Actors are increasingly using performance-enhancing drugs like human growth hormone due to pressure to have ripped muscles for on-screen roles.

HGH has become the PED of choice in Hollywood.

Trainer Happy Hill, who has worked with Ryan Phillippe and Jake Gyllenhaal, estimated that around 20% of actors use PEDs. She said six-pack abs are difficult to maintain beyond age 40, "unless you are extremely genetically gifted."

Hill claimed that the body of an unnamed 40-something actor in a recent studio film was not possible without a cycle or two of steroids.

Charlie Sheen told Sports Illustrated that he took steroids while preparing for his role in 1989's "Major League," and both Mickey Rourke and Arnold Schwarzenegger have admitted to using PEDs.

Henry Cavill, who starred as Superman in the recent "Man of Steel," said his build in the movie was 100% natural.

Read more: Hollywood Reporter