Most drugs produce a sense of euphoria so Fentanyl just gets sold as whatever and because it's illegal it's impossible to understand the potency of what you're buying.

Besides the issues caused by dealers adding adulterants, drug lab products have varying purity levels and a tiny mistake can create something totally different to what you intended with no way to test it.

From Wikipedia: "In 1976, a 23-year-old graduate student in chemistry named Barry Kidston was searching for a way to make a legal recreational drug... Kidston successfully synthesized and used desmethylprodine for several months, after which he suddenly came down with the symptoms of Parkinson's disease and was hospitalized. Physicians were perplexed, since Parkinson's disease would be a great rarity in someone so young, but L-dopa, the standard drug for Parkinson's, relieved his symptoms. L-dopa is a precursor for dopamine, the neurotransmitter whose lack produces Parkinson's symptoms. It was later found that his development of Parkinson's was due to a common impurity in the synthesis of MPPP called MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), a neurotoxin that specifically targets dopamine producing neurons." https://en.m.wikipedia.org/wiki/Desmethylprodine