Monteith was honored at the Emmy awards last month.
Monteith's death recalled the lives of Heath Ledger, Corey Haim and River Phoenix actors who battled substance abuse and died in their 20s and 30s.
Monteith had talked bluntly about struggling with addiction since he was a teenager, calling it a serious problem and telling Parade magazine in 2011 he was "lucky to be alive."
In that interview, he said he was using marijuana and drinking by age 13, and his drug use was "out of control" by the time he was 16. "Anything and everything, as much as possible," he said. "I had a serious problem."
"Glee," with its catchy song-and-dance numbers and high-profile guest stars like Gwyneth Paltrow and Britney Spears, became an instant hit when it debuted in 2009. Monteith served as the show's resident hunk with a heart of gold.