I’m a former heroin addict. I was considered less hardcore than most, as I’m not big on needles and mostly smoked H. I’ve met more than a few other addicts along the way. Far more than you would think live very normal lives. I’d bet money that most people have met one in their life and never noticed. If we expand the discussion to include people who abuse proscribed opiates, I’d guarantee it.
Also, pretty much everything CosmicManiac has said about H is wrong. Your pupils don’t dilate, they constrict on opiates (also called miosis). Also, heroin isn’t a poison, it’s a psychoactive drug. Like every drug ever, it’s the dose that makes the poison.
And speaking of the dose, if you put $80 worth of H in your arms, it’s game over. That’s about a half a gram in my parts, and unless your tolerance is quite high or you weigh 400 lbs, that’s a lethal dose. The tolerance is a huge part of what makes H so dangerous to use. Once you’ve used long enough to drive up your tolerance, the line between euphoria and death is razor thin. Combine that with it being impossible to accurately know how strong the drug you are using is, and it gets real scary, real fast.