The TV cook said she took cocaine with her late husband John Diamond when he found out he had terminal cancer, and in July 2010 when she was being "subjected to intimate terrorism by Mr Saatchi".
She said: "I have never been a drug addict. I've never been a habitual user.
"There are two times in my life when I have used cocaine."
Ms Lawson, 53, went on to say the first was when her late husband Mr Diamond found out his cancer was terminal, and she had it with him on six occasions.
"It gave him some escape," she said.
She said she had responsibility for looking after him and the family and earning a living.
Ms Lawson said she spoke to a doctor at the time.
Speaking about the second time in her life, she said it was when a friend gave the drug to her.
"There was another time I took cocaine. In July 2010 I was having a very very difficult time.
"I felt subjected to intimate terrorism by Mr Saatchi," she said.
Adding that she felt "isolated and in fear of...just unhappy".
"A friend of mine offered me some cocaine. I took it," she said.
Ms Lawson said it "completely spooked" her.
She said: "The idea that I am a drug addict or habitual user of cocaine is absolutely ridiculous."
Ms Lawson said this was her evidence under oath.
She faced dozens of photographers and television crews from around the world as she arrived at Isleworth Crown Court in west London this morning.
Her former PAs, Francesca Grillo and her sister Elisabetta, sometimes referred to as Lisa, are accused of committing fraud by abusing their positions by using a company credit card for personal gain.
Prosecutors claim the Italian sisters lived the ''high life'', spending the money on designer clothes and handbags from Louis Vuitton, Christian Dior and Vivienne Westwood.
The pair are accused of using credit cards loaned to them by the TV cook and her ex-husband Mr Saatchi to spend more than £685,000 on themselves, the jury has heard.
Ms Lawson said Mr Saatchi "told everyone" he was taking cocaine out of her nose after he was photographed holding her neck at Scotts restaurant.
She claimed the incident was actually sparked when she commented on a person walking by with a baby.
"I said 'I'm so looking forward to having grandchildren'," Ms Lawson told the court.
"He grabbed me by the throat and said 'I'm the only person you should be concerned with'."
Ms Lawson admitted taking cannabis in the last year of her marriage to Mr Saatchi.
"I have to be honest, I have smoked the odd joint," she said.
"I found it made an intolerable situation tolerable.
"It's a false friend and not a good idea.
"I found the answer was in changing the situation and trying to create a tolerable situation for me and my family.
"I have to say, since freeing myself from a brilliant but brutal man, I'm now totally cannabis, cocaine, any drug-free."
Ms Lawson claimed she did not know how to roll a cannabis joint but had instead asked others in her house to do so.
"This was not behaviour I'm proud of," she said.
Ms Lawson said she was not a "habitual drug user and drug addict, or a snorter of cocaine for 10 years".
Asked by Anthony Metzer QC, representing Lisa, where these drug claims may have come from, Ms Lawson replied: "I believe some of it came from your clients and Mr Saatchi - not the three most reliable witnesses."
Ms Lawson said Mr Saatchi was unaware that she was smoking cannabis and admitted she had not told him she took cocaine once during their marriage.
"It was a one-off," she said.
"I felt by going to my GP I didn't feel a need to tell him."
Ms Lawson told the jury that Lisa was aware she had taken cocaine in the past.
But she said claims that credit cards and envelopes containing white powder were left around the home she shared with Mr Diamond were "completely false".
She admitted that Mr Diamond used rolled-up notes to take the drug.
"John did but he carefully ironed them out," she said.
"There is some misery you cannot escape."
Elisabetta Grillo, 41, and co-defendant Francesca, 35, both of Kensington Gardens Square, Bayswater, west London, deny the fraud charge against them.
Ms Lawson said she believes Mr Saatchi had a mindset of "Get her, I don't care what it takes" in relation to her and the current legal proceedings.
She told the court he feels betrayed by her.
In reference to Mr Saatchi possibly suing her if she did not appear as a witness in this trial, Ms Lawson said it was "just another form of bullying".
She said he is on a campaign to "ruin me in any way", whether financial or otherwise.
"I think he likes everyone to do what he wants," she said.
She added that she feels there is a "witch hunt" against her, and that she is on trial with no counsel and "no rights".
Ms Lawson told the court that Mr Saatchi "punished" her on one occasion for going to a friend's party.
When she was asked by Mr Metzer about the word "pass" used in the now infamous email sent to her by Mr Saatchi in which he accused her of being "off her head" on drugs, she said: "I had once made reference to a 'pass' because I had been punished for going to a girlfriend's birthday."
Ms Lawson said she had not been beaten, but said she was left with "emotional scars", which were "very wounding and very difficult and of course we know how things accelerated".
She described the allegations of her drug use in the email as being "part of his plan of attack", and said she thought it was "very odd" that he knew what was in the witness statements before they were published.
Ms Lawson said she believed Mr Saatchi intended to damage her interests, adding: "And look how it's played out."
The celebrity cook said she had hidden any drug use from Mr Saatchi, adding: "I could have weathered a conversation about it. I just wouldn't have wanted to."
This afternoon, Judge Robin Johnson and Mr Metzer clashed when Mr Metzer wanted to show Ms Lawson a photograph of the incident at Scott's.
Ms Lawson pleaded: "Please don't make this go on for two days."
Judge Johnson said: "I'm not having photographs put to this witness."
When asked again about the incident, Ms Lawson said she had already given a direct, "almost verbatim" account of what happened, and said she did not intend to speak about it again.
Judge Johnson told Mr Metzer: "You do not have the right to string this out", twice adding firmly: "Move on."
Ms Lawson said that, despite changing her number and blocking communication with Mr Saatchi in her email account, she still received the email containing the drug allegations.
She said it came from a "completely different email address".
Ms Lawson was told by the judge she must return to the court in the morning to complete her evidence.
The trial was adjourned until tomorrow.