Maybe because it's "depressing " or, repressing what chaos that has been running your brain, making it all stop/subside....though, also, the after effects are probably the depressant part, after you've come down from the initial euphoria. This is the beginning of a vicious cycle

It works great...until it doesn't.

Depressant as in reducing brain function, the opposite of a stimulant. Not depression like extreme sadness xD you can't be depressed if your nervous system is depressed, can't be sad if no think