Because so often these athletes use PED's in very, very high doses, many times what a Doc would prescribe in order to increase the effect. So he may have been taking it in higher than "normal" doses and may be addicted to it because of that.
I know with HGH a Doc would tell you or I to take 1 or 2 IU's per day, bodybuilders and athletes take 10-20. If he was using Adderall he may have been using mega-doses and could very well be addicted. Although usually drugs that effect the central nervous system cause major health problems if taken in too large of a quantity. I don't if Adderall is in that group.
Either way, he cheated and IMO the NFL and the players union need to change the drug rules. They aren't stiff enough.
