While "sober" fits the dictionary definition for not being high, it holds too many connotations of "not being drunk". I'd like to find a word that specifically relates to cannabis. I made one up, "abstonent", which I think is pretty good. I'd like to hear other members suggestions.
I like your thinking. Sober is definitely, at least for me, identified with alcohol. As for a word that means not high, I usually hear people saying I'm "straight," though I don't really like the term. Abstonement is pretty clever. I like the prefix play. A-, Anti-, Dis-, Il-, Im-, In-, Ir-, and Un- all mean some sort of not, or without. So "ilstoned" or "unhigh."
Back when I started smoking, in the late '60s, "straight" was the uncontested term for "not stoned". More recently, when I've used that term, some people didn't know what I was talking about. It's further complicated because heroin users get "straight" when they get a fix. At least they used to. I have no contacts in the hard drug using community anymore and terminology changes so rapidly.
We could say "low" when we aren't "high". Or maybe we could use the word "level"... Like straight.... You're not high, so you're level, like flat. Or maybe "empty"...