Sources familiar with the situation tell TMZ ... Zac has been doing cocaine for more than 2 years -- he was also into Molly. He tried to clean himself up in March by getting professional help. We're told he left California and stayed in a private residence where he received intensive outpatient care for several weeks.
In April he returned to L.A. and began shooting Seth Rogen's film "Neighbors." During the shoot Zac relapsed in a big way. TMZ broke the story ... Zac was not showing up on the set and it was common knowledge he was using again.
Our sources say after the film wrapped up he got a second dose of outpatient care.
As for how Zac plunged into the depths of coke, we're told there are 4 reasons:
-- Zac has significant issues with his parents, who are still trying to control his life and decision-making.
-- Zac is upset about his movie career, which was once white hot but after "High School Musical" he hasn't had real success.
-- Zac has "girl issues" ... although we're told it has nothing to do with his break-up with Vanessa Hudgens. That, our sources say, is ancient history for both.
-- Zac had been hanging around with "a bad group of friends" who introduced him to the world of cocaine. We're told Zac has 86'd most of them from his life in the last few months.