Monteith, 31, was found dead in his Vancouver hotel room on Sunday. The toxicology test showed the actor "died from a mix of drug toxicity."
The "Glee" star had been public about his battles with drugs, saying that he had undergone rehab when he was 19 after beginning to drink and smoke marijuana at age 13. He readmitted himself to rehab in April and seemed to be doing well before he was found dead.
Monteith's toxicology report was sped up after the publicity of his death. Costar Naya Rivera released a statement on his death Tuesday.
The cast, including his girlfriend Lea Michele, will have to act out losing him in the show as they mourn. The next season of "Glee" may be delayed until November.