The "morphine effect" reportedly caused respiratory failure, leading to her death, reported TMZ.
The retired actress' body was discovered in her Colorado apartment on July 5, two days after her death. She was 43.
According to the Weld County Coroner in Colorado, Peterson had a variety of prescription drugs in her system, including anti-anxiety medication, anti-psychotic medication, opiates and marijuana.
The '80s star had a long history of substance abuse. She was allegedly arrested four times over the last 20 years in her hometown of Greeley, Colo. Most recently, she was booked in May 2012 on a DUI and drug paraphernalia charge.