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Sylvester Stallone's son, Sage Stallone, died of natural causes, ET has learned.

RELATED: Sage Stallone's Death Certificate

The Los Angeles County Coroner ruled Atherosclerotic Coronary Artery Disease (heart disease) to be at the root of Sage's untimely death in early July.

Although drugs were not attributed to the 36-year-old's passing, "sub-therapeutic" levels of the painkiller Hydrocodone were found in his system.

Earlier this month, Sylvester opened up about his son's death, telling Good Morning America that coping with the loss has been "very tough."

VIDEO: Sage Stallone on His Rocky V Film Debut

"If you have good friends and family support [you can get through it]," he said. "It's a horrible situation, but time hopefully will heal. It's a reality of life."

Sage was among Sylvester's five children, and one of two sons from his first marriage to Sasha Czack.