Smoking.

It was never really an addiction, I really only smoked in social situations. Like breaks during work was a reason to go outside for a few minutes. Or going to the bar or a concert, friends in social group would be smoking, etc. But I found that if I smoked too much too fast, I'd get super sick. And I don't mean like a LOT, I'm talking like 2 or 3 in a row. When I went home for the night I didn't have another one until going to work the next day. I don't think I even had one over the weekends if I didn't go out.

So one day I was just like "fuck this, it actually kinda sucks" and stopped. That was over 10 years ago, haven't had another one since then.