Investigators at the scene found drug paraphernalia, including a spoon with drug residue and a used hypodermic needle, along with two empty champagne bottles and glasses, the report said.
Monteith died from mixed drug toxicity involving heroin and alcohol.
- Read more about Cory Monteith's death
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The coroner said the fact that Monteith went to rehab, stopped using drugs for some time and then started up again, may have contributed to his death, which has been ruled an accident.
A drug that was previously tolerated may become fatal after a period of time, the report explained.
Monteith was found dead in his hotel room in July at the Fairmont Pacific Rim in downtown Vancouver.
The investigation confirmed that hotel video captured Monteith returning alone to his room at 2:16 a.m. PT on July 13, and that he was not seen alive again. He was found dead the same day by hotel staff when he missed his checkout time.
Monteith, who played the quarterback Finn Hudson on the Fox TV series about a high school glee club, was born in Calgary and raised in Victoria.