I can think of two scenarios as to why he took Aderall or Ritalin. Neither of them are good, at all. (And a third, very unlikely scenario that would be the positive scenario).
One (most likely) - People use Aderall and Ritalin recreationally. It's known as a "poor man's cocaine." He was partying, probably with a girl(s) and they decided to sniff some crushed up pills. This, coupled with his chatroulette adventures, calls his maturity into question and makes me wonder if he's really ready to be a professional athlete. He's also 22 so let's hope he learns from this.
Two - There is a reason that amphetamines are banned in the NFL. If he was taking these drugs before games to give himself a little bit of extra zip, it's cheating. It's also very dangerous. I hope that's not what the case is here, but the fact is that Aderall helps people stay focused so maybe he thinks it helps him stay sharp during games.
Three (homer glasses) - He takes it to keep himself focused when learning the playbook. The reason why the drug exists is to help people study. Maybe he takes it to help learn a complicated defense. Anyone who believes this, I got some beans to sell you.
The bright side is that it's not straight up steroids or HGH (assuming we believe him) so I doubt we'll see a production decline unless he was using it before games. This is more serious than marijuana, though, and calls his judgment into question big time. A guy has a smoke with a friend, I give him a pass and hope he waits until the offseason next time. This circumstance isn't good for Brandon's reputation and I hope it's a real wake up call for him. This is not the University of Florida, it's time to get with the program.
