Apparently MLB gave him an excemption to use Adderall, in the past (last year), for mental health issues. This year they took the excemption away and it looks like he started using it again.
http://www.cbssports.com/mlb/eye-on-baseball/24706357/chris-davis-suspended-25-games-for-positive-amphetamine-test
Davis has released the following statement:
"I apologize to my teammates, coaches, the Orioles organization and especially the fans. I made a mistake by taking Adderall. I had permission to use it in the past, but do not have a therapeutic use exemption (TUE) this year. I accept my punishment and will begin serving my suspension immediately."