Sage was found dead in his Hollywood apartment last weekend, drawing tributes from father Sylvester and Terminator star Arnold Schwarzenegger.
The 36-year-old's autopsy was completed earlier this week, but the results are being deferred for extensive toxicology testing.
A cause of death for Sage will not be officially announced until toxicology tests are completed in the coming weeks.
Family and friends of Sage will gather this Saturday for his funeral at the St Martin of Tours Catholic Church in Los Angeles, reports ET Online. Sage's mother Sasha Czack has asked for donations to Autism Speaks in lieu of flowers.
Sage notably starred in his father Sylvester's films Rocky V and Daylight in the 1990s.
> Sage Stallone 'on something' months before death?