Blood Clots & Blood Thinners

Excessive blood clotting is a condition in which blood clots form too easily or don't dissolve properly. Normally, blood clots form to seal small cuts or breaks on blood vessel walls and stop bleeding.

Slow blood flow in the blood vessels also can cause blood clots to form. For example, if a blood vessel narrows, blood may slow down as it moves through the vessel.

Excessive blood clotting has many causes. Problems with the blood, blood vessel defects, or other factors can cause the condition. Regardless of the cause, blood clots can limit or block blood flow. This can damage the body's organs and may even cause death.


Excessive blood clotting can be acquired or genetic. Acquired causes of excessive blood clotting are more common than genetic causes.

"Acquired" means that another disease, condition, or factor triggers the condition. For example, atherosclerosis (ath-er-o-skler-O-sis) can damage the blood vessels, which can cause blood clots to form. Atherosclerosis is a disease in which a fatty substance called plaque (plak) builds up inside the arteries.

Other acquired causes of excessive blood clotting include smoking, overweight and obesity, and being unable to move around much (for example, if you're in the hospital). 

If excessive blood clotting is genetic, it’s caused by a faulty gene. Most genetic defects that cause excessive blood clotting occur in the proteins needed for blood clotting. Defects also can occur with the substances that delay or dissolve blood clots.

Although the acquired and genetic causes of the condition aren't related, a person can have both. People at highest risk for excessive blood clotting have both causes.


The outlook and treatment for excessive blood clotting depend on the cause of the blood clots, how severe they are, and how well they can be controlled.

Life-threatening blood clots are treated as emergencies. Medicines that thin the blood are used as routine treatment for blood clotting problems. Some people must take these medicines for the rest of their lives.

With medicines and ongoing care, many people who have excessive blood clotting can successfully manage it.

To read more about this topic, from this and related documents, please see the National Heart Lung and Blood Institute.


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