I think the parent comment is silly, but there’s a pretty fundamental difference between something you are and something you do.

Is being gay something you are or that you do? If drinking alcohol off the job is protected other recreational chemicals should be just as protected. And a significant portion of Marijuana use is medical regardless of opinion to the contrary.

There are clinical uses of medical marijuana to treat cancer pain and epileptic seizures. We’ve now crossed into “discrimination against a legally protected class” territory depending on the definition of disabled.

Yes, but I dislike that I do. It's absolutely true that I'm going against what I consider principals here; I think the harm from allowing racial discrimination in the workplace is greatly and demonstrably worse than other forms of discrimination. It's a practical trade-off that I don't think we need for marijuana.

I'd also like to see employers discriminate against alcohol users, even though I consume myself. There's plenty of fields that would benefit from employer enforced teetotaling. I don't think that either marijuana or alcohol make you a bad person, just a worse one. They're called vices for a reason. Everyone should have one, but I'd encourage people to play 40k or practice trainspotting or eat too much chocolate, all things shown to have less effect on mental health.

I would not like to see employers discriminate (per se) against alcohol or cannabis users, even though I don't consume myself.

If alcohol (or cannabis) in fact cause problems, then it is not necessary to discriminate against all users, just those who manifest the problems.

I am unclear on why many people (of a libertarian tilt) think that discrimination based on stereotypes is to be defended.* It seems to me to be a way of shifting internal costs of evaluating people onto society, and therefore we should not expect an unregulated market to provide an optimal amount of it.

* Provided it is not against a legally protected class.