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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.
Too many red blood cells is referred to by doctors as polycythaemia and erythrocytosis.The main problem caused by polycythaemia is that the high number of red cells increases the blood's 'thickness' (viscosity).