smoked cannabis before accepting her Oscar in 1991.
The 55-year-old actress - who won the Academy Award for Best Supporting Actress in recognition of her role as Oda Mae Brown in 'Ghost' - was so nervous before the prestigious ceremony, she smoked some pot in the hope of "relaxing herself".
She said: "I learned a great lesson though. Never smoke pot before there's the possibility of having to talk to one hundred million people. Smoking cigarettes and pot every now and then are one thing that relaxes me and I thought, 'I've got to relax.'
"So I smoked this joint that was the last of my home grown and honey, when he said my name and I popped up, I was like, 'Oh f**k, oh f**k, up the stairs one two three four five, onto the podium. There's millions of people, OK grab the statue.' I knew I should have never had done it."
However, the 'Sister Act' star admitted as soon as her mother saw her accept the award she realised she was under the influence of a drug.
Speaking during a voiceover session for her 1992 movie 'The Pagemaster' in a video obtained by gossip website TMZ, Whoopi added: "My mother called me afterwards and said, 'You smoked didn't you? Your eyes were just glistening.'
"So I panicked and got the Oscars tape to see whether you could tell and you just couldn't. I know you're not supposed to admit that you smoke pot but I don't drink alcohol!"