I think the problem here would be that currently drug testing is for illegal drugs, not things that impair. I don’t think there is anything an employer can do about employees use of something that is legal. If marijuana is legalised, and employment was based on simply drug use, then that would eventually have to involve alcohol and tobacco use, I would think.
Certainly you could be fired if you come to work (or to your employer-mandated drug screening) drunk.