The allegations of drug-taking emerged at the start of the trial in an email written by Saatchi, 70.
The self-styled "domestic goddess" said she used cocaine with Diamond, her first husband and father of her two children, on six occasions because "it gave him some escape" when he was suffering from terminal cancer.
The second time was in 2010 when a friend gave her the drug as Lawson was having a "very, very difficult time".
"I felt subjected to intimate terrorism by Mr Saatchi," she said.
Lawson said she also took cannabis in the last year of her marriage to Saatchi.
"I have to be honest, I have smoked the odd joint. I found it made an intolerable situation tolerable," she told the court.
Earlier, Lawson said Saatchi wanted revenge after she refused to back him over paparazzi pictures that showed him gripping her by the throat at a London restaurant earlier this year.
"He had said to me if I didn't get back to him and clear his name he would destroy me," Lawson said.
"I have been put on trial here, where I am called to answer, and glad to answer the allegations, and the world's press, and it comes after a long summer of bullying and abuse."
Lawson, who has made a fortune with a series of cookery books and television shows in Britain and the United States, split with Saatchi this year after 10 years of marriage following the throat-grabbing incident at the upmarket Scott's restaurant in London.
Wearing a black fitted coat, an impassive-looking Lawson was flanked by seven police officers as she walked into the court building past a scrum of photographers and television cameramen on Wednesday.
In her evidence, Lawson told the court that after the "awful incident at Scott's", false allegations of drug use began circulating on a "PR blog".
The allegations on the blog were "dedicated to salvaging Mr Saatchi's reputation and destroying mine", Lawson said.
She painted a bleak picture of the end of her marriage to the former advertising guru, describing him as being quick to become angry.
"He did have a temper and I don't think that anyone can be in any doubt he had a temper," Lawson said.
Saatchi "didn't like to take part in family life" and her independence tended to "irritate" him, added Lawson.
Saatchi last week told the court he had no proof that his ex-wife had taken drugs but admitted he had sent an email claiming she was "off her head".
He said he was "utterly bereft" that the private email had been made public.
Saatchi accepted a police caution over the throat-grabbing incident but said he was disappointed Lawson had refused to say publicly that he had not abused her.
Lawson meanwhile said Elisabetta Grillo had once been her "rock" but lacked "a very strong moral compass".
But Lawson said she was "flabbergasted" by the extent of the alleged spending by the two sisters, which is said to have been on luxury goods, first-class transatlantic flights and hotels.
- AFP
