I'd oppose it.
Tough to find scripture that actually deals with DRUGS, so I'll fall back to this one:
Most drugs will turn up on a drug test long after the effects have gone away, so it's not like you can determine if someone is "sober" while at work the same way you can show drunkenness with a breathalyzer. If someone smoked a joint at the start of the weekend, and then came to work Monday morning with their game face on, then I don't see why they should lose their job over that.
Clearly, I oppose people being incapacitated while at work. There should be limits as to how far into one's off-duty life an employer can intrude, and mandatory drug testing is invasive in ways beyond the pitting of personal scruples of the employer against the employee. For example, if an employee fails a drug test because of a prescription he or she is on - now they must divulge personal medical information to their employer or risk termination.
There may be some jobs where the messy scruples that can come up are outweighed by safety or health concerns. But MOST jobs are not those jobs. The job where I work is certainly not one of those jobs. I don't know too many of my co-workers who would be comfortable with divulging our medications and medical records to HR, and I don't see how the few employees who might have smoked a joint over the weekend are possibly incapable of doing their jobs effectively the rest of the week.
Also, if there IS some kind of issue with someone coming to work hung-over, high, or otherwise incapacitated... It's not exactly rocket science to figure that out. You don't need a drug test to say, "You are clearly not able to do your job right now. Go home, and if this happens again, you are fired."
