Zac Efron dyed his hair after losing a bet

Zac Efron was forced to dye his hair bright blond after he lost a bet with a friend.

The Hollywood heartthrob is currently promoting the Netflix movie Extremely Wicked, Shockingly Evil and Vile, in which he plays American serial killer Ted Bundy.

While Zac naturally has brown hair, he shocked fans when he unveiled his new platinum 'do earlier this year (19), and he has now revealed that the dye job isn't for an upcoming role.

"I lost a bet with a friend. It's kind of a double dare sort of thing," he admitted to Entertainment Tonight.

When quizzed if the blond hair is here to stay, he quickly replied, "No, I mean, that would be so much work."

However, Zac confessed that he also had the same colour hair when he was younger, after being inspired by Eminem's signature look.

"Eminem was cool when I was growing up. This is my Stan coming out," the 31-year-old explained, referencing the rapper's 2000 hit that was influenced by the multiple letters he had received from obsessed fans.

Despite his new blonde makeover, Zac flexes his acting muscles as the infamous serial killer in Extremely Wicked, Shockingly Evil and Vile, and credits his co-star Lily Collins, who portrays his girlfriend, Elizabeth Kloepfer, for helping him "raise the bar".

"I'm ready for the day and it was the first warm-up scene, and Lily was ready to go right then at rehearsal, perfect," he recalled. "Gave a perfect performance on the rehearsal, and I literally walked outside and I almost had a mental breakdown.

"I was like, 'We didn't even get to rehearse and Lily is going for awards already?' (She's) so good! And I just sat at home and said, 'Zac, you gotta be better, you gotta be better, how can you do it?' And I figured out how."