The purpose of the drug test for a new patient would be to assess your honesty firstly. And if you’re seeing the therapist for drug related issues a base line is needed. I would expect a therapist to ask about your drug use both legal and recreational as these have an effect on our mental health as well as our physician health. So if you provide incorrect info regarding this question and your drug test shows differently it will come up in treatment. Establishing a trusting relationship with your therapist is paramount to receiving good therapy to reach your goals. The testing is not meant to cause you problems but only to assist you to getting better. The therapist will not stop or decide to not see you because you got this wrong, It’s only a fact now known only between you and your therapist and a beginning of a positive relationship.