Postmortem and Toxicological Findings in a Series of Furanylfentanyl-Related Deaths

Over the course of 4 months in 2015 and 2016, a cluster of seven fatal intoxications involving the opioid-analogue furanylfentanyl occurred in Sweden; toxicological analysis showed presence of furanylfentanyl either as the only drug or in combination with other illicit substances. Previous publications have only reported non-lethal furanylfentanyl intoxications. In the cases presented here, furanylfentanyl intoxication-alone or in combination with other drugs-was determined to be the cause of death by the responsible pathologist. All victims were young (24-37 years old) males, five of which had a well-documented history of drug abuse. Femoral blood concentration of furanylfentanyl ranged from 0.41 ng/g to 2.47 ng/g blood. Five cases presented a complex panel of drugs of abuse and prescription drugs. Moreover, in five cases the concurrent presence of pregabalin corroborates previous observations indicating pregabalin as a possible contributing factor in polydrug intoxications. We conclude that it is difficult to establish a specific lethal concentration of furanylfentanyl, due to incompletely known effects of possible pharmacokinetic and pharmacodynamic interactions with other drugs, as well as to the unknown degree of tolerance to opioids. We suggest that a full toxicological screening-to assess the possibility of drug interactions-together with segmental hair analysis regarding opioids-to estimate the level of opioid tolerance-be carried out to assist in the interpretation of cases involving synthetic opioids such as furanylfentanyl.