After he shot to fame at age 17 after starring as squeaky-clean basketball star Troy Bolton on Disney Channel's High School Musical franchise, it appeared the heartthrob had avoided the pitfalls of youthful stardom.
But after it was revealed Monday that Efron, now 25, had checked into rehab earlier this year, sources tell PEOPLE the seemingly straight-arrow actor – who has never run into trouble with the law and has a sterling reputation as a professional on set – partied much harder than his public image let on.
"He surrounds himself with a group that enjoys partying," an Efron insider tells PEOPLE. "He got caught up in the partying lifestyle the past few years."
Says another source who has socialized with Efron: "Like most people at clubs, he liked to drink and have a good time. He definitely liked to party."
Trying to Avoid the Wrong PathJust a year ago, the California native – who grew up in a non-Hollywood family (his father is an electrical engineer and his mother a secretary) – told PEOPLE he avoided going down the wrong path by "appreciat[ing] the work I've done."
"The stakes are very high," he said at the time. "I care too much about those around me and everything I've worked for to just go out and blow it. It's just a personal integrity."
While acknowledging he does "like to have fun," the actor pointed out that "some people can handle it and some people can't."
Though he was struggling in private, several nightlife sources say he didn't appear out of control while clubbing and in fact tried to keep a low profile.
"He would make sure that his friends surrounded him and was clearly concerned with his privacy … almost to a paranoid level," says one insider who has seen him frequently at L.A. hotspot Bootsy Bellows drinking vodka soda.
After-Hours PartiesEfron also often turned up at after-hours parties at friends' homes, sources say.
Enablers are often crippling for those with addiction issues, says William Oswald, director of Summit Malibu Treatment Center, who has not treated Efron.
"Codependents who are living off of [a star] and keeping the party going can really hinder things," says Oswald, whose treatment center provides "hand-carved treatment in a non-institutionalized" environment. "There are lots of predators who would feed off of someone like him because they're getting the party with it."
What a recovering addict needs is people who will "stand up and say, 'This is not a good idea. You're going down the wrong path,'" he adds. "[Someone like Zac] is in a pretty vicious business. … Friends make you feel better and drugs and alcohol make you feel better."
Additional reporting by JENNIFER GARCIA, RAHA LEWIS and PATRICK GOMEZ