Nigella Lawson told Isleworth Crown Court that the drug allegations were "dedicated to salvaging Mr Saatchi's reputation and destroying mine."
She said that her ex-husband Charles Saatchi started spreading false rumours as a way of "getting back at me."
She admitted to taking cocaine "a handful of times", but insisted she was not a habitual user.
She also admitted smoking cannabis in the last year of her marriage to Charles Saatchi, but said she has now given up.
It's alleged that Italian sisters Elisabetta and Francesca Grillo used credit cards loaned to them by Lawson and her ex-husband Charles Saatchi to spend more than £685,000 on themselves.
She told the court she took out £7,000 from her savings to pay for Elisabetta Grillo to have her teeth fixed.
She said her children were "hurt to the quick" by an article describing her as a "surrogate mother." She said she had faced a long summer of "bullying and abuse."
The case continues.