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The extra red blood cells make your blood thicker than normal. As a result, blood clots can form more easily. These clots can block blood flow through your arteries and veins, which can cause a heart attack or stroke. Thicker blood also doesn't flow as quickly to your body as normal blood.
Thick blood is problematic if there are symptoms that are not being treated. Some people with hypercoagulability do not show signs or symptoms. When blood is thickening within the body, however, it has a difficult time circulating and carrying necessities, such as oxygen, nutrients and hormones throughout the body.