Polycythemia is an abnormal increase in the number of red blood cells produced by the bone marrow. The extra numbers of red blood cells cause blood to thicken and may increase the risk for blood clots that may cause heart attacks or strokes.

Treatment for polycythemia (also called erythrocythemia) sometimes involves removing red blood cells (much like donating blood) from the blood. However, this procedure may decrease iron levels in the body.