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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 caused by heavy proteins, or by too much blood in the circulation. Too many red cells, white cells, and platelets will result in blood thickening. Another cause is an imbalance in the blood clotting system.