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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.
Cells & protein: Blood may become too thick if there too many cells or too much protein. Increased red cell count, polycythemia, can make the blood viscous. High levels of immunogolobulins, usually due to cancer, also increse viscosity. Rarely leukemia with a very high white cell count may make the blood thick.