EPO

Erythropoietin (EPO) is a natural hormone produced mainly by your kidneys that stimulates the production of red blood cells in the bone marrow. Red blood cells carry oxygen from your lungs to various parts of your body. When oxygen levels are low, such as at high altitudes or due to certain medical conditions, EPO production increases to boost red blood cell count and improve oxygen delivery. Synthetic versions of EPO are used medically to treat anemia, especially in kidney disease or cancer patients, but misuse in sports to enhance endurance is considered doping and is banned.