Actor Richard Dreyfuss wrongly feared marijuana caused his bipolar disorder because he smoked so much weed in his youth.

The Jaws star has spent years struggling with bipolar disorder and anxiety issues, and he was initially convinced his mood swings were self-inflicted because of his drug use.

Dreyfuss admits a candid conversation with his sister set him straight on the issue.

He tells Tv host Piers Morgan, "(I first realised I was bipolar) when I was about 14. I said to my sister, 'I wonder what would've happened to me if I had never taken any drugs' and she said, 'Why are you asking?' I said, 'Well before that I didn't, you know, before I smoked marijuana and all that, I was pretty normal'. And she said, 'Don't revise history. You were nuts when you were born'... (She always thought I was a little bit crazy)... And I was."