The answer is:
stunted growth
increased muscle mass
paranoia
irritability
kidney damage
Using performance enhancing drugs will definitely increase users' muscle mass and improve their athletic performance. But if it is used during puberty, their body's organ performance and the bone structure may not still developed enough to fully accommodate the weight of the muscle and increased function . This will lead to stunted growth and kidney damage.
Beside the physical negative effects, PED could also create a psychological defect. Such as getting angry over small things (irritability) , or paranoia (like believing that their body is 'not big enough'.