The review, conducted by researchers at the Universidade Federal do Rio Grande do Sul in Brazil, searched PubMed/Medline, Scielo and Google Scholar for articles and technical notes published between 2020 and 2024. From an initial pool of 108 works, 41 studies were included in the final analysis. The authors note that the United Nations Office on Drugs and Crime had recorded more than 120 opioid receptor agonists by 2024, divided into five subgroups: fentanyl analogues, U-series substances, nitazenes, piperazines and a miscellaneous category. Nitazenes stand out within this crowded field because of the sheer affinity of some analogues for the µ-opioid receptor. The first fatal case involving isotonitazene, one of the most notorious members of the family, was reported in the United States in August 2019, only months after the compound first appeared on the recreational market. Since then, metonitazene, butonitazene, flunitazene, etodesnitazene, protonitazene and N-pyrrolidinoetonitazene have all been identified in forensic casework, and between 2019 and 2025 a total of 32 nitazene analogues were reported by 36 countries.

What makes the current wave of nitazene activity particularly hazardous is the way these substances reach consumers. They are rarely sold as standalone products. Instead, they are frequently detected as adulterants in heroin, benzodiazepines, synthetic cannabinoids and even cocaine, often without the knowledge of the people who buy them. This adulteration broadens the population at risk far beyond habitual opioid users. In Brazil, a country where opioid misuse had historically been considered statistically insignificant, the arrival of nitazenes came through an unexpected channel: herbal smoking blends sold as synthetic cannabinoids. An analysis of 140 opioid samples seized by São Paulo state police over roughly one year found that 95 percent contained nitazene analogues, with metonitazene present in 72 percent of the nitazene-positive samples. In most of these cases the compounds appeared in combination with other drugs, meaning users were inhaling a potent opioid hidden inside a product they believed contained something else entirely.

The chemistry behind this potency is instructive. Nitazene analogues share a benzimidazole nucleus linked to an amine group, and small modifications to the side chains produce compounds with markedly different pharmacological profiles. Etonitazene, first synthesized in 1957, proved in animal models to be between 100 and 1000 times more potent than morphine, making it the most powerful compound of the 2-benzylbenzimidazole subclass. Isotonitazene ranked second in potency, followed by protonitazene, metonitazene and butonitazene. The electron-withdrawing nitro group at the 5-position of the ring, the structural feature that gives the nitazenes their name, reduces the basicity of the heteroaromatic ring and appears central to receptor binding. The basic properties of these molecules generally dominate over their acidic ones, owing to two basic moieties: the benzimidazole ring and a tertiary amine in the side chain. These subtle structural differences translate into dramatic differences in how strongly each compound activates opioid receptors and how long it stays bound to them.

Once inside the body, nitazene analogues follow a metabolic route that creates its own set of problems. Biotransformation is mediated primarily by cytochrome P450 enzymes in the liver, particularly the CYP2D6, CYP2B6 and CYP2C8 isoforms. Phase I metabolism oxidizes the parent compounds, introducing reactive and polar functional groups, while phase II conjugation adds glucuronides and sulfates to facilitate urinary excretion. Crucially, some of the metabolites generated along the way are pharmacologically active in their own right. The main urinary metabolite of isotonitazene, N-desethyl-isotonitazene, is actually more potent than the parent drug and has itself appeared for sale on the drug market. Metabolite profiling shows that the O-desalkyl metabolites of isotonitazene and metonitazene are the same molecule, complicating the interpretation of toxicological results, and in silico analysis of etazene suggests it is three times more lipophilic than morphine, crosses the blood-brain barrier readily and inhibits several major CYP enzymes. Genetic variation in CYP450 activity means metabolism can differ substantially between individuals, adding further unpredictability to overdose risk.

Clinically, nitazene exposure produces the effects expected of powerful opioids: euphoria, pain relief and relaxation, followed by drowsiness, dizziness, nausea, constipation and dry mouth. The life-threatening effect is respiratory depression, which can progress to cardiac arrest. Prolonged exposure leads to tolerance, dependence and a characteristic withdrawal syndrome that begins with anxiety, drug craving and lacrimation before escalating to tremors, muscle pain, tachycardia, hypertension and gastrointestinal distress. What distinguishes nitazenes from earlier opioid threats is the difficulty of reversing their overdoses with naloxone, the standard opioid antidote. Laboratory work suggests that nitazenes and fentanyl analogues dissociate slowly from the µ-opioid receptor, reducing naloxone sensitivity. A systematic review found that nitazene overdoses typically required naloxone doses in the range of 1 to 6 milligrams, compared with the 0.4 to 0.8 milligrams usually sufficient for heroin. In one Brazilian clinical case involving N-pyrrolidinoprotonitazene, a patient with a blood concentration of 3.0 nanograms per milliliter required three ampoules of naloxone, equivalent to 1.2 milligrams, before respiratory function improved. The short half-life of naloxone relative to these synthetic opioids also means respiratory depression can relapse, demanding repeated dosing and sustained monitoring.

Detecting nitazenes in biological samples presents a formidable analytical challenge. Because these compounds are so potent, they are consumed at microgram doses and clear the body rapidly, so timely sampling and highly sensitive instruments are essential. Liquid chromatography coupled with tandem mass spectrometry has become the method of choice, typically using whole blood as the primary matrix and liquid-liquid extraction for sample preparation. Related techniques, including GC-MS and LC-QTOF-MS, allow detailed chemical differentiation between analogues. Forensic scientists emphasize that robust documentation of exposure requires detecting the parent compound alongside N-desethyl, O-desalkyl and N-desethyl-O-desalkyl biomarkers, since metabolites are often more abundant than the original drug. Adding to the confusion, illicit production by the original synthetic route can generate positional isomer impurities that confound analytical interpretation. The review also highlights a promising harm-reduction tool: nitazene test strips, a lateral flow immunoassay commercially available since early 2024 and calibrated against isotonitazene. However, a Belgian laboratory evaluation found the strips failed to detect two analogues, metodesnitazene and etodesnitazene, even at high concentrations, underscoring how quickly the market evolves beyond the tools designed to monitor it.

The regulatory response has been uneven. Brazil formally controlled metonitazene in 2023 and has since listed ten more analogues, while its national rapid alert system for new psychoactive substances was only permanently formalized in early 2025. In a potentially significant shift, China introduced generic control of the entire nitazene structure in June 2025, meaning any new analogue containing the core scaffold is now illegal. The downstream effects on the illicit market remain uncertain, but similar generic controls on fentanyl precursors have previously reshaped supply chains in unpredictable ways. The review’s authors stress that underreporting remains a major obstacle to understanding the true scale of the problem, since many poisonings go unidentified without specialized testing and standardized reporting protocols are still lacking.

The core message from the toxicologists is that health systems must adapt before the crisis deepens. The combination of extreme potency, active metabolites, polysubstance adulteration and naloxone resistance creates a clinical scenario for which many emergency services are not adequately prepared. The authors call for specific intervention protocols, training for health professionals on the risks of synthetic opioids, and urgent advances in toxicokinetic and toxicodynamic research to support detection, treatment and rehabilitation programs. Without that foundation, they warn, public health authorities will continue to underestimate both the prevalence and the lethality of these compounds. As nitazene analogues spread through adulterated drug supplies worldwide, the gap between what users think they are consuming and what is actually in the sample may prove to be the deadliest variable of all.

Subject of Research: Toxicology of emerging nitazene analogue synthetic opioids and their overdose risks

Article Title: Toxicological aspects of emerging nitazene analogues as a new threat to the synthetic drug landscape

Article References: Martins, J. N., dos Santos, B. P., & Arbo, M. D. (2026). Toxicological aspects of emerging nitazene analogues as a new threat to the synthetic drug landscape. Discover Toxicology, 3(1), Article 5. https://doi.org/10.1007/s44339-026-00050-4

Image Credits: AI Generated

DOI: 10.1007/s44339-026-00050-4

Keywords: nitazenes, synthetic opioids, isotonitazene, metonitazene, fentanyl, naloxone, overdose, new psychoactive substances, toxicokinetics, respiratory depression, forensic toxicology, drug detection