Abstract
Despite the current shift toward the so-called novel synthetic opioids (NSOs), such as nitazenes, fentanyl and its analogs (fentalogs) are still being extensively studied, since they represent a serious problem in the drug market. Fentanyl is notable for its rapid onset of action and a potency approximately 100 times greater than morphine. These properties, combined with the simplicity and low cost of its synthesis, have favored its rapid global spread. Fentalogs were initially studied for their pharmacological properties, but then their use as illicit drugs has expanded. More than 1400 fentalogs have been described in patents and scientific literature. They differ for only minor modifications. The existence of all these analogs shows how the illicit drug market is sadly faster than the actual drug market. The most detected NSOs belonging to fentalogs are acetylfentanyl, carfentanyl, furanylfentanyl, cyclopropylfentanyl, and acrylfentanyl, but some other compounds are increasing in use. Since different names correspond to the same compound, we tried to dissipate confusion and bring some order to this jumble of names. Furthermore, different compounds are sometimes confused; for instance, thiafentanil is occasionally referred to as thiofentanil, despite the two compounds differing in their structures. In this review, we summarize the most cited fentalogs, along with their structure and characteristics. The CAS (Chemical Abstracts Service) Registry number was also reported for each compound, as it is a unique feature to indicate a substance. To find pertinent information on fentanyl and fentalogs, searches were conducted using PubMed, SpringerLink, Science Direct, Wiley Online, Scopus, Reaxys, and Web of Science.