The problem is not actually the production of the fentanyl itself, but the way it’s mixed in with “heroin” or pressed into pills (plus all the likely accidental contamination from using the same equipment to process fentanyl and stuff like cocaine). If the mixture is not done well, it creates hotspots that can easily cause an overdose due to a much higher density of fentanyl.

And also, this part is not necessarily done pharmaceutically, so there is no standardization of dosage or strength. Which can in part incentivize producers to progressively increase the ratio of fentanyl to <other> so theirs is “stronger”.

With no consequences for chemically altering their products... or getting people killed...