Monteith, 31, was found dead in a Vancouver hotel room in July 2013.
Authorities said he died of a lethal mix of heroin and alcohol. Traces of morphine and codeine were also found in his system.
He was known for his starring role as Finn Huck on Fox’s hit TV show, “Glee.”
McGregor told ABC News in 2014 that Monteith first began using drugs at age 15.