We're told the problem spiraled out of control during the filming of Seth Rogen's movie "Neighbors," which was shot during a narrow time frame beginning in April.

Sources tell us Zac -- who also starred in the movie -- was a no-show on a number of days.  As one source connected with the film tells us, "It was common knowledge he was struggling with cocaine."

Three other sources confirm Zac's problem was rooted in cocaine, but say he also dabbled in Molly -- a pure form of Ecstasy, and a popular drug in clubs and raves.