He didn't intentionally take a known banned substance. According to four separate Saints players (including Zach Strief) a trace amount of a substance picked up from a tainted supplement that is approved by the league was found in his sample. And according to his agent the banned substance wasn't even one recognized as "performance enhancing" and could be taken with a Therapeutic Use Exemption (TUE) ... (ie. fertility meds, hair growth meds, etc.).