Unless it’s a protected class you can discriminate on it, from what I understand.

If they suspect that you’re using it as a stand-in for a protected class, you may be in trouble (not hiring smokers because blacks smoke, for example).

There may be edge cases where something is effectively a protected class. And it may vary from state to state.

If you want to work at BYU, abstaining from coffee is a condition of employment. This goes for even for people who are not members of its sponsoring church. Just a few years ago, BYU almost lost its ROTC program because a non-LDS colonel didn't want to abide by the university's code [1].

[1] https://www.airforcetimes.com/news/your-air-force/2017/08/29....

I dabble in adult content creation and could absolutely get fired for it. It's not exactly a vice though.

Some states bar or limit employers from regulating off-duty conduct by employees. Some don't. Smoking is a common one because it affects insurance costs, but the same legal theory could probably be applied to alcohol or caffeine.

https://www.workplacefairness.org/off-duty-conduct

yes, in college I worked for a christian company that had a policy of firing you if they found out you drank at all.

at-will means at-will.