Do you screen for cigarettes and alcohol use as well? Smoke breaks take up a lot of time, and someone with a hung over or slight buzz can be a safety issue.

What I've seen is that most companies don't care about those. They only care about illegal drugs. Even legal prescriptions that could have nasty effects are not judged harshly.

In short, there are more defensible reasons to drug test than what I listed, but they don't show up often in practice.