"Simply put, PEDs have the ability or potential to drastically alter the human body and biological functions, including the ability to considerably improve athletic performance in certain instances. "
http://www.usada.org/substances/effects-of-performance-enhancing-drugs/
http://www.mayoclinic.org/healthy-lifestyle/fitness/in-depth/performance-enhancing-drugs/art-20046134
first PEd mentioned are Anabolic steroids, including testosterone.