Cory Monteith spent a month in rehab shortly before his death.

The 'Glee' actor passed away from a heroin overdose in July 2013, but his mom Ann McGregor worries ''massive'' dental treatment he'd had done in the weeks following his stint in a clinic that April - which came four months after he told her he was using drugs again - affected his sobriety.

She said: ''He had little teeth and they were all capped. He had a lot of medication in his system, which was not good for his body coming out of rehab.

''He didn't have enough drugs in his system to kill him, but for some reason it did because of his intolerance [built up by periods of intermittent sobriety].''

Cory was an ''advanced'' youngster who skipped two grades at school, so was hanging out with older boys when he was in his early teens, which Ann thinks contributed towards his problems.

She told People magazine: ''There was this disconnect. Because he was pushed so far ahead and always associated with older children, but he was still a boy.''

Cory began drinking and smoking marijuana when he was just 13, and Ann checked him into rehab when he was 15, and again at 19, even though he'd tried to keep his problems secret from his mother, who raised him alone.

She recalled: ''He tried to keep everything from me, because he loved me and wanted to protect me. He was just always so curious. And the darker world just drew him in.''

Though the actor eventually got sober and launched a successful career in Los Angeles, Ann doesn't think her son could cope with the showbiz life.

She said: ''He wasn't ready for the Hollywood world. Drugs were his way of checking out.''

And Ann still hasn't come to terms with the loss of her youngest son.

She said: ''I still can't pick up the pieces. My world totally stopped. And I'm a different person than I was before...

''Cory was such a loving and genuine person. And the best part of his life was still ahead.''