Robert Downey Jr. insists prison didn't change him.

The 'Iron Man' star - who was jailed twice for drugs offences in the late 1990s - is astonished his reckless lifestyle didn't get him into trouble sooner and it has taken him until now to believe his punishment had consequences for how his life panned out.

He said: "I do think there's something Divine about large-scale humiliation, but no, it never worked for me. It never produced enough of a blow back for me to catch the drift. It doesn't so much produce results as just get your attention - you become a fish in a barrel and there's really something quite beautiful about that.

"Jail didn't help me any more or less than anything else. But I know I created that experience for a reason, and I'm only just starting to understand those reasons. Remember this is California, 'Come on vacation, leave on probation.' I mean, I can't believe I stayed out of the pen for as long as I did."

The 46-year-old actor insists conditions in jail were not much different from being at school or on a film set.

He added to Britain's GQ magazine: "And I didn't mind it inside really; it wasn't so bad.

"Listen, a prison is just like a public school. Did you ever go on a film set on location? It's just like prison. Have you ever been in a bad relationship? It's just like having a cellmate who wants to kill you. Have you ever been in a street fight? It's just like chow-time meal time. Have you ever been to a rave-up? It's like a yard riot. I wouldn't wish it on anybody, but it neither fazed me, nor changed me, nor informed me - but maybe it toughened me up a little bit."