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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.
However, if the hematocrit gets too high (above 60 percent), it thickens the blood, causing circulatory difficulties that may result in strokes and heart problems. To avoid these problems, the patient can have phlebotomies (blood-letting) to reduce the hematocrit.