It's not, although rules vary by state. Medical professionals can prescribe it even though it's a Schedule III drug. I get ads for these presumably legitimate therapy services online all the time on major social networks.

We carry ketamine on our ambulance where I work part-time. I wouldn't be surprised if some people were legally receiving drug therapy for depression

Firing someone for taking a prescribed therapeutic medication for a medical condition? Yeah, that’s illegal.

It's not illegal if it's through a licensed channel.