You've probably heard of fentanyl, which can be up to 50 times stronger than heroin and 100 times stronger than morphine. What you might not know is that there's another synthetic opioid out there that is structurally related to fentanyl called ocfentanil or A-3217 that's even more potent than fentanyl. It was developed in the early 1990s as a naloxone-reversible drug, with the hopes that it would have more therapeutic benefits than fentanyl while having fewer cardiovascular effects and less chance of causing respiratory problems. Keep in mind that fentanyl isn't only a drug responsible for a lot of addiction and overdoses; it's used legally as a painkiller and for anesthesia. Ocfentanil was also studied as an anesthetic and showed some promise in being more effective than morphine for pain relief after surgery, but it was never actually developed for medical use. Around 2013, it started being used as a club drug.

Ocfentanil is an opioid analgesic that is an analogue of fentanyl was developed in the early 1990s. It is similar to fentanyl in effects, producing strong analgesia and sedation, but is slightly more potent. Side effects of fentanyl analogues are similar to those of fentanyl, and include itching, nausea and potentially serious respiratory depression which can be life-threatening.

World Health Organization 2017:
Ocfentanil, a compound structurally similar to the opioid analgesic fentanyl, was developed in the early 1990's with the hope that it would provide a better clinical safety profile than fentanyl. The receptor pharmacology of ocfentanil is not available publicly, but it appears to share pharmacodynamic effects with fentanyl and other U opioid agonists, including analgesia, sedation, and respiratory depression. In rodents, ocfentanil was approximately 2.5 times more potent as an analgesic than fentanyl and had a shorter duration of action. Because the preclinical research suggested that ocfentanil had a better safety profile than fentanyl, it was selected for clinical evaluation. In humans, however, ocfentanil had a similar potency (3kg ocfentanil produced effects that were comparable to 5kg fentanyl) and side-effects profile as fentanyl so further clinical development was discontinued. Ocfentanil is not approved in any country for medical use and is under national control in Canada, the United Kingdom, and China. At least 3 deaths in Belgium and Switzerland related to ocfentanil have been reported. Ocfentanil is currently being sold as heroin but no user discussions of the effects of ocfentanil specifically were found on the Internet. It does not appear to be a common adulterant in heroin, but its high potency is a public health concern in terms of opioid overdose risk.

OCFENTANIL Critical Review Report (PDF 17 pages):
https://www.who.int/medicines/access/controlled-substances/CriticalReview_Ocfentanil.pdf?ua=1

2015 Article:
More potent than heroin, Ocfentanil has similar effects to fentanyl, producing strong analgesia and sedation and presents a clear risk for overdose. Side effects include itching, nausea, and potentially serious respiratory depression which can be life-threatening. Ocfentanil is easy to overdose on as it is so powerful. The major issue with fentanyl and analogs is that they are very potent and active at microgram levels, the spokesman said. As such taking an amount of the substance believing it to be something else, for example heroin, could be life-threatening.

Between March 19, 2015, and July 16, 2015, Wedinos received six samples in powder form that upon analysis were found to be Ocfentanil. Three of the substances were white while three were brown. The route of administration for these samples was snort/sniff in two cases, smoked in one, and the remaining three samples did not have any details around the route of administration, the spokesman said. Effects experienced included euphoria, being relaxed, nausea, chest pains, paranoia and agitation. The one substance in all of them is caffeine. How are they going to ban that? Ocfentanil might not be being used by people normally associated with taking heroin. You would not normally mix heroin with caffeine, Mr Blakebrough said. o this is probably used more on the party scene than by those that are taking heroin to chill out. I would imagine this is being used in the club scene more than by those on the methadone programme. He urged people to steer clear of the substance. The clear message is not to take anything if you have no idea what it is, he said.

Ocfentanil, a compound structurally similar to the opioid analgesic fentanyl, was developed in the early 1990's with the hope that it would provide a better clinical safety profile than fentanyl. The receptor pharmacology of ocfentanil appears to share pharmacodynamic effects with fentanyl and other u opioid agonists, including analgesia, sedation, and respiratory depression. In rodents, ocfentanil was approximately 2.5 times more potent as an analgesic than fentanyl and had a shorter duration of action. Because the preclinical research suggested that ocfentanil had a better safety profile than fentanyl, it was selected for clinical evaluation. Like other u opioid agonists, ocfentanil has been reported to produce itching, nausea, sedation, and severe respiratory depression. Chest pain, psychosis, and agitation have also been reported. In humans, however, ocfentanil had a similar potency (3 ug/kg ocfentanil produced effects that were comparable to 5 ug/kg fentanyl) and side-effects profile as fentanyl so further clinical development was discontinued. Ocfentanil is not approved in any country for medical useand is under national control in Canada, the United Kingdom, and China.

Ocfentanil:
https://drugs.ncats.io/drug/MX52WBC8EV

Ocfentanil was never developed for medical use despite reasonable results in human clinical trials, but subsequently started to be sold as a designer drug starting in around 2013. Ocfentanil being approximately 2.5 times as potent as fentanyl. Fentanyl analogs have killed hundreds of people throughout Europe and the former Soviet republics since the most recent resurgence in use began in Estonia in the early 2000s, and novel derivatives continue to appear.