Speaking at the fraud trial of two ex-aides, Lawson said she took cocaine when her first husband John Diamond was dying and again in 2010 when she was being subjected to "intimate terrorism" by multi-millionaire art dealer Saatchi.
Italian sisters Elisabetta and Francesca Grillo, who used to work as personal assistants to the celebrity couple, each deny fraudulently spending USD 1.1 million (820,000 euros) on Saatchi's company credit cards.
"I have to say, since freeing myself from a brilliant but brutal man, I'm now totally cannabis, cocaine, any drug-free."
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.
"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.
"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.
