U-47700. An old opioid becomes a recent danger

U-47700 is a synthetic opioid analgesic and a potent, short acting structural isomer of the earlier opioid AH-7921 that has recently invaded the drug arena in Europe and the Unites States. Although the drug was synthesized and patented in the 1970s, it was first identified in October 2014, as a powder sample that was seized by Swedish Customs. Sweden formally notified the European Union Early Warning System in January 2015. Animal studies proved that U-47700 is a strong μ-opioid receptor agonist and has a morphine-like analgesic action, being 7.5 times higher than morphine. The drug has a much lower affinity for the κ-opioid receptor. This newly appearing psychoactive substance has already led to more than 25 confirmed fatalities associated with U-47700 in Europe and the United States and to six non-fatal intoxications reported in the United States. The aim of this review is to summarize the current knowledge about this drug, regarding its chemistry, synthesis, pharmacology, toxicology and metabolism, as well as its international legal status. The existing analytical methods for the determination of U-47700 in biological samples are also presented. Published or reported U-47700 related cases, fatalities or intoxications, and self reports from drug users are reviewed.