Make it legal, let them pay tax on it.
Then, if they need to be drug free to comply with safety at work standards, let them be randomly tested once per week.
I have no qualms whatsoever about the use of weed, grass, smoke, hash etc etc etc.
I do draw the line at having to inhale the shit, and subsidising work shy feckers who buy it because they can't or wont get a job.