Giving evidence in the fraud trial of two former personal assistants, the celebrity chef said that claims she was a habitual cocaine user were "absolutely ridiculous".
"There are two times in my life when I have used cocaine," she said, detailing how she used the drug with her late husband John Diamond when he was diagnosed with terminal cancer.
"It gave him some escape," she said, adding that there was also a second time in her life when she turned to cocaine.
"In July 2010, I was having a very, very difficult time. I felt subjected to intimate terrorism by Mr Saatchi," she said.
She also admitted to smoking cannabis during the last year of her marriage to Mr Saatchi.
I have smoked the odd joint - I found it made an intolerable situation tolerable. I have to say, since freeing myself from a brilliant but brutal man, I'm now totally cannabis, cocaine, any drug-free.
A large press pack had been waiting on Wednesday morning, when she arrived at Isleworth Crown Court in west London.
Taking to the witness stand, the 53-year-old said that she had effectively been "put on trial" herself over the drug allegations, but that she was glad to answer them.
"It comes after a long summer of bullying and abuse. I find it's another chapter in that," she said.
Nigella Lawson claimed that the drug allegations against her were "dedicated to salvaging Mr Saatchi's reputation and destroying mine", but later said that she had used cocaine.
Mr Saatchi was previously cautioned over pictures which emerged of him appearing to grasp his then wife by the throat during a row at a restaurant.
Nigella Lawson told the court that Mr Saatchi "told everyone" he was taking cocaine out of her nose, but said that the incident was actually as a result of a comment about a passer-by with a baby.
"I said: 'I'm so looking forward to having grandchildren'," she claimed.
"He grabbed me by the throat and said: 'I'm the only person you should be concerned with'."
He (Charles Saatchi) had said to me if I didn't get back to him and clear his name he would destroy me.
The trial of two Italian sisters, 41-year-old Elisabetta Grillo (sometimes referred to as Lisa) and 35-year-old Francesca Grillo, centres on allegations that they committed fraud by abusing their positions as personal assistants by using a company credit card for personal gain.
The court heard last week that they had allegedly spent more than £685,000 of Nigella Lawson and Charles Saatchi's money on themselves, funding a lavish lifestyle.
Speaking about Lisa, Nigella Lawson said: "It's very difficult when you find out that someone you have loved and trusted could behave that way.
"In my heart of hearts I do not believe Lisa to be a bad person. I believe her not to have a very strong moral compass."
She referred to Lisa as a "stalwart" who had helped her through when her first husband died.
"I loved Lisa. My children loved Lisa. She came to me at a very difficult time in my life. She was a rock. I would have done anything for her," Nigella Lawson said.
The chef outlined how she had once taken £7,000 out of her own savings for Lisa to have dental work which boosted her confidence. She also continued to pay her for three months after she left her position for a time, so she would not get "too anguished" about finding a new job.
But Nigella Lawson added that, on her return, Lisa became "increasingly bitter".
"She broke our heart," she said.
Both sisters, from Kensington Gardens Square in west London, deny the charges against them.