So you're selling heroin and figure "hey, I'll cut it and make more money". You add something inert and then add a tiny bit of a fentanyl and oops you added too much, now it's poisonous for users to take a "normal" dose.
It is ridiculously potent. We’re talking a normal dose of 10’s of micrograms.
However. I would argue diversion of Dsuvia will likely reduce overdoses as the quality control means you know exactly how much you’re taking.
Then you get cases where one distributor dilutes their product so much that their customers get used to taking very large amounts to get a high. Then one of those customers switches to another supplier with a "more normal" concentration and unwittingly overdoses.
Then there's the people that develop a tolerance to the drug over time, and keep trying higher and higher doses to get the same high.
And all of that is before you get into measuring errors and other mistakes caused by people doing complex chemistry without any formal training.
> This substance is extraordinarily potent (i.e. active in the microgram range), to the point that the pure powder can result in a fatal overdose if spilled on one's skin. [...] It is strongly encouraged to wear gloves while handling, use volumetric dosing combined with a milligram scale
https://psychonautwiki.org/wiki/Fentanyl (notably heroin doesn't have the same warning)
https://www.drugabuse.gov/about-nida/noras-blog/2018/05/true...
Imagine you have 100g of this stuff, and you have to dilute it with sugars. You need a ratio of something like 50ug fentanyl per 300mg of sugar.
Your 100g contains 2,000,000 doses of fentanyl. 2m * 300mg = 600kg.
So you need 600kg of sugar to correctly dilute your 100g of fentanyl.
Dilution is hard.
Now, there are always a few "garbagehead" addicts that'll use whatever you got, but that's not common in my experience, since opiate addicts chase calm, sleepy euphoria and meth/cocaine addicts chase stimulating, energetic euphoria. Different scenes, different personality types. Probably different genetics as well.