Lt. Cheryl MacWillie with the coroner's office said the results may not be known for at least 10 weeks.
ABC News has learned the 36-year-old was planning to get married this weekend in Las Vegas. Friday, police and a housekeeper found Sage Stallone's body in his Studio City home.
There were no signs of trauma or foul play, but investigators did find prescription drug bottles.
Family attorney George Braunstein said the 36-year-old appeared to be doing well.
"He never had any serious health problems that I knew of,"said Braunstein. "He didn't have a history of drug or alcohol abuse. We all think it was some sort of tragic accident."
Braunstein called Sage Stallone a creative genius who often secluded himself to write screenplays or watch movies.
Sage Stallone made his big screen debut with his father in "Rocky V" in 1990. In 1996, he was back on screen in "Daylight," again paired up with his high-profile father.
City News Service contributed to this report.