I've read what happens often is that when heroin addicts relapse they often don't account for the dropped tolerance while they were sober and they start back at the same levels as when they were using and because of that OD. I'm pretty sure I saw that this was what happened with Hoffman.
One reason for overdose is that dealers increase the potency of their product by cutting it with Fentanyl, and other synthetic opioids.
These opiates, while easy to produce in a lab, are active in very small amounts (at the microgram level.)
As a result, if a batch of heroin isn't fully mixed, you end up with "hot spots" in some doses, where the fentanyl content is high enough to kill you.
This is how one of the first young women I met at university died. We grew apart, and one day she stopped responding to my text messages. A few days later I found out via Facebook.
