Erythropoietin (EPO) is a hormone produced in the kidneys. The existence of this hormone has been known since 1906, when scientists first started isolating it, and since the 1980s, a recombinant version of the hormone known as Epogen or Betapoietin has been available for use in medical treatment. People can use erythropoietin in treatments for anemia, typically in the case of anemia caused by chemotherapy, chronic kidney disease, or heart failure. A doctor usually supervises use of the hormone, with periodic blood tests to monitor patient progress.

This hormone stimulates the production of red blood cells in the bone marrow. It is produced by the kidneys when specialized cells in the kidneys sense that the oxygen level in the body's tissues is low. When introduced synthetically by injection, erythropoietin can help treat anemia by boosting the number of red blood cells in the body. Injections may be required because the kidneys are not producing enough erythropoietin on their own or when a patient is experiencing severe anemia, and on occasion they must be supplemented with iron tablets.

The gene for erythropoietin production is located on the seventh chromosome. In a normal, healthy person, levels of the hormone can vary, with levels tending to be higher in athletes and people living at high altitude because these individuals experience more oxygen deprivation in their muscle tissue. If someone has anemia, a erythropoietin test may be requested to explore the cause of the anemia, with the level of the hormone in the blood being used as an indicator.

In addition to being used in medical treatment under the supervision of a doctor, this hormone is sometimes utilized by athletes as a performance enhancing drug. Taking erythropoietin is supposed to increase endurance for athletes, and the drug is difficult to detect in blood doping tests because the substance is naturally present in the body. High levels of erythropoietin can, however, increase blood pressure, and some doctors have suggested that overuse of this hormone may explain an increase in exercise-related deaths among some running and cycling communities, as the high blood pressure strains the heart and exercise pushes the body over the limit.

For people interested in the chemical structure of this hormone, erythropoietin is what is known as glycoprotein. Glycoproteins are proteins with a sugar attached. Numerous reproductive hormones are glycoproteins, as are antibodies, and some parts of connective tissue like muscles. These proteins often increase viscosity, which is why they appear in secretions such as lubricating fluids along with structures like egg whites.