I don't know if anyone is sure at this point based on the way it was worded in
the tweet. He certainly could have failed due to other substances but if that's the case why even mention the Adderall? The tweet below also implies it was for Adderall. Guess we will find out.
USC LB Porter Gustin was issued a temporary therapeutic use exemption for Adderall after the NFL Combine, per league source. While the test came back positive, the exemption is retroactive, per source.