For heroin, the major risk is variation in potency. Heroin is highly cut at consumer level, for the most part. But occasionally, some relatively pure shit hits the street. And then there's the risk of boosting by Fentanyl and other high-potency opiates.
Also, it's pretty clear that the increasing popularity of heroin has been driven by decreasing availability of prescription opiates. New heroin users tend to be clueless about risk management. So they tend to overdose.
I'm curious about increasing deaths from prescription drugs. I wonder how much of that is driven by acetaminophen toxicity, as oxycodone etc have become harder to get than mixtures of acetaminophen with codeine and hydrocodone. I'm also wondering whether these figures include deaths for prescription drugs obtained informally.