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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.
Excess Blood Cells. Certain diseases, namely blood cancers, thicken the blood because they lead to abnormally high levels of blood cells. While rare, one of the more common reasons for thick blood is called polycythemia vera (PV), in which the body produces excessive blood cells -- mostly too many red blood cells.