The High School Musical actor is said to have quietly completed a voluntary stint at a treatment centre around five months ago.
Reps for Zac have yet to comment on the speculation but sources told People magazine that the 25-year-old heartthrob is doing well.
Another source told Us Weekly that Zac is "cutting those toxic people out of his life."
Zac looked in good spirits at the Toronto International Film Festival earlier this month, where he attended the premiere of his new movie Parkland.
Parkland is a historical drama focusing on the aftermath of John F. Kennedy's assassination. Other stars include Tom Welling, Billy Bob Thornton and Marcia Gay Harden.
It's released in the UK on 8 November.
Watch a trailer for Parkland below: