"I'm not moaning because I chose to do it but it was my downfall. I was stroppy and drinking too much. I never thought it was a problem - it's only with hindsight you realise you were a bit out of control.
"I still turned up for work - even though I had a face like a slapped a**e. My eyes were vague and dead and my mouth was smiling without feeling in a false pose I'd become accustomed to."
She admits she experimented with drugs: "I was so green and that's what saved me - otherwise I may have got into it more. I had a very dark moment at 16 when I didn't want to be here any more but I did nothing about it, thank God."
Piper will quit Dr Who at the end of this series and said: "The longer I stayed the more scared I'd be of leaving because it's so comfortable and nice."
Of her autobiography, she said: "I hope it's an inspiration to young girls who constantly ask how I did it. It will be warts and all."
The full interview appears in the new Radio Times, out tomorrow.