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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.
Such a hemorrhage may be caused by a sudden or severe sneeze or cough, or due to hypertension or as a side effect of blood thinners. It may also be caused by heavy lifting, vomiting, or even rubbing one's eyes too roughly.