One thing I picked up on, though: the OP refers to people dying from OD. I believe it's quite unusual to die from an overdose; my understanding is that nearly all "OD" deaths are not poisonings, but the consequence of inhaling your own vomit. Hendrix, Joplin and Jim Morrison all died from inhaling vomit.
Opiates depress your respiratory reflexes, so you breathe "less well". And they make you dozy, or even pass out. If you puke in your sleep, and then breathe in, bad stuff (like, hydrochloric acid) goes into your lungs, you fail to wake up, so you "drown" i.e. you can't breathe. Death by respiratory failure.
Smack in itself isn't really that harmful. Smackhead doctors have been dosing themselves with clinical heroin for decades, while carrying on their jobs and family lives. Scoring on the street, though, is super-high-risk.
/me never used smack.
I did at one time hang out around losers; a few of them were smackheads, and they robbed the rest of us rotten. Most of these losers were into downers - I once held the tourniquet for a guy that was injecting Tuinal (a discontinued barbiturate combination). Injecting barbs is much more dangerous than injecting smack, if you can get smack that isn't adulterated.