He was never on roids. Mirko is too much of an egomaniac to take them, because even though he can credit all of his success to himself and downplay the role of coaches and people around him, he couldn't lie to himself about this shit. He also seems afraid of taking steroids. Guy is just a fanatic for training with good genetics, lives an insanely boring life and training is all he knows. He never even tested positive for HGH when he took a therapeutic dose to try to heal his shoulder because he didn't want to back out of the fight, but in the end they suspended him anyway. He also fought injured a bunch of times because of his ego or perhaps he didn't want to miss out on cash.