Do some research. What legit OTC or foods(poppy seeds) come up positive for the same thing in your blood. Make a point of eating or using that in front of people including your boss and when it comes back positive, swear up and down it was the other substance. You can always raise reasonable doubt. It may not save your job but doing nothing is not going to save it for sure.
Always have a backup plan. What other job do you have available. Always be fishing for other opportunities at other companies and you may be able to quit one job and start another before the results come back.
Something I have wondered about is keeping a packet of some outrageous mix in my pocket. Just a pinch of anything OTC or food or otherwise that can set off false positives. if I have to do a test, simply dip my finger in the packet then "wet" my finger as I am filling the bucket. When it comes back you will easily be able to deny you had 50 illegal substances in your blood at high concentrtions because you would be dead.
It is a sucky system. No differentiation between medical use and recreational use, no consideration between someone who uses on the weekends and is clean during the week. We can't have dope heads driving semi-tractors but there has to be some better option than this. Why is it someone can get drunk every weekend, drink themselves to sleep every night alone in a studio apartment, go to work and everyone thinks that is great but if the person in the next cubicle takes a performance enhancer or uses MJ on friday night he is fired.
I am sure there are more people with better advice but those are the first things I would think of.
Some googling should give more suggestions. You are not the first in this position.
Good luck.