The simple act of increasing the number of red blood cells in blood may be associated with hyperviscosity syndrome which is characterized by increased blood viscosity and decreased cardiac output and blood flow velocity which results in the reduction of peripheral oxygen delivery. For instance, an overdose of EPO can thicken blood into a highly viscous and artery-clogging sludge. This increases the chances of heart attack, stroke, phlebitis, and pulmonary embolism, which has been seen in cases where there is too much blood reintroduced into the blood stream. Because blood doping increases the volume of red blood cells, it effectively introduces a condition called polycythemia, a blood disorder that has known adverse outcomes such as heart attacks or strokes.