Justin Bieber -- Drug Use Out of Control in Atlanta

Justin Bieber is going harder than ever on sizzurp and weed since living in Atlanta ... multiple friends involved in his life tell TMZ.

TMZ broke the story ... Justin has had an alarming problem with sizzurp for well over a year, and he's been a big fan of weed for a long time. But since moving to Atlanta, we're told he's "constantly high" ... rarely leaving the house. As one source said, "Drinking more lean, smoking more weed than ever."

When Justin lived in L.A. he had a schedule set up by people connected to his business. Since moving to Atlanta, he has no schedule and no rules.

Some of the people in Bieber's life that have confronted him about his drug use have been pushed away since Bieber made his move. And we're told -- one by one -- Bieber is removing people he says have "ratted" him out over his drug use.

Justin does spend a fair amount of time in the studio in Atlanta ... working on hip hop ... but we're told rarely is he sober during those sessions. And the thing is ... that's part of the rap game.