The Rocky star was determined to have a "very limber and athletic" look but became so focused on his appearance that he stopped "keeping down" food in order to stay as slender as possible.
He told The New York Times newspaper: "I started to think: Design your body like a jaguar, a cat. Not big like Arnold (Schwarzenegger), but more feral and aggressive and very limber and athletic.
"It became an obsession. Really bad. It got to the point where I became bulimic.
"I got down to 2.6 percent body fat when I was doing Rocky III (in 1982).
"I couldn't hold anything down. Or I didn't want to keep it down because I became so obsessed with staying at that point of fitness, literally an obsessive body-worship kind of thing."
The 80-year-old actor also admitted he used steroids in a "sensitive" way and argued "nobody" can achieve their fitness goals "without additives and drugs, period."
He said: "I don't care what they say.
"It isn't as though you're a junkie, but if you're going to work out and try to stay in shape for six months straight, and you're eating yoghurt and pudding, it's not happening."
Asked if he had used steroids, he replied: "Oh, my god, you have to. But you do it in a sensitive way - six weeks on, six weeks off. People go, 'Oh, you don't get that way naturally.' No, you don't."
Stallone admitted using the synthetic hormones "took a terrible toll" on his body but he spent decades worrying about the pressure to look a certain way.
He said: "It wasn't until I did (1997's) Cop Land, when I had to play the weakest guy in the room - partially deaf, overweight - that I felt somewhat liberated.
"Then things went sideways and I just fell out of grace with Hollywood."
The Expendables star admitted he has "abused" his body over the years, resulting in "nine back operations, two shoulder operations, and three neck fusions" to repair the damage.
He added: "Everything is held together with baling wire, but I still want to stay in good shape...
"It's come at a terrible price. If I could do it over again, I would say, 'Let someone else carry the banner and don't do your own stunts.' I don't know what it was about hurting myself all the time."
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