The cause of the younger Stallone's July death was disclosed Thursday after the results of an autopsy and toxicology tests were evaluated, People.com reported.
The coroner cited coronary artery disease as the cause of death and said a drug test was negative except for a small, "sub-therapeutic" level of hydrocodone, the magazine said.
Sage Stallone was a horror-movie distributor and occasional actor. Friends told People.com he was a smoker who loved junk food and avoided exercise.
Sylvester Stallone, whose latest movie, "Expendables 2," has been No. 1 at the U.S. box office for two weeks, has endured a summer of personal tragedy. Just weeks after his son died, his younger half-sister lost her battle with cancer.