I don't know about coke and how ppl get overdose on it , cuz I had addiction almost for two years and I did a lot but never even felt something close to heart attack or so.
One day, about 12-14 months ago, I've got an ounce and decided push my limit as much as I can. I don't know how many grams I snored in 24 hours, but it was a lot. I was basically doing it nonstop, hoping to get some black out at least. But nothing. I was tryna till some point when my nose stopped working, and I just didn't feel anything at all.
Can't say anything about heavy drugs like heroin or so, cuz never tried.