People with thick blood have too many red blood cells in their blood, which increases their risk of blood clots. Medication and lifestyle choices can help lower the risk of clots and other complications.

People often use the term “thick blood” to refer to polycythemia vera, but there are several possible causes.

Having too many red blood cells can hinder the movement of blood and therefore the movement of oxygen, hormones, and nutrients throughout the body, preventing them from reaching tissues and cells. It can result in low oxygen levels, hormonal imbalances, and nutritional deficiencies.

Below, we look at how thick blood affects the body, what can cause it, and how doctors treat it.

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Thick blood is a common term for polycythemia vera, but there are other possible causes. A high proportion of red blood cells makes the blood thick and increases the tendency of blood to clot more easily. Blood needs to coagulate to stop bleeding, for instance, from a wound. However, if it coagulates too easily, it may not move freely through the body.

Blood coagulates when certain factors, or proteins, change the balance of ingredients in blood.

Health conditions that involve thick blood may be inherited or genetic, or they may develop over time.

Here, we look at some medical conditions that cause the blood to thicken.

Polycythemia vera

Polycythemia vera (PV) is a blood disorder that originates in bone marrow, the soft center of the bone where new blood cells develop.

PV involves the bone marrow making too many red or white blood cells and platelets, causing the blood to thicken. Blood clots sometimes occur, but the sluggishness of the thick blood can also lead to symptoms such as:

Various symptoms may appear over time, including:

  • fatigue
  • headaches
  • dizziness
  • weakness
  • sweating, especially at night
  • itchy skin
  • blurred vision and ringing in the ears
  • abdominal fullness or bloating, due to an enlarged spleen

Experts believe that PV results from genetic changes that occur after conception. It is not generally inherited, but genetic changes occur slowly over many years.

PV affects 44–57 people in every 100,000, and it usually appears after the age of 60 years, according to the National Organization for Rare Disorders.

Find out more about polycythemia vera — and secondary polycythemia.

Waldenström macroglobulinemia

Waldenström macroglobulinemia is a rare type of non-Hodgkin lymphoma. The body produces large amounts of the antibody protein called immunoglobulin M. This can cause the blood to become thick, resulting in a range of symptoms.

The symptoms vary, but they may include:

  • nosebleeds
  • bleeding in the gums and the retinas of the eyes as small blood vessels become damaged
  • anemia
  • tingling and numbness in the fingers and toes

There are about 1,000 to 1,500 new diagnoses of Waldenström macroglobulinemia in the United States each year, according to the American Cancer Society.

Deficiencies in proteins C and S and antithrombin

The body produces natural anticoagulants, such as proteins C and S and antithrombin, which help prevent the blood from becoming thick. Some people have low levels of these substances, leading to a higher risk of blood clots. This may result from an inherited health issue or develop over time.

The first sign of this issue is often a blood clot. The doctor may find this during a test. If a clot forms in a deep vein, the doctor may diagnose deep vein thrombosis (DVT). If a clot breaks off and travels to block a blood vessel that supplies blood to the lungs, this is called a pulmonary embolism (PE), and it is life threatening.

Protein C deficiency affects around 1 in 200–500 people, protein S deficiency 1 in 500 people, and antithrombin deficiency 1 in 2,000–5,000 people.

Learn more about protein C deficiency.

Prothrombin gene 20210A mutation

People with this genetic feature have too much of the blood-clotting protein factor II, also called prothrombin. Prothrombin is one factor that enables blood to clot correctly, but too much prothrombin can increase the risk of clots forming, including those responsible for DVT and PEs.

The mutation affects 2–4% of Americans of European ancestry and around 0.4% of African Americans, according to the National Blood Clot Alliance.

Factor V Leiden

This results from a genetic change that increases the risk of blood clots, especially in deep veins.

In most people, a protein called activated protein C regulates the activity of the clotting factor called factor V. In people with this condition, protein C cannot regulate factor V’s activity, resulting in thick blood and a risk of clotting.

Unregulated factor V activity leads to excessive clotting and thickened blood.

Thick blood can develop over a lifetime due to environmental factors or health conditions.

Risk factors include:

  • smoking
  • hormonal changes, for example, during pregnancy
  • hormonal medications, such as some forms of birth control, hormone replacement therapy, and testosterone therapy
  • inflammation, possibly caused by SLE or inflammatory bowel disease
  • trauma and injury
  • some diseases, such as cancer

Thick blood often produces no symptoms. The first sign may be a blood clot, or a person may experience symptoms related to the underlying health condition.

A person may wish to consider testing if they:

  • develop a blood clot of unknown origin
  • have repeated blood clots
  • experience recurrent pregnancy loss
  • have a family history of thick blood or blood clots

With testing, a doctor can assess the risk of some medical conditions related to thick blood.

What are the symptoms of a blood clot?

For health conditions that affect blood clotting, doctors may prescribe drug-based treatments, such as:

  • Antiplatelet therapy: Medications such as aspirin can help prevent blood cells from forming clots.
  • Anticoagulation therapy: Some drugs, such as warfarin (Coumadin), act on coagulation factors to prevent clots.
  • Thrombolytics: Doctors may use these”clot-busting” drugs in emergency situations.

Also, protein C and antithrombin factor may be suitable for some people.

A doctor will only prescribe medication if a dangerous blood clot has formed or they suspect that the person has an increased risk of clotting.

Doctors may also recommend:

  • quitting smoking and avoiding secondhand smoke
  • losing weight, if necessary
  • doing regular physical activity
  • avoiding sitting for long periods
  • moving the legs and feet frequently to support blood flow

Learn more about treatments for blood clots.

Thick blood disorders can lead to a range of problems, including DVT and PE.


When a blood clot affects a deep vein, a doctor may diagnose DVT. These clots tend to affect the large veins of the lower legs, but they can form in the thighs, arms, abdomen, or pelvis.

In the area of a clot, a person tends to have pain and circulation problems.

Specific symptoms of a DVT include:

  • swelling, redness, and warmth in the area
  • a sudden or gradual onset of pain
  • leg pain when bending or stretching
  • leg cramps in the calf
  • bluish or pale skin discoloration elsewhere in the body

Learn more about DVT.


A PE can occur when part of a clot detaches and travels to the heart or lungs. If this happens, the fragment can cause a new clot, block blood flow, and interfere with gas exchange in the lungs. A PE is a life threatening emergency.

Symptoms of a PE include:

  • shortness of breath and rapid breathing
  • a rapid heartbeat
  • pain with deep breaths
  • blood when coughing
  • chest pain

Find out more about PEs.

Other complications

Other possible complications of blood clots include:

Here are some questions people often ask about thick blood.

Is having thick blood serious?

Thick blood can be serious as it increases a person’s risk of a developing a blood clot.

How do you treat thick blood?

Treatment will depend on the cause and severity of thick blood. Medications can address an underlying problem and balance out substances needed for blood to flow or coagulate as it should. Avoiding smoking and exercising regularly may also help thick blood in some cases.

Can polycythemia be cured?

There is no cure for primary polycythemia, but there are ways to treat and manage it. Secondary polycythemia stems from an underlying condition. Treating the condition can help manage polycythemia.

How rare is polycythemia vera?

Polycythemia vera, or primary polycythemia, is a rare disease. It affects fewer than 200,000 people in the United States.

Certain medical conditions can cause the blood to become thick, increasing the risk of clots and other complications.

Anyone with a family history of thick blood or blood clotting disorders should speak with a doctor about the risks and possible complications. Also, seek medical advice about any unusual symptoms.

Medications and lifestyle changes can help manage thick blood and reduce the risk of blood clots and other potential problems.

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