So let me tell you about the stupid thing I did...
I had to schedule that physical & drug screen as part of my enrollment tasks for school, and when I called to make the appointment, I expected that it would be sometime next week before I could go in. Surprisingly, they had a spot open for me the next day. About three hours before the appointment, I realized that I had made a serious goof - I ate poppy seed chicken the day before! In a panic, I started looking up stuff on the internet, and sure enough, it is no urban legend - poppy seeds can and do cause people to fail drug screens all the time.
Luckily, my doctor's office does the drug screens in-house, so the results are almost instant. After a bit of discussion, we decided to postpone it until the first of next week, lest it look like I am on opiates and get kicked out of school before I even start!