EXCLUSIVE

Zac Efron did NOT go to rehab for alcohol abuse -- it was far more serious, because multiple sources tell TMZ he had a serious cocaine addiction.

Zac completed his stint in rehab earlier this year. 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.

We're told ... early this year, Zac and some friends went on a coke rager in a room at the SkyLofts at the MGM Grand in Vegas ... and caused around $50k in damage. No word on who took care of the bill.