alt Hacker News> Fentanyl will kill you the first time you're exposed to it.
Its lethal dose is a lot smaller than that of heroin, but it isn’t guaranteed to kill. If it were, why is it used so much in medicine?
https://en.wikipedia.org/wiki/Fentanyl: “Fentanyl is a highly potent synthetic opioid of the piperidine family, used primarily as pain medication
[…]
In 2015, 1,600 kilograms (3,500 pounds) were used in healthcare globally.
It is on the World Health Organization's List of Essential Medicines.”
There's always a continuity there ("the dose makes the poison"), there's no known substance that will reliably kill you if you're exposed to a single molecule of it.
The issue with potency is that the more potent a substance, the more precise equipment and quality control you need to stay on the safe side. With fentanyl, healthcare and pharmaceutical industry can handle the precision and QC demands (which includes guarantees and maintaining public trust). Criminal enterprises cannot.