Zac Efron -- who was partying in a bar in Ibiza with Michelle Rodriguez -- is not only back on the bottle ... we're told he was never off it.
Zac has had big issues with substance abuse ... going to rehab twice last year for cocaine addiction. We're told he's also a big drinker.
People involved in Zac's life tell us ... his biggest problem is that he always believes he has the ability to stop. As one source says, "He'll stop cold for 3 or 4 days when he has to work and then he's back."
It's no secret Zac loves to party. We're told he's "climbing the walls" just before his various projects end ... waiting for the moment he doesn't have to show up for work in the morning so he can party hard.
Here's the good and the bad.
The good -- Zac's younger brother Dylan is a big influence on him. We're told Dylan has counseled him about the importance of sobriety and Zac doesn't blow him off.
The bad -- Zac is 26 but he's acting like an 18-year-old. The way it was put to us ... "When Zac was 18 and 19, when most kids go crazy, he was kept on a short leash [also known as Vanessa Hudgens]. When they broke up he went off the rails."