Yes, for the most part you can't as employer do thing about people on legal meds with Dr's script. Good luck even finding out why(say phantom back pain) they have the script since with HIPPA code you can't even delve into that, union or no union. I see it all the time at my place (govt.).
Only would be for public safety ones like airline pilot or something.
Things like below no chance in hell as employer you going to do anything: