MM says EPO is a vital drug when used properly: it is a hormone we all have in our bodies and helps us make red blood cells. EPO saves tons of lives every year, but its properties for making red blood cells was also recognized by athletes who saw it as an ability to boost their red blood cells and able to carry more oxygen in their bloodstream so they can go farther and for longer and faster. Its use became endemic during the 1990s, so it's been a big problem in sports for many years. (Are there particular games where more athletes take it?) In swimming, cycling, cross country skiing. For the 2000 Olympics they rushed a test out but that test is complicated and expensive and requires a great deal of skill in terms of interpreting the results. A number of athletes challenged their tests in courts which made the process more complicated. But now athletes have become better at using the drug and avoiding positive tests, so it's driven the drug and its use under ground. There are companies in India and China who are manufacturing new versions of EPO which give a slightly different test result so people using it can get away with it. in some tests it's been found that athletes increased their performance by up to 50%.