I am super nervous because I really need this job. I have a good job already, but the new one is a big promotion and pays more. I already turned in my resignation at my current job. I'd have a hard time explaining to my current job why I changed my mind if I did fail a drug test for the new job.

Since youve expressed that you really need this job, why not just cut to the chase and substitute, instead of taking chances with your own urine?