In the drama, the actor plays a climber who is forced to amputate his own arm with a penknife after becoming trapped under a boulder during a hike in Utah.
Franco suffered a number of injuries throughout the challenging shoot in a canyon fissure, leaving him with marks resembling drug injection scars.
He tells Britain's Total Film, "I had a lot of scars. I guess they weren't as bad as I thought... I guess it just proves that Danny (Boyle, director) knows what he's doing because it didn't scar me for life.
"I went to a doctor for insurance purposes for another movie and he saw these scars and he was like, 'Errr...?' He soon assessed that they were not track marks."