People magazine on Sunday quoted the 30-year-old actor's representative as saying that Monteith is seeking treatment for substance addiction, and "graciously asks for your respect and privacy as he takes the necessary steps towards recovery."
Monteith had said in a 2011 interview that he had also been in a rehab centre when he was 19.
At the time, the actor said he hoped being honest about his struggle with substance addiction would help others.
"I don't want kids to think it's OK to drop out of school and get high, and they'll be famous actors, too," he said. "But for those people who might give up: get real about what you want and go after it."
Monteith, who was born in Calgary, and his brother were raised by their mother in Victoria, B.C., after their parents divorced.
Cory Monteith is said to be dating Lea Michele, his co-star on the critically acclaimed Fox series Glee, which had its debut in 2009, and is the winner of numerous Emmy and Golden Globe awards.
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