Non-fentanyl-derived synthetic opioids emerging during recent years

PURPOSE: Since the appearance of fentanyl followed by its many kinds of analogues around 1988, North America has been exposed to fierce synthetic opioid pandemic resulting in more than 130,000 deaths due to their overdoses until May 2019, when China declared to prohibit the licit fentanyl analog production. However, the Chinese announcement did not go into force in USA due to the adroit strategies of tough traffickers. Thus, contrary to the expectation, the number of synthetic opioid products and their poisoning cases in USA has increased by about 30%; especially, various benzimidazole synthetic opioids have revived on the illicit drug market during a recent few years. In this article, the recent abrupt changes in the situations of illicit synthetic opioid market and their current abuses are described. METHODS: Various databases, such as SciFinder, Google, and Google Scholar, were utilized to collect relevant reports referring old but newly appearing synthetic opioids. RESULTS: At the present time, there are several families of new synthetic opioids, which are not fentanyl derivatives; MT-45 and its analogs, benzamide and 2-phenylacetamide opioids (U-series opioids), and benzimidazole opioids. Most of the above substances had been developed in 1950s to 1970s, but had never been used as analgesic medicines, because of their severe adverse effects, such as respiratory depression, physical dependence, and resulting deaths. However, there is possibility that these drugs will become main illicit synthetic opioids in place of the fentanyl analogs during coming several years from this time. CONCLUSIONS: All of the above non-fentanyl-derived families had been developed 50–70 years ago to establish them as analgesic medicines, but had been unsuccessful. These drugs largely appeared in the illicit drug markets in North America, Europe, and Australia, during recent years. Pharmacological, toxicological, and metabolic studies are insufficient for benzamide and 2-phenylacetamide opioids, and are very scant especially for benzimidazole opioids. This time we should start studying pharmacotoxicology of the newly emerging synthetic opioids to alert forensic toxicologists in the world and to suppress their rapid and wide spread in the world. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s11419-022-00624-y.