Justin Bieber -- Drug Use on Trial in Paparazzi Assault Case

Justin Bieber acts like an animal because of massive drug abuse ... that's what the lawyer in Bieber's paparazzi assault case believes and he's dragging in Biebs' closest friends to prove it.

We've learned the lawyer repping the paparazzo who claims Bieber ordered his goons to beat him up will subpoena Lil Za, Lil Twist, Khalil, Scooter Braun and others ... and the focus will be drugs and booze.

The lawyer believes if Bieber's buds are honest ... they'll fess up about the connection between the singer's bizarre, aggressive behavior and his consumption of sizzurp, weed and alcohol.

Something interesting ... Bieber's trainer, Patrick Nilsson, made the list ... and that could backfire. Sizzurp and muscles generally don't go hand-in-hand.