These days the only time people seem to notice anything off is when I'm not on anything due to my depressed withdrawn mood. When I have my daily supply of opiates and benzos people don't really seem to notice much, and I can function normally, better than normally usually, but I keep my use pretty moderate these days.
When I used to binge on dexedrine and alcohol it was obvious. weight loss, up for days at a time, never eating, always talking, then sleeping for three days straight.
I've always been strange when compared to "normal" people, quiet, always stayed up super late, I've always been thin, I've used some kind of mind altering substance or another since I was 15 and I've never made this a secret. I guess people expect me to be a little different. I got accused of being a drug addict long before I ever touched the things.
People accused me of being a stoner years before I smoked pot.
People accused me of being a heroin addict years before I ever did opiates.