A big part of the problem is that fentanyl is so concentrated.. and carfentanyl(for anesthetizing animals like elephants ) is even worse.
It takes a good compounding chemist or pharmacist to safely measure the proper amounts and the mixing MUST be VERY accurate and uniform.
The Police Chief’s Guide to Carfentanyl
https://www.policechiefmagazine.org/police-chiefs-guide-carfentanyl/
Although there are proper uses for this, the world would be better off if this did not exist.