Zac Efron has shut down rumours that he’s had plastic surgery, saying his jaw muscles “grew” because of an injury.

The actor was subject to social media speculation following an appearance on last year’s Earth Day, where he collaborated with Bill Nye The Science Guy.

Speaking to Men’s Health, however, Efron explained that an accident and a lapse in physical therapy caused his masseter muscles, used for chewing, to grow.

The actor said he was running through his house in his socks when he slipped and smacked his chin against the granite corner of a fountain. After he lost consciousness, Efron woke up to find his chin bone “hanging off his face”.

Following the injury, Efron said he began physical therapy with a specialist to stop his muscles on the inside of his face and jaw from overcompensating. When he took a break from his therapy, however, “the masseters just grew,” Efron said. “They just got really, really big.”

As noted in the interview, the actor was unaware of the online speculation until his mother called to ask if he had actually had some surgery.

Speaking about keeping his distance from social media, Efron added: “If I valued what other people thought of me to the extent that they may think I do, I definitely wouldn’t be able to do this work.”

Efron’s next film is The Greatest Beer Run Ever, which follows the true story of John ‘Chickie’ Donohue who ventures from New York in 1967 to deliver beer to his army friends in Vietnam. The film also stars Russell Crowe, Bill Murray, Jake Picking and Will Ropp.

The Greatest Beer Run Ever will be released in select cinemas and on Apple TV+ from September 30.