Is nitazene-related mortality underestimated? Findings from an <i>in vivo</i> and <i>ex vivo</i> rat study and pharmacoepidemiological analysis of coroner-reported deaths

Introduction Nitazenes are potent synthetic opioids. Following reports questioning their post-mortem stability, nitazene-related deaths may have been underestimated in the United Kingdom. We investigated this using a rat model and regional coronial data, and also present national pharmacoepidemiologic trends in nitazene-related deaths.Method In vivo/ex vivo study: Anaesthetised Wistar rats (n = 12) received intravenous nitazene (metonitazene, N-desethyl isotonitazene, or N-pyrrolidino etonitazene). Rats were euthanised 15 min post-administration if cardiorespiratory arrest had not already occurred (n = 8). Blood and urine were collected, with repeat blood samples taken following cadaver refrigeration (4 °C) for one week. All samples were immediately frozen (−80 °C). Upon defrosting, half were analysed immediately by liquid chromatography tandem mass spectroscopy with half stored at 4 °C for 1 month before analysis.Pharmacoepidemiology Nitazene deaths were extracted from the National Programme on Substance Use Mortality in March 2025 along with all deaths from the Birmingham & Solihull coronial area (2019–2023). Descriptive analyses were conducted along with exponential smoothing models to compare observed and forecasted deaths in Birmingham & Solihull in 2023.Results In vivo/ex vivo study: A small fraction of the nitazene detected in the immediate post-mortem blood sample remained in the post-mortem day 7 blood sample that had been refrigerated for 1 month.Pharmacoepidemiology In Birmingham and Solihull, non-nitazene drug deaths rose 33% in 2023 compared to 2019–2022 (predicted n = 107, actual n = 142). By March 2025, 285 deaths with nitazene detections were reported to the National Programme on Substance Use Mortality, with small clusters in 2021 (n = 23) and 2022 (n = 15) before markedly increasing in 2023 (n = 131). Twelve nitazenes were detected with the predominant nitazene shifting over time (2021: isotonitazene; 2022: N-pyrrolidino etonitazene; 2023: N-desethyl isotonitazene). Coroners deemed nitazenes causative in 90% of cases (n = 256/285).Conclusions Nitazene-related deaths have increased in England, Wales, and Northern Ireland, and may have been underestimated due to post-mortem instability. Urgent public health action is required to reduce nitazene-related harms.