with your doctor about dietary recommendations, as certain foods, particularly those that are high in vitamin K, can affect how warfarin works.

Direct oral anticoagulants (DOACs)

Direct oral anticoagulants (DOACs) like apixaban (Eliquis®), betrixaban (Bevyxxa®), dabigatran (Pradaxa®), edoxaban (Savaysa®), and rivaroxaban (Xarelto®) directly block the blood’s ability to form clots. They are the newest class of anticoagulants, having been first introduced into the market in 2010.

These medications offer some advantages in contrast to other anticoagulants. Unlike warfarin, they do not require regular lab work. They are also less likely to interact with food, so there aren’t any dietary restrictions. There are FDA-approved antidotes for dabigatran, apixaban, and rivaroxaban to stop excessive bleeding in the event of trauma or emergency surgery.

On the other hand, DOACs are shorter-acting than vitamin K antagonists, so missing even one dose can make you vulnerable to blood clot formation.

The most serious side effect to watch for is uncontrolled bleeding, though it is rare. Contact your healthcare provider if you experience heavy or unusual bleeding or bruising.

Some studies also suggest that DOACs may be unsafe to take while pregnant. If you are planning on getting pregnant, speak with your healthcare provider about alternate medications.

Avoid taking nonsteroidal anti-inflammatories (NSAIDs) like aspirin, ibuprofen, and naproxen while taking a DOAC, and wear a medical ID bracelet to ensure proper medical treatment in the event of an emergency.

Low-Molecular Weight Heparins (LMWHs)

This medication class, which includes dalteparin (Fragmin®) and enoxaparin (Lovenox®), is primarily used for short periods of time to treat or prevent a deep vein thrombosis — a blood clot in the body’s deep veins, often in the legs. It can also reduce your risk of pulmonary embolism, a blockage in your lung arteries as the result of a blood clot.

In contrast to other anticoagulants, LMWHs are often used as a bridge to longer-term use of another anticoagulant such as warfarin, though some people take LMWHs for longer periods of time. They can be administered via IV drip in hospitals or doctors’ offices or through subcutaneous injections at home once or twice a day. No blood tests are required.

Other potential side effects include:

  • Uncontrolled bleeding
  • Redness, irritation, and bruising at the site of infection
  • Loss of bone strength
  • Elevated liver enzymes

Consult your doctor about the potential risks and benefits of taking an LMWH, and make sure they are aware of any other medications you take. In particular, NSAIDs can increase your risk of bleeding while taking LMWHs.

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This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or another qualified health provider with any questions you may have regarding a medical condition.

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