Why is fentanyl so deadly? What was it originally for?

In the early 1960s the synthetic opioid was manufactured and approved by the FDA a few years later, for a specific purpose: to treat pain.

Fentanyl, Inc.' Tracks Opioid's Dark Web Path From China To U.S.

What is Fentanyl? And Why is it So Deadly? — Mirror Inc

Fentanyl is everywhere, increasing overdoses in the South - NC

If You Can See It, It Can Kill You

What is fentanyl? The little-known but deadly drug that killed

Fentanyl: What is it, and why is it so dangerous

Why is fentanyl so deadly?

DEA Washington Division Announces the Seizure of Over 8 Million of

Fentanyl Invades More Illicit Pills, With Deadly Consequences - WSJ