The case involves two personal assistants to Lawson and ex-husband Charles Saatchi who are set to go on trial for fraud.
The assistants' lawyers want to introduce allegations that Lawson used drugs at the time. The assistants deny wrongdoing.
Judge Robin Johnson told a pre-trial hearing on Tuesday that Lawson and other witnesses could be questioned about drug use during the trial.
The judge also read an e-mail from Saatchi in which he said his wife abused drugs extensively.
Saatchi and Lawson divorced earlier this year after a newspaper published photos of Saatchi grasping his wife's throat during an argument at a London restaurant.
A spokesperson for Lawson declined to comment.
