There is a bit of a grey area when it comes to performance enhancing drugs. There are some obviously wrong drugs, like some steroids, but then there are other drugs like nutrients that are part of a healthy diet, just in pill form. One of the most difficult drugs to detect is something called human erythropoietin (EPO). This is a hormone naturally created by the human body to increase red blood cell levels. Some athletes will inject themselves with EPO to increase their stamina, and it is impossible to determine if someone has used the drug, or just has naturally high EPO levels.