Xylazine, a Drug Adulterant Whose Use Is Spreading in the Human Population from the U.S. to the U.K. and All Europe: An Updated Review

Abstract

Xylazine, commonly called “tranq” or “sleep cut”, is a strong α2-adrenergic agonist used in veterinary practice as a sedative, analgesic, and muscle-relaxing agent. It has never been approved by the Food and Drug Administration for human use, but its use by people is on the rise. In the last decades, due to its low cost and ease of availability, it has often been illicitly used due to its abuse potential as a drug for attempted sexual assault and intended poisoning. In addition, xylazine’s presence in the human body has also been related to domestic accidental events. Generally, it is combined with multiple other drugs, typically by intravenous injection, potentiating the doping effects. Xylazine’s mechanism of action is different from that of other illicit opioids, such as heroin and fentanyl, and it has no known antidote approved for use in humans. The combination with fentanyl prolongs the euphoric sensation and may heighten the risk of fatal overdose. Furthermore, it may cause adverse effects, including central nervous system (CNS) and respiratory depression, bradycardia, hypotension, and even death. Recent reports of xylazine misuse have risen alarmingly and describe people who become “zombies” because of the drug’s harmful effects on the human body, including serious wound formation that could even lead to limb amputation. This paper is an extensive review of the existing literature about xylazine and specifically deals with the chemistry, pharmacokinetics, pharmacodynamic, and toxicological aspects of this compound, highlighting the most recent studies.

1. Introduction

Xylazine (Rompun^®, Proxylaz^®, Chanazine^®, Xylazin^®), generally referred to as “tranq” or “aka tranq”, is a potent non-opioid sedative and is currently used as an adulterant in combination with fentanyl in drug intoxications, with significative implication for public health, safety, fatalities, and criminal investigation [1,2,3,4,5,6,7,8,9]. Xylazine was first identified in Puerto Rico (2001) and Philadelphia (2006), where about 90% of fentanyl was contaminated with this adulterant [10]. After rapid diffusion in the U.S. illicit drug market [11,12,13], it has now penetrated the U.K. and even Europe [14,15]. Xylazine was initially developed as a tranquilizer and analgesic in veterinary medicine and animal experimentation since it is an α2-adrenergic agonist for producing antinociception, sedation, and muscle relaxation. It is still used for these purposes in animals, generally in association with ketamine, as evidenced by recent studies in different fields [16,17,18,19,20,21,22,23]. Xylazine is also used as a horse doping agent in equine and racing practice, and its use in this field is highly regulated [24,25,26]. Actually, it is classified as a Class 3 performance-enhancing agent (penalty class B) [27] by the Association of Racing Commissioners International (ARCI) [28]. However, an acceptable daily intake for xylazine has never been established, as assessed by the 47th Joint FAO/WHO Expert Committee on Food Additives meeting [29,30], since one of its metabolites, specifically, 2,6-xylidine is considered “genotoxic and carcinogenic” [31,32,33]. The immunomodulatory effect of xylazine has been described both in vitro and in vivo [34]. In recent years, a significant increase in xylazine abuse potential has been demonstrated, together with its use as a drug for attempted sexual assault and as a source of accidental or intentional poisoning with criminal intent [35,36,37,38,39,40]. Xylazine has earned the nickname “zombie drug” for its severe sedative effect and necrotic skin ulcerations (Figure 1) [41,42]. It is generally used as an adulterant in association with opioids, stimulants, benzodiazepines, and cannabinoids [43,44]. It is often referred to as “tranq dope” [45], “anestecia de caballo” or “horse anesthetic” on its own. It is also used in combination with illegal opioids, including heroin, fentanyl, or speedball (a mixture of heroin and cocaine) [43,46]. In humans, xylazine may cause respiratory depression, bradycardia, hypotension, as well as symptoms of xylazine withdrawal [47]. These deleterious effects on humans cannot be fully reversed by the α2-adrenergic receptor antagonists, suggesting the possibility that xylazine targets receptors other than α2-adrenergic. Recently, α7-nicotinic acetylcholine receptors have been suggested as targets of xylazine in studies in Xenopus oocytes [48].

The unintentional ingestion of xylazine by consuming a spiked drink containing it or suicide attempts can lead to unconsciousness and death. The first cases of human intoxication with xylazine date back to the 1980s [35,49,50] but have increased considerably in recent years [51,52]. Reports of adulteration of heroin and illicit fentanyl with xylazine continue to increase worldwide, particularly in Western countries [53]. However, the absence of specific symptoms often delays the diagnosis and treatment of xylazine poisoning. In addition, the role of xylazine in causing death is often complicated by the presence of other toxins. Several factors, including the age of the victim, pre-existing morbidities, and co-exposure, are considered in xylazine poisoning. Thus, the exact lethal concentration of xylazine has not been yet established [54,55]. In addition, people with xylazine wounds frequently engage in heterogeneous wound self-treatment practices [56]. Moreover, no antidote against xylazine poisoning in humans has been approved. Although naloxone is still recommended in cases of opioid overdose, it is not yet known whether the effects of xylazine can also be reversed by naloxone [57]. However, α2-receptor antagonists have been suggested as xylazine antidote in humans [58]. Considering the significant public health concern because of its increasing misuse, in this review, we summarize the chemical aspects of xylazine, also underlining the pharmacokinetic properties and involvement of this molecule in intoxications and overdoses. A literature search was conducted on the PubMed/MEDLINE, Scopus, and Google Scholar search engines, using the keywords “xylazine”, “intoxication”, “abuse”, “tranq”, and “zombie”, mainly focusing on the last five years published papers.

2. History and Chemistry of Xylazine

Xylazine (Figure 2) was synthesized in 1962 and identified by the code Bay Va 1470 [59]. Its use as an antihypertensive in animals was promoted by Farbenfabriken Bayer Ltd. (Leverkusen, Germany). However, it was not approved for human use due to severe hypotension and high CNS depression. Hence, it was registered with the name Rompun^®, and it was only for veterinary use in farm animals such as cattle and horses (European Medicines Evaluation Agency, EMEA) [60]). Later, its use was expanded to other animals, including sheep, goats, llamas, and buffaloes [61,62].

Xylazine [N-(2,6-dimethylphenylamino)-4H-5,6-dihydro-1,3-thiazine or N-(2,6-dimethylphenyl)-5,6-dihydro-4H-1,3-thiazin-2-amine or 2-(2,6-dimethylphenylamino)-4H-5,6-dihydro-1,3-thiazine or 2-(2,6-xylidino)-5,6-dihydro-4H-1,3-thiazine, C₁₂H₁₆N₂S, CAS Number: 7361-61-7] is a white crystalline solid, developed as a clonidine analog acting as an α2-adrenoceptor agonist [63]. It bears a 2,6-xylyl moiety and a thiazine one, so it belongs specifically to the group of 1,3-thiazines, which are structurally similar to 1,4-thiazines (phenothiazines) [64]. The 2,6-xylyl (or 2,6-dimethylphenyl) moiety can be found in numerous clinically relevant drugs [65], whereas the dihydro-1,4-thiazine cycle is related through cyclomethylene homology and isology to the 2-imidazoline ring, a well-known privileged scaffold (Figure 3) present in the structure of the α₂-adrenergic agonist clonidine.

The synthesis of xylazine is relatively simple and has been reported in several papers, mostly patents [66]. It is commonly used as hydrochloride salt and has four well-known polymorphic conformations (A, M, Z, and X) and a monohydrate form (H), which is the most stable [67]. A novel polymorph (Y) was recently reported by Abdulla et al. (2023) [68]. Different partition coefficients (logP) have been reported for xylazine: logP = 2.37 [69], XlogP = 2.8 [70], Calc logP = 1.34 [71,72]. Xylazine as a free base is not water-soluble, whereas xylazine hydrochloride is very soluble both in water and MeOH, while it is by far less soluble in hexane and Et₂O [73]. The pK_a value reported for xylazine is 6.94 [74,75]. The melting point depends on the considered xylazine hydrochloride polymorphic form. Specifically, the xylazine hydrochloride form A melting point was reported to be 165.3 °C by Krūkle-Bērziņa et al. [76]. In his Master’s thesis, Abdulla [77] reported the melting points for all the forms of xylazine. Polymorphs X and Z have lower melting points (105–109 °C), whereas the other forms have melting points varying between 165 and 169 °C. The monohydrate melting point was 166–167 °C.

3. Pharmacokinetics and Pharmacodynamics of Xylazine

The intramuscular or subcutaneous injection of xylazine in horses leads to rapid absorption and produces a fast onset of action with a short duration [78]. Xylazine is quickly metabolized by liver cytochrome P450, specifically cytochrome P4503A (CYP3A) [66,79], and eliminated by the kidneys with a biological half-life ranging between a few minutes to three hours [40,60,80,81]. The severity and duration of sedative or analgesic effects depend on the dose and the examined species [28,82,83,84,85,86,87]. Xylazine is 10–20 times more potent in ruminants than in other species [88]. Specifically, cattle are the most sensitive [81,89]; the dose used is about 1/10 of that adopted in horses or dogs to achieve the same sedative or analgesic level [82]. The rapid absorption of xylazine and spread in the body lead to the accumulation in the brain and kidneys a few minutes after the intravenous administration compared to other organs. The usual dose in animals is 0.5–2.0 mg/kg [90]. The sedative effect may appear immediately (5 min) and last up to 4 h [49,91]. Xylazine is also often used in animals for anesthetic premedication [92,93,94,95]. Intramuscular administration of xylazine (0.6–1.4 mg/kg) in large animals, such as sheep, leads to the achievement of the maximum plasma concentration within 0.2~0.3 h [96]. The rate of excretion depends on the species considered. Garcia-Villar et al. (1981) [82] reported that less than 1% is excreted unchanged in cow urine and approximately 8% in rat urine. When injected into the subarachnoid space, xylazine causes prolonged analgesia with a long-lasting duration of action and antinociceptive effect [97], even though side effects may occur [98]. The analgesic effect is mediated by α2-adrenergic receptors in the dorsal horn of the spinal cord [99,100]. The agonism of α-2-adrenergic receptors present at the presynaptic axon terminals, exerted by xylazine, blocks the neurotransmitters release into the synaptic cleft [101]; thus, reducing the release, for instance, of norepinephrine and serotonin into the synaptic cleft and, consequently, the injection of xylazine produces sedation and relaxation in the CNS [11]. Furthermore, norepinephrine is involved in arousal and, most importantly, in regulating blood pressure through binding to α-1 receptors on blood vessels, causing vasoconstriction [102]. This implies that a xylazine injection potentially impairs arterial blood pressure [103]. In humans, the clinical effects of α-2 agonism are hypotension (sometimes preceded by hypertension), lethargy, drowsiness, hyperglycemia, bradycardia, and potential apnea requiring intubation.

Table 1. Pharmacological parameters of xylazine in humans.

t1/2	Tmax	Fatal Dose Range	Approximate Effect Duration
23–50 min	12–14 min	40–2400 mg	4–6 h

Data taken from Hoffman et al. (2024) [66].

summarizes the available pharmacological parameters of xylazine in humans [66].

Studies on the Metabolism of Xylazine

The first studies on xylazine metabolism date back to 1973, when Putter and Sagner (1973) [104] demonstrated that the major metabolite excreted in cattle urine, in both free and conjugated forms, was 2,6-xylidine (or 2,6-dimethylaniline, 1, R = H), most likely produced via CYP450 through N-S-dealkylation. Then, other metabolites were identified in horses [105], and the existence of 4-hydroxy-2,6-dimethylaniline (2, R = H; R¹ = 4-OH), 2,6-dimethylphenylisothiocyanate (3) and 3- and 4-hydroxyxylazine (4, R¹ = 3-OH or 4-OH) in rat urine was also established [106]. The most common phase I metabolites are presented in Figure 4.

The formation of 2,6-dimethylaniline in equine urine was reported for the first time by Spyridaki et al. (2004) [78]. Meyer et al. (2013) [37] reported that xylazine was N-dealkylated and S-dealkylated oxidized and/or hydroxylated to twelve phase I metabolites, and the phenolic metabolites were partly excreted as phase II glucuronides or sulfates. All the metabolites were identified both in rat and human urine. Recently, Matos et al. (2020) [107] described some other metabolites, including those deriving from oxidation (5, Figure 3) and acetylation at the thiazine ring, in zebrafish (Danio rerio), a human-like surrogate, used as an alternative model for the study of the metabolism of xylazine. Recently, Hoffman et al. (2024) [66] summarized all the main metabolites derived from xylazine metabolism in animal studies. However, in humans, there is still a limited understanding of the clinical pharmacology and effects of xylazine [108,109,110]. The lipophilic nature of xylazine allows it to cross the blood–brain barrier, enabling the α2-receptor agonism not only in the peripheral nervous system but also in the CNS. The clinical relevance of the possible biological activities of the main xylazine metabolites, apart from the well-known toxicological potential of anilines as genotoxic, carcinogenic [31,32,33], methaemoglobinemia inducers [111], and the general toxicity of aryl isothiocyanates [112] has been poorly explored until now. Very recently, a complex pattern of Gi agonistic properties at the opioid nociception receptors (µOR, δOR, κOR) and dopamine D2R heteroreceptor was disclosed for 3-hydroxy- and 4-hydroxyxylazine [57]. 3-Hydroxyxylaxine shared the same G protein biased agonism at κOR and α-adrenergic receptor, subtype 2A (α_2A-AR) with its parent compound. The pharmacological profile and clinical impact of these metabolites in humans are still under study [108].

4. Methods for the Detection of Xylazine and Its Metabolites

Several analytical methods [113] have been described for the identification of xylazine in animal-derived food, including high-performance liquid chromatography (HPLC) [114,115,116], gas chromatography (GC)/mass spectrometry (MS) [78,117,118,119,120], and LC-MS [121,122]. Park Choo et al. (1991) [106] demonstrated the presence of 4-hydroxy-2,6-dimethylaniline, 2,6-dimethylphenylisothiocyanate, and 4-hydroxyxylazine in urine by comparison with chemically prepared standards with GC/MS. The simultaneous analysis of xylazine and some of its metabolites in animal-derived food has also been addressed. Holland et al. (1993) [123] described the use of an HPLC method for the detection of xylazine along with 2,6-xylidine in bovine and swine kidneys, with 25 ppb as the limit of detection (LOD). Zheng et al. (2013) [124] reported a sensitive, accurate, and reproducible LC-MS/MS method for the detection of xylazine along with 2,6-xylidine in animal tissues (liver, meat, kidney, and fat). Due to its increasing prevalence, xylazine is now often studied along with classical opioids and novel synthetic opioids (NSO) for their detection in the blood and urine of humans [125]. The most recent methods described for the detection of xylazine are summarized in

Table 2. Methods used for the detection of xylazine.

Method of Detection	LOD	Sample	Ref.
Electroanalytical method using glassy carbon electrode	120 nmol/L	Pharmaceutical and Urine samples	[126]
Portable electrochemical devices integrated with a smartphone	0.06 μg/mL	Beverage samples	[75]
Laser-scribed graphene on polyetherimide substrate	1.39 × 10−7 mol/L	Urine and Beverage samples	[127]
Monoclonal-antibody-based lateral flow immunoassay	0.1 ng/mL	Milk	[128]
Colorimetric assay using molecularly imprinted polymers	1.36 mM *	Gin and tonic drinks	[129]
Enzyme-linked immunosorbent assay	0.1 ng/mL **	Oral fluid	[130]
Portable electrochemical test strips based on Au/graphene	0.02 µg/L	Raw milk	[131]
Portable amperometric sensor with anti-fouling properties	~1 ppm *	Alcoholic and Non-alcoholic beverages	[133]

* Results obtained in 20 s; ** Results obtained in 24 h.

. Mendes et al. (2019) [126] reported the first electroanalytical method for xylazine quantification in pharmaceutical, veterinary formulations, and urine, using a glassy carbon electrode, which is simple, sensitive, and accurate for clinical and forensic applications. The LOD was 120 nmol/L. Saisahas et al. (2022) [75] described, for the first time, a portable electrochemical device for xylazine detection, consisting of an electrochemical paper-based analytical device (ePAD) integrated with a smartphone, which leads to the detection of xylazine by using differential pulse voltammetry. The sensor is low-cost, single-use (disposable), and able to evaluate traces of xylazine in beverage samples, demonstrating excellent accuracy in the detection of xylazine. The LOD was 0.06 μg/mL. De Lima et al. (2022) [127] reported a laser-etched graphene on a polymeric polyetherimide substrate as an electrochemical device for the detection of xylazine in urine and beverage samples and suggested its use in forensic studies as a low-cost, disposable device. Chen et al. (2022) [128] described a monoclonal-antibody-based lateral flow immunoassay and suggested its use for the detection of xylazine in milk, with an LOD of 0.1 ng/mL. Recently, Marroquin-Garcia et al. (2024) [129] described the use of a colorimetric assay using molecularly imprinted polymers with high selectivity for the detection of xylazine in alcoholic beverages. The assay required minimal sample pretreatment. A dip-stick-like device was developed and tested for xylazine detection in gin and tonic drinks, yielding positive results in just 20 s, with an LOD of 1.36 mM. Levitas et al. (2024) [130] described an enzyme-linked immunosorbent assay for rapid screening of xylazine in oral fluid. The quantification of the drug was as low as 0.1 ng/mL. Zheng et al. (2025) [131] reported a portable electrochemical test strip based on a gold nanoparticles/reduced graphene oxide-modified screen-printed carbon electrode which is capable of rapid detection of xylazine in in raw milk samples with a detection limit as low as 0.02 μg/L. He (2025) [132] studied the use of Raman hyperspectral imaging analysis combined with the multivariate curve resolution-alternating least square algorithm for the detection of xylazine in drug mixtures, including acetaminophen, dipyrone, and mannitol. The method successfully resolved xylazine at concentrations as low as 5%, 10%, and 25%. Vinnikov et al. (2024) [133] described the use of a portable amperometric sensor with anti-fouling properties to detect xylazine in beverage samples and/or from the residue of consumed beverages. It showed high sensitivity, rapid response times (<20 s), and low estimated detection limits (~1 ppm). These sensors can effectively detect xylazine at <10 ppm in both common alcoholic and non-alcoholic beverages, requiring only a minimal volume (20 µL) of the spiked beverage for a standard addition analysis.

5. Combinations of Xylazine with Other Drugs

Xylazine is often combined with ketamine as a sedative and analgesic during surgery in animals [134,135], such as equines [136,137], and with lidocaine [138,139,140,141] since xylazine greatly enhances the anesthetic and sedative properties of these drugs. A synergistic interaction between ketoprofen and xylazine on the level of cyclooxygenase-2 in mice has been reported [142]. Moreover, the administration of carprofen in combination with xylazine in the castration process of male goats is likely beneficial, as it enhances plasma concentration of carprofen and decreases clearance in castrated male goat kids [143]. In a recent study in West African Dwarf goats, the combination of xylazine with propofol provided an analgesic duration higher than propofol alone [144]. Some diseases might be exacerbated by the use of xylazine in combination with ketamine, and some animals might be affected by its use. It has been recently reported that the administration of ketamine/xylazine in mice has increased the severity of influenza (A/Puerto Rico/8/34) [145].

In the unregulated market, xylazine is generally used as an adulterant in recreational drugs, with synthetic opioids, mainly fentanyl [146], as it allows the illicit production costs to be lowered without losing the euphoric effects of the adulterated compounds [146]. The attraction of the mixture for drug addicts is that the xylazine-doped fentanyl drugs extend the relatively short-lived “high” of fentanyl. The first signs of xylazine occurred when clients noticed an unidentified adulterant in the unregulated opioid supply that produced rapid, tranquilizer-like sedation that was unlike fentanyl. Initially, it was thought that the new substance “was just good fentanyl” that produced more intense sedative effects. However, clients then began experiencing a more rapid onset of withdrawal symptoms; then, necrotic wounds were also evidenced [147]. Between 2015 and 2020, the presence of xylazine in toxicology testing of fatal opioid overdoses in the U.S. was greatest in Philadelphia, Maryland, and Connecticut [148]. Friedman et al. (2021) [35] reported that 98% of the cases in which xylazine was detected in fatal opioid overdoses in the U.S. were fentanyl. Moreover, in Philadelphia, there is an increasing presence of xylazine in the local fentanyl/heroin drug market [149]. In addition, it may be combined with cocaine and methamphetamine and, more recently, with oxycodone and alprazolam [46]. Teoh 2022 [26] summarized several examples of drugs reported in co-presence with xylazine, which included analgesics (heroin, fentanyl, carfentanyl, codeine, morphine, butorphanol, methadone, phenacetin, stimulants (cocaine, methamphetamine, methylphenidate, caffeine), anesthetics (ketamine, combination of tiletamine and zolazepam, detomidine, lidocaine, procaine, phencyclidine), antiepileptics (phenytoin, pentobarbital), antipsychotics (haloperidol), antimalarials (quinine), benzodiazepines (alprazolam, etizolam, clorazepate), antihistaminic drugs (diphenhydramine, hydroxyzine), selective serotonin reuptake inhibitors (citalopram), tetracyclic antidepressant (mirtazepine), calcium channel blockers (diltiazem), non-steroidal anti-inflammatory drugs (ibuprofen and naproxen).

6. Toxicity of Xylazine

6.1. Adverse Effects

Several cardiovascular and pulmonary adverse effects and difficulties in controlling hypotension and diuresis have been related to exposure to xylazine. In humans, xylazine may trigger CNS depression, hypoventilation, bradycardia, hypotension, and even death. Cases of biventricular systolic failure, valvular dysfunction, fibrosis, and cardiac necrosis have been reported [150]. In 2014, Ruiz-Colón et al. [46] reported 43 cases of xylazine intoxication in humans, of which about half were non-fatal cases, and the other half resulted in fatalities. Recently, a rare case of xylazine-induced cardiomyopathy was described in a 22-year-old woman with a history of polysubstance abuse [151]. In 2023, Malaca et al. [47] reported a total of 48 fatal cases related to xylazine consumption. Xylazine can also cause significant tissue damage, skin ulceration, and necrosis at injection sites [152,153]. This may be related to the vasoconstriction caused by xylazine, which hinders wound healing [154], or to the direct binding to kappa opioid receptors [57]. Reconstruction in xylazine-induced skin necrosis is often required [155], and often, the injuries may be so serious that amputation is required. Avoidance of the use of the term “zombie drug” has been suggested as it may amplify the stigma of people who use this drug (PWUD) and people who inject the drug (PWID) [41]. The toxicity of xylazine is generally related to the metabolite 2,6-xylidine, even though this issue is quite controversial since several data available from xylazine metabolism studies in rats do not indicate a significant formation of this metabolite [30]. 2,6-Xylidine is a chemical intermediate mainly used in the dyeing industry or as a component of tobacco smoke. It may also derive from the degradation of aniline-based pesticides and represents a metabolite of local anesthetics bearing the 2,6-xylidide moiety (lidocaine and xylocaine). According to the IARC Cancer Agency, 2-6-xylidine is classified as a carcinogenic compound belonging to Group 2B (“human carcinogen”). It can enhance the incidence of nasal and urinary bladder tumors and has been identified by the Committee for Medicinal Products for Veterinary Use as a potential genotoxin capable of inducing DNA alterations [156,157].

6.2. In Vivo Toxicity Studies in Animals

Recently, Acosta-Mares et al. (2023) [158] demonstrated that the combination of fentanyl with xylazine caused an increase in lethality in comparison to fentanyl alone in Swiss Webster male and female mice, using the conditioned-place preference test. The lethality of the fentanyl/xylazine combination was prevented by naloxone but not yohimbine. Choi et al. (2023) [159] studied the brain-specific hypothermic and hypoxic effects of both xylazine and a mixture of fentanyl/heroin with xylazine in freely moving rats. The authors suggested that xylazine may exacerbate the potentially lethal effects of opioids due to worsening cerebral hypoxia in an overdose. In 2024, Choi et al. demonstrated that treatment with naloxone and atipamezole was more effective than naloxone alone in reversing the hypoxic effects of fentanyl-xylazine mixtures in rats (male and/or female). Thus, the use of this combination has been suggested for the prevention of overdoses induced by mixtures of fentanyl and xylazine in humans [160].

6.3. Toxicity Studies in Humans

In humans, toxicity from xylazine generally consists of CNS depression, bradycardia, and hypotension. Various methods for the administration of xylazine have been reported in humans, including subcutaneous, intramuscular, intravenous, and inhalation. The doses known to produce toxicity are in the range of 40 to 4300 mg [161], with an average dose of 1200 mg in fatal cases and 525 mg in non-fatal cases [162]. The first documented case of overdose from xylazine inhalation was reported by Capraro et al. (2001) [163], with symptoms related to the α2-agonistic activity, e.g., coma, miosis, apnea, bradycardia, hypothermia, and dry mouth 2 h after exposure. The effects were not reversed by naloxone. Hoffmann et al. (2001) described a case of self-injection of 1.5 g xylazine in a 27-year-old farmer [164]. The man became comatose, hypotensive, bradycardic, and mildly hyperglycemic, and intubation and ventilation were required. After 30 h, a full recovery was obtained. The second case of abuse by the inhalation of xylazine was reported by Elejalde et al. (2003) [90] in a healthy 18-year-old boy, with antecedents of drug abuse with various substances, including cocaine and amphetamines after the intentional inhalation of xylazine. The boy experienced an episode of chills, disorientation, and dizziness followed by sweating, gait instability, dysmetria, ataxia, dysarthria, reactive pupils of intermediate size, palpitations, and two syncopal episodes with hypotension and bradycardia. In 2023, the Hindustan Times reported the appearance of a tiny injection site between the thumb and forefinger of a 51-year-old woman that had turned into a necrotic brownish-green lesion [165], likely caused by xylazine, which she had been using for years as an animal tranquilizer. Warp et al. (2024) [166] described a case of wound care for a xylazine-induced wound in the state of Florida. The detection was obtained by using a xylazine test strip (XTS) in urine. The use of XTS for the detection of xylazine has been also described by several other authors [167,168,169,170,171]. Dai et al. (2024) described a case of fatal hyperpyrexia in a 41-year-old man caused by xylazine in China [172]. The first fatal case in the U.K. was reported in Birmingham in May 2022 [173] and the first fatal intoxication case of xylazine-adulterated heroin in the E.U., specifically in Italy, was reported by Di Trana et al. (2024) [174]. Recently, toxicity related to xylazine has also been reported in children. Three cases of synthetic opioid intoxication were complicated by the concomitant presence of xylazine. The first was a 15-month-old boy with a medical history of prenatal substance exposure (maternal use of amphetamines, fentanyl, cocaine, methadone) and neonatal opioid withdrawal syndrome, who was found to be unresponsive, limp, and blue, with bradycardia and apnea. It was found that the mother had suffered a near-fatal synthetic opioid overdose one week before. The second case was a 7-month-old infant with a medical history of prenatal substance exposure (maternal use of amphetamines and methamphetamines) who was found unresponsive on the floor. Deutsch and De Jong reported a case of a 19-month-old infant with a medical history of prenatal substance exposure (maternal opioid use) and neonatal opioid withdrawal syndrome who was found to have impaired breathing and cyanotic [175]. Finally, there are limited data directly addressing the risks of xylazine exposure during human pregnancy. However, there has been an increase in pregnancy-related overdose deaths caused by the use of illicitly manufactured, high-potency synthetic opioids, such as fentanyl [176].

7. Incidence of Xylazine Drug Overdose Events in the U.S.

The Department of Public Health of Philadelphia (Pennsylvania) reported that between 2010 and 2015, xylazine was detected in less than 2% of cases of fatal heroin and/or fentanyl overdose. This percentage increased to 31% in 2019 [177]. Xylazine-fentanyl deaths also remarkably increased in Connecticut from 2019 to 2023 and often involved multiple substances (e.g., cocaine, ethanol, benzodiazepines, and oxycodone) [178]. In Cook County, Illinois, xylazine in fentanyl overdoses have more than doubled from 2019 to 2022 [179]. In a recent study in Ohio [180], it was reported that the number of age-adjusted overdose deaths involving xylazine was about 35 times higher in 2021 than in 2018. The U.S. Drug Enforcement Administration (DEA) seized xylazine in 48 states, and about 23% of fentanyl powder and 7% of fentanyl pills seized were found to contain xylazine [181]. Recently, the DEA stated that “xylazine is making the deadliest drug threat our country has ever faced, fentanyl, even deadlier” and that “people who inject drug mixtures containing xylazine also can develop severe wounds, including necrosis—the rotting of human tissue—that may lead to amputation” [182]. In 2022, a striking prevalence of xylazine among opioid-involved overdose deaths was found, and DEA reported that almost a quarter of seized fentanyl powder contained xylazine [11,183]. In addition, in 2023, the White House Office of National Drug Control Policy (the Biden Administration) declared fentanyl combined with xylazine as an emergent threat to the U.S., with the justification stated as “xylazine combined with fentanyl is being sold illicitly and is associated with significant and rapidly worsening negative health consequences, including fatal overdoses and severe morbidity” [65,184].

8. Harm Reduction Initiatives for Xylazine

Marshall and Nelson (2024) [9] recently reported the harm reduction strategies for xylazine and xylazine-adulterated fentanyl that can be provided as an overdose prevention strategy. In this context, the pharmacists’ and clinicians’ assistance and expertise may be essential. There are different points that must be considered. Xylazine cannot be visually identified in products. Xylazine or xylazine-adulterated fentanyl must not be used for health and general safety reasons. Consider using a “test” or lower dose initially. Intravenous use is probably less safe than inhalation or smoking. In injection practices, it is not recommended to reuse or share needles. Before injecting, swab the area with alcohol, change injection sites, and avoid injecting into areas of skin wounds. When using xylazine, access to naloxone is requested, and the knowledge of how to use it. Seek medical help after an overdose, even if the patient responds to naloxone, as overdoses associated with xylazine-adulterated products may require additional care. Pharmacists and clinicians must explain self-administered wound care management and how to reduce the risk of infection in skin and soft tissue wounds. Finally, screening for HIV [185], hepatitis, and sexually transmitted diseases must be promoted, and the use of medications for opioid use disorder (MOUD) must be described and encouraged [186]. Xylazine test strips became commercially available in March 2023, and patient education is needed for the proper use of test strips in the community setting. Pharmacists can educate people who use drugs about xylazine-adulteration.

9. Conclusions and Perspectives

Xylazine is a non-narcotic sedative proposed for analgesia and muscle relaxation in veterinary medicine. It is still used in animal studies, generally in combination with ketamine. At the beginning of the 21st century, its use as an adulterant of heroin in street drugs spread to Puerto Rico and, a few years later, to Philadelphia before 2010. Its rapid circulation is due to several factors, such as reduced cost, the improvement in its addictive properties, and the ability to prolong the duration of the opioid’s effect. Recently, a surge in its use as a drug adulterant, most often in association with illicitly manufactured fentanyl, has been experienced. The number of individuals using xylazine has dramatically increased in recent years, spreading from the U.S. to the U.K. and all of Europe. The heavy adverse effects triggered by xylazine include cardiovascular and pulmonary issues, depression, the appearance of skin lesions beyond the site of injection, eschar formation, and wound cratering. Several deaths due to xylazine overdoses have also been reported. The vasoconstriction induced by adrenergic receptor stimulation seems to be the main cause of wound occurrence and also prevents further treatment that results in the progression to necrotic wounds. This event, and the subsequent wound infections, could increase, as well as the high mortality incidence, together with the acute overdose. Another important aspect resides in the absence of a proper antidote approved for use against xylazine in humans. Thus, more studies directed toward the identification of a treatment for acute xylazine overdose are needed. Moreover, the metabolites that are derived from this drug have not been completely established. Thus, new studies are required in order to better understand the pharmacokinetics and pharmacodynamics related to this compound, which may disclose other hidden aspects involved in its dangerousness. There is an urgent need to increase the use of XTS among PWUD and PWID and to develop clinical practice guidelines for the treatment of xylazine-related wounds in outpatient settings. Furthermore, the availability of new tests for determining the presence of xylazine and/or identifying its potential toxicity at various exposure levels are more than desirable, together with implemented mitigation measures directed to prevent harm and, lastly, more rigid measures and sanctions to hamper illicit distribution. As the xylazine epidemic expands, clinicians need to understand its impact on vulnerable populations, such as pregnant women and individuals at both extremes of age. New studies are needed to better elucidate the pharmacokinetic properties of xylazine in humans and to estimate the doses of this drug that are typically being used in the community and the toxic doses.