You couldn’t. And I don’t even care if they’re an addict. If they smoke meth daily or drink twelve beers when they get home, so what? As long as they can do their job, fine. I worked with a major alcoholic. He was an asshole, but he got his job done and did it well. So apart from it being unpleasant to work with him, who cares? And if it was cocaine or meth or whatever instead, again, who cares?