Cory Monteith's father says his son's drug addiction was worse than ''anybody knew''.

The 'Glee' star was died at the age of 31 last week from a toxic mix of heroin and alcohol and his father Joe - who split from his mother when the actor was a child - has claimed his son's battle with his demons may have been harder than people realised.

He told Entertainment Tonight Canada: ''I guess he was involved a little more in drugs than anybody knew at the time.

''His mother who was there with him all the time, she started picking up on it, you know stuff was missing, money here, stuff like that.''

Cory's parents were distraught when they heard the news, and his mother Ann McGregor ''almost fell'' when she was told of her son's death.

Joe added: ''When they did break the news to my wife, she almost fell over, in fact I had to grab her. And I did ask them again what was that you said? It didn't really sink in.''

Meanwhile, fellow star Demi Lovato - who was in rehab in 2011 for emotional and physical issues - has spoken out in hope that Cory's tragic story will make people aware of the dangers of addiction,

She recently said: ''All it takes is one moment of vulnerability to get slipped into your addiction. He didn't choose to die.

''It was the disease. It can creep up on you at any moment. It only takes one moment of relapse to potentially die.

''It's really scary, but I'm really hoping that from this, people are able to see this is a very, very, very dangerous disease.''