Some may undergo cosmetic surgery - for example, to speed recovery from an injury. Or in the case where Castillo shaved his flesh on his eyebrow straight down to the bone as he has suffering cuts mostly from head butts during his matches. They may even opt for performance- enhancing surgery: laser eye surgery for a golfer or archer is one possibility. Athletes from countries near sea level use 'altitude tent' treatments to acclimatize themselves to compete at higher altitudes. And some use drugs to increase their muscle mass, allow faster oxygen delivery to the muscles or allow them to train longer.
Because there is such a grey area it is hard for certain tampering of the body to construed as illegal as they vary from little advantage to a possible high advantage. I don't understand how laser surgery in archery is acceptable (high level of possible advantage) but fail to understand to see how marijuana (lower possibility of an advantage) is considered a PED in MMA. When it truth marijuana would probably effect the athletes performance and hinder their health simultaneously. Due to fine lines, regulations and rules based on PED's being created a long time ago is why they seem odd. The problem is sometimes phrased as the distinction between therapy (treating an illness) and enhancement. The examples above seem straightforward at first, but when we try to define a single point at which an acceptable 'therapy' becomes an unacceptable 'enhancement', problems arise.
Because of the natural competitiveness of leagues these days there is need to stay on top and thus some resort to enhancements such as PED's. With a quick look at the prisoners dilemma we can see how taking PED's just ends up harming your body and you're better off not taking it at all. If both players don't take the PED in the dilemma, then both are unharmed from the use of steroids which are proven and there is no unfair advantage to a single player or team.