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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.
This makes the blood vessels narrower and makes it more difficult for blood to get through. Your blood pressure could go up. The buildup can also cause a blood clot to form. If a blood clot breaks off and travels to your heart, it causes a heart attack.