As with anything that happens in the States, it depends. I’m sure different states have different laws regarding testing, but what I’m most aware of is that some employers do pre-employment testing, and others will test after events.

I work in Illinois where weed is legal, so they’re less testing for that and more for other substances. I work for a hospital, they probably want to know if a new nurse hire is positive for unprescribed opioids. Likewise, if a forklift driver runs into something causing damage, they’re going to get tested as well.