Are we talking a lightly medicated level or recreational level?
My boss pays me to be ready to perform any task in my job description at any time. So I would never consider coming in after smoking to the point where I can actually feel it. I manage a lab. It could be absolutely DISASTROUS to show up stoned, and I'd rather call in sick or even use up a personal day than risk the safety of myself or others by working impaired. One bong hit at 7am before I go to work for 9 will cut my anxiety down to a manageable level without any 'high' feeling.
If I had employees, I'd be RIPSHIT if they showed up noticeably high, or even hung over. Anything self-inflicted and avoidable that would make them less able to focus on working with deadly chemicals would also make me send them home and write up a report in their personnel file, and if it happened again, they'd be GONE.
Now, the dish washer at the diner or the chick whose job it is to water plants at a nursery is not likely to kill anyone by making a mistake...