Many psychoactives have medium term effects on the brain. Regular marijuana consumption causes memory and attention deficits even when not stoned. That matters.
There is even an argument that caffeine should be restricted in such roles. I certainly wouldn’t want ATC operating on too many energy drinks or coffee.
If we are banning people who take antidepressants, we should also be banning people who take edibles on their day off.
If we are going purely performance-based, then there should be a mandatory cognitive function test before the start of every shift, testing working memory, reaction times, coordination, etc. “sober” is a wide spectrum and encompasses “ate tons of edibles 48 hours ago and came to work on 3 hours of sleep and a five hour energy shot on the way in”.
I’d prefer my ATC has a cup of coffee if they want it.
Aside from that, what does what happens in their off hours matter provided they’re coming to work sober?