Lawson was testifying at the trial of two of her personal assistants who are facing fraud charges relating to the alleged misuse of 685,000 pounds ($1.12 million) on a credit card belonging to her ex-husband, millionaire art dealer Charles Saatchi.
Last week the assistants' defence counsel said there was a tacit understanding that they could spend what they liked as long as they did not tell anyone about Lawson's drug use.
The court has also heard that Saatchi wrote her an email in October in which he accused her of being "off her head" on drugs.
But the chef, who is well known in Britain and the United States, told the jury at Isleworth Crown Court in west London: "The idea that I'm a drug addict or a habitual user of cocaine is ridiculous. I don't have a drug problem, I have a life problem."