Does joining the US Digital Service require a clearance, and if so does occasional marijuana consumption automatically disqualify you?
Protip: Contractor or FTE, make sure you specifically ask HR whether or not there will be an additional screening, and whether or not it is a 'public-trust' position and what level that would be following your hiring.
I was pretty miffed when 'they' told me after the fact that my position required an additional background check and that my continued employment would be predicated on the outcome.
A valid prescription (the little white pads with a doctors signature, not a pot club membership card) for medical marijuana protects you from criminal prosecution, but not from employer drug testing programs. Executive Order 12564, signed in 1986, explicitly makes the federal government a zero tolerance drug free workplace.
For a basic public trust it was just a credit, background and drug test (the same treatment you would get at any employer who didn't trust their employees).