Most things are out of your system in days, unless you've got a high BMI and are smoking marijuana like it's your job. Drug tests don't tend to be as random as they make them out to be and typically get asked for when someone seems off at most places. The random testing pool tends to not include non-employees in most environments so they need to go out of their way to test non-employees. I don't work at the VA and couldn't tell you how their process works for sure, but it is likely low risk so long as you're not currently using anything since the chances of a test lining up with the time period where you happen to also have a substance pop positive is low.
To anyone reading this thread, I would strongly recommend avoiding substances throughout training, as it can become an almost insurmountable barrier to practice on the small chance you get tested by your school or a rotation site and come up positive for something. You didn't make it this far just to lose it all for a quick high