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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 (also known as hypercoagulability, Antiphospholipids, and Hughes Syndrome) is a condition where the blood is more viscous (thicker and stickier) than normal. The unusual increase in the thickness of blood is due to an abnormality in the clotting process.