The 63-year-old actor died on August 11 at his home in Tiburon, California in Marin County.
Toxicology reports reveal antidepressants, caffeine and levodopa, a drug used to treat Parkinson's disease, in his system.
Alcohol and illegal drugs were not involved, the statement from the coroner's office said. Prescription drugs were found in therapeutic concentrations, the coroner said.
The Oscar-winning actor had battled severe depression for years and had recently been diagnosed with Parkinson's, his widow, Susan, revealed after his death.