The movie-star father and his firstborn often clashed, say friends of Sage's
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In 1976, Sylvester Stallone proudly brought the world both his biggest movie character and his first-born son.

Problem was, Sage Stallone found it hard to reconcile himself with his father’s most enduring creation, Rocky Balboa.

Since Sage’s untimely July 13 death of unknown causes his friends have come forward to claim that the 36-year-old had a prescription drug problem – and a complicated relationship with his famous father.

The friends say Sage appreciated that his father was a hugely successful moviemaker who gave him a house in the Hollywood Hills and enough money that he didn’t have to work. But Sage – who chain-smoked and seemed to subsist on little more than Doritos and Mountain Dew – didn’t like the ageless, muscle-bound action heroes embodied by Sylvester Stallone.

These same friends say that Sylvester helped fuel these feelings by openly disapproving of his son’s physical appearance – skinny as a kid and overweight and out of shape in recent years.

“The love-hate would go back and forth all the time,” recalls longtime friend and fellow actor Stephen Wozniak. “[Sage] would say, ‘I loved that rare ending in Nighthawks that my dad was really sticking up for.’ And then a few minutes later he would look in a magazine and say, ‘Look at these tribal tattoos that my dad is getting at age 66. What is he doing?’ ”

Sylvester may have further exacerbated the situation a few years ago by casting the much trimmer Milo Ventimiglia in Rocky Balboa to revive the Rocky Jr. role that Sage had played as a teenager in Rocky V.

“Sage wanted to do it,” says another longtime friend, director David DeFalco, who cast Sage in his horror movie Chaos.

Sylvester and Sage Stallone publicly aired some of their differences back in 1996, when Sylvester told PEOPLE: “He avoids the gym like the plague, and is interested in abstract, art-house films. He has put himself in a position in which he’ll never have to be compared to me.”

At the same time, Sage told PEOPLE that, when his character yelled at his dad in Rocky V for not being there for him, it wasn’t all acting.

“When I was screaming: ‘You never spent time with me! You never spent time with my mother!’ that was true. I was looking into my father’s face and really saying that,” Sage, whose parents split in 1985 amid rumors of Sylvester’s affairs, said at the time. “I got a lot of things out. We broke into tears a few times. After the film, everything changed. We’d do anything for each other.”

Prescription Drug Use

DeFalco and Wozniak say Stallone routinely took prescription medications like Vicodin and Xanax for anxiety and other issues, and they warned him to be careful. “He’d done this on and off for as long as I knew him,” Wozniak says. “I’m certain he was addicted to those medications.”

Investigators confirm that there were large, empty prescription drug bottles when they found Sage’s body, but they are awaiting results of toxicology exams before saying how he died.

Stallone family friends say the grief-stricken screen star saw no indications that his son had a prescription drug problem.

“If there was any rumor at all that Sly had heard, he would’ve been right over there,” says one family source. “Sly isn’t the kind of person who would let somebody self destruct, especially his loved one. He’s very caring.”