Etonitazene is a potent and selective mu-opioid agonist. It was developed in CIBA. Administration of etonitazene may induce respiratory depression, and therefor etonitazene is not used in humans. Etonitazene is explicitly listed as an illegal drug under UN convention and is illegal throughout the world.

Etonitazene:
https://drugs.ncats.io/drug/9U3GT3353T

It has been suggested that etonitazene is the most addictive substance on the planet.

As an opioid, it exhibits 1,500 times the potency of morphine, but it's also a stimulant on par with ecstasy. Basically, it's a molecular speedball. Although etonitazene has appeared as a street drug on a few occasions, the vast majority of the world's supply is consumed by mice and monkeys in addiction studies. Scientists know that rhesus monkeys lick their little furry lips with delight when you spike their water dish with etonitazene, but monkeys like a lot of shit, and as the old saying goes, never send a rhesus monkey to do a man's job.

Thomas Highsmith worked at a prestigious laboratory in Salt Lake City designing low-friction laminates for high-performance skis. In 2003, he started spending long nights in his lab secretly manufacturing a personal supply of etonitazene. Shortly after completing the synthesis, he became hopelessly addicted. He would show up for work clutching a 12-ounce spray bottle of etonitazene and fiendishly snort it throughout the day. Over the course of a couple of months, his tolerance escalated to the point where he was taking 300 times his starting dose. A coworker became suspicious of Highsmith's odd behavior and reported him to the police. His etonitazene supply was seized, and he was prescribed methadone to combat the withdrawal. At that point, his addiction equated to 500 bags of heroin a day, and the methadone umbrella did nothing to deflect the 10,000-pound etonitazene anvil hurtling toward his head. Highsmith never received a criminal sentence because he was found dead in his home before his first court date. The withdrawals were so severe that he had killed himself to escape the pain.

Drugs-Forum:
I know that etonitazene (and its analogs) was invented before fentanyls. But it has no go in human treatment, but fentanyl does. In Moscow (second half of 1990 s), it was used in some circles. It was used as neat powder (some salt of it), cotton needle threads were rubbed with it, so they absorb powder in there pores, then thread was inserted in ciggarette with the help of thin wire, prodding it along cigarette length. Then it was smoked. Someone who isn't me, found that it has somewhat different action, it was more stimulating then heroin, and it produced less dizzyness. Addicts prefer it to heroin. So one cancer patient used it instead prescribed morphine. After one use he trashed all morphine, and did not used it any longer. Etonitazene helped him to live until his death from cancer without pain, and be happy. Someone who isn't me just wants to know why when there exists such a miracle drug, it is not using in pharmacopea? Who knows some more facts related to humans (not monkey and rats).

The reason etonitazene never gained medical use was that it has to low of a safety margin unlike drugs like fentanyl, alfentanil, remifentanil and sufentanil. Etonitazene is listed on the UN list illicit drugs (i can't remember what the list is called) as well as Clonitazene which is only 3x stronger than morphine.

A synthetic opioid. Etonitazene has been shown to have approximately 1000 - 1500 times the potency of morphine in animals, but only 60 times in man. It has a strong dependency potential similar to that of morphine, and a strong tendency to produce respiratory depression, and is therefore not used in humans. It appears to have a steep dose-response curve making it particularly hazardous, more so than even fentanyl. Etonitazene and its related opioid agonist benzimidazoles were discovered in the late 1950's.