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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.
Typically, your body breaks down the clot. However, sometimes blood clots form too easily or do not dissolve properly. This excessive clotting -- hypercoagulation -- also causes thick blood. This can be dangerous since clots can form inside your blood vessels and block blood flow to tissues or organs.