In Europe there are a lot of these so-called "research chemicals". They are basically illegal drugs but chemically they are different enough that they aren't illegal. I think the most well-known example is 1P-LSD.
1. Fent is not an RC, it is used in medicine 2. It's strongly regulated
Caption: "Yan Xiaobing was indicted for distributing these similar compounds; making slight changes to the molecular structure can have unpredictable effects on potency."
Also: [I]n recent years, rogue chemists have unearthed instructions for analogues that researchers discovered decades ago but never put into legitimate use.
While Chinese authorities control fentanyl, they’ve been slow to ban new analogues.