Zac Efron: 'Partying made it hard to work'

Actor Zac Efron knew he had substance abuse issues when his hard partying left him struggling on film shoots.

The High School Musical star entered a rehab facility for treatment last year (13) to battle his drug demons, and he now realises his problems had nothing to do with his heavy workload but were caused by his social life.

Speaking to adventurer Bear Grylls on his new TV show Running Wild, Efron says, "It was getting to the point where I was caring less for the work and waiting more for the weekend, where I couldn't wait to go out and sort of let loose and have fun. But when Monday and Tuesday were difficult to get through, then I was like, 'Ah, this is bad.'"

He further explains, "The challenging part was never the work. That was never it. It was the sort of in-between work."

Running Wild With Bear Grylls features the adventurer taking an A-list celebrity into the wild each episode to survive for a weekend.