Its lethal dose is a lot smaller than that of heroin, but it isn’t guaranteed to kill. If it were, why is it used so much in medicine?
https://en.wikipedia.org/wiki/Fentanyl: “Fentanyl is a highly potent synthetic opioid of the piperidine family, used primarily as pain medication
[…]
In 2015, 1,600 kilograms (3,500 pounds) were used in healthcare globally.
It is on the World Health Organization's List of Essential Medicines.”