Polycythemia (say “paw-lee-sy-THEE-mee-uh) is an abnormal increase in red blood cells. It happens when the tissue inside your bones (bone marrow) makes too much blood. It also can occur if your blood does not have enough liquid, or plasma. This can make the number of red blood cells seem higher than normal. The extra red blood cells make your blood thicker than normal. This may raise your risk for blood clots that can cause heart attacks or strokes. Clots can form in the deep veins of the body, a condition called deep vein thrombosis. Or, a clot can travel through the blood to a lung (a pulmonary embolism).

Your doctor may treat you by taking out some of your blood (phlebotomy). The process is like donating blood. Your doctor may even recommend that you donate blood. You may take pills to stop your body from making red blood cells. You also will get treatment for any other conditions that may cause your body to make too many red blood cells.