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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.
High levels of abnormal white blood cells can cause anemia, tiredness, weight loss, bone pain and frequent infections. Most seriously, people with thick blood -- whether from excessive blood cells or hypercoagulation -- are at high risk for clots that block or limit blood flow to vital organs.