re: Why does fentanyl even exist

Military medics use or used a fentanyl "lollypop" as a pain killler.
This lollipop is basically a white lozenge on a stick. The corpsman or medic places the lozenge either under the tongue or inside the cheek and the medication is taken up through the tissue in the mouth. Unlike morphine which takes 15 to 20 minutes to relieve pain, the lollipop is effective within five minutes.

It’s also much easier to administer than morphine, which must be injected into a large muscle, and if the dosage is too strong the lollipop can be rapidly removed. The effects of morphine cannot be halted.