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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.
Sudden unexplained bruising or blood spots under the skin or a sudden increase in the frequency of bruising may be caused by: 1  A medicine, such as aspirin or blood thinners (anticoagulants).2  Infection that causes the buildup of toxin in the blood or tissues (sepsis).udden unexplained bruising or blood spots under the skin or a sudden increase in the frequency of bruising may be caused by: 1  A medicine, such as aspirin or blood thinners (anticoagulants).