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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.
Blood that is too thick can lead to blood clots, and blood that is too thin can lead to easy bruising or bleeding. Problems with blood thickness can occur from birth, or develop later in life. Blood thickness may be affected by foods, drugs, and various medical conditions.