I was in a situation very close to yours. Graduating in May, had a fucking awesome offer I accepted, and all contingent upon passing the drug screen. I can tell you a few things: 1. Do not take this lightly at all. If you can sub, DO IT. Don't be naive or lie to yourself thinking you can pass naturally. It's not worth the risk. If you are going to have to change into a gown for the physical, take the private changing time to hide your sub stuff. 2. They will only run the GC/MS if you fail the initial test, yes.