Xylazine

# Research Output: Xylazine

## Chemical Identity

**Verified via PubChem CID 5707 and REST API confirmation [cross-referenced with StatPearls Ref 48]:**

- **IUPAC name:** N-(2,6-dimethylphenyl)-5,6-dihydro-4H-1,3-thiazin-2-amine
- **Canonical SMILES:** CC1=C(C(=CC=C1)C)NC2=NCCCS2
- **Molecular formula:** C₁₂H₁₆N₂S
- **Molecular weight:** 220.34 g/mol
- **InChIKey:** BPICBUSOMSTKRF-UHFFFAOYSA-N
- **PubChem CID:** 5707
- **CAS number:** 7361-61-7 (web-retrieved; not returned by PubChem API call)
- **Stereochemistry:** No chiral centers; no stereoisomers
- **Trade names:** Rompun (Bayer), AnaSed, Sedazine (noted in brief — confirmed from standard references)
- **Street names:** Tranq, Tranq Dope, "the Zombie Drug" (media term)

**Note:** The brief seed data lists SMILES, molecular formula, molecular weight, InChIKey, and PubChem CID as blank. All have now been populated via PubChem and are confirmed above. No discrepancies found between PubChem data and the brief's pharmacological class designation ("Alpha-2 adrenergic receptor agonist").

**Physical properties (from literature):**
- High lipophilicity → rapid CNS penetration and concentration in CNS and kidneys [Ref 48]
- Available in veterinary formulations at 20, 100, and 300 mg/mL for injection [Ref 48]
- **Metabolites:** 2,6-dimethylaniline (DMA) and 4-hydroxyxylazine are the primary metabolites detected in urine and blood [Ref 45]

---

## Pharmacology

### Primary Mechanism: Alpha-2 Adrenergic Receptor Agonism

Xylazine is a potent agonist at presynaptic alpha-2A and alpha-2C adrenergic receptors, producing sympatholytic effects including sedation, analgesia, and muscle relaxation [Ref 48]. Alpha-2 receptor activation inhibits adenylyl cyclase via Gi/Go proteins, reducing cAMP levels and suppressing noradrenergic tone. At high doses or concentrations, xylazine also activates alpha-1 and alpha-2B receptors, causing peripheral vasoconstriction and, potentially, brief hypertensive crises before the predominant hypotensive phase [Ref 33, 48].

The alpha-2 antagonists yohimbine and idazoxan block xylazine's antinociceptive effects, confirming the alpha-2 receptor as the primary pharmacological target (Cakir et al. 2025 [Ref 2]). The veterinary reversal agent atipamezole is a selective alpha-2 antagonist that fully reverses xylazine sedation in animals but is not approved for human use (Puzovic et al. 2025 [Ref 38]; Choi et al. 2024 [Ref 37]).

### Secondary Mechanism: Kappa Opioid Receptor Agonism

A landmark finding published in 2024 established that xylazine is a **full agonist at kappa opioid receptors (KOR)** in vitro and in vivo (Bedard et al. 2024 [Ref 19]). This was confirmed and extended by Huang et al. (2026 [Ref 25]), who screened 320 GPCRs and found that KOR agonism is **unique to xylazine** among all FDA-approved alpha-2 adrenergic agonists (dexmedetomidine, clonidine, guanfacine, brimonidine, etc.). This KOR activity likely accounts for several of xylazine's distinctive features: naloxone-precipitable withdrawal (Bedard et al. 2024 [Ref 19]), skin and tissue pathology via KOR-mediated keratinocyte effects (Robertson et al. 2026 [Ref 1]), and female sex-specific withdrawal enhancement.

A parallel line of research (Romero et al. 2013 [Ref 4]) had suggested that xylazine's central antinociception involves endogenous opioids and mu-opioid receptors — however, this earlier animal study used mu-OR antagonists that may have disrupted downstream endogenous opioid signaling rather than directly implicating mu-OR in xylazine's mechanism. The KOR findings from Bedard 2024 and Huang 2026 represent more mechanistically direct evidence.

**⚠️ FLAG for human review:** There is tension between the 2013 Romero finding (mu-OR involvement in central antinociception) and the 2024 Bedard finding (full KOR agonism). The generation agent should note both findings and acknowledge that KOR agonism is the more recent and mechanistically direct characterization.

### Tertiary Mechanism: Norepinephrine Transporter (NET) Inhibition

Park et al. (2013 [Ref 3]) demonstrated that xylazine competitively inhibits the norepinephrine transporter (NET) in neuronal cells via a site overlapping the nisoxetine-binding domain. This effect is independent of alpha-2 receptor signaling and contributes to elevated synaptic norepinephrine. The clinical significance of NET inhibition at illicit doses has not been characterized in humans.

### Antinociception: NO/cGMP Pathway

Romero & Duarte (2009 [Ref 5]) demonstrated that xylazine's peripheral antinociceptive effects are mediated through the L-arginine/nitric oxide/cyclic GMP pathway in rats, with the effect blocked by NOS inhibitors and soluble guanylyl cyclase inhibitors.

### Metabolic Effects: Hyperglycemia via Insulin Inhibition

Xylazine inhibits glucose-stimulated insulin release via alpha-2 adrenergic receptors on pancreatic beta cells, causing hyperglycemia and hypoinsulinemia in dose-dependent fashion. This was established in isolated rat islets (Abdel el Motal & Sharp 1985 [Ref 46]) and confirmed in livestock (Hsu & Hummel 1981 [Ref 47]). Yohimbine abolishes this effect. Clinically, human users exposed to xylazine-contaminated drugs may experience unpredictable glycemic shifts; anemia and hyperglycemia have been reported anecdotally in chronic users, though a direct causal link with xylazine (vs. other factors) is not firmly established (Zagorski et al. 2023 [Ref 31]).

### Anti-inflammatory Activity

Xylazine and other alpha-2 agonists reduce neutrophil extravasation by stabilizing endothelial adherens junctions via alpha-2 receptor-mediated pathways (Herrera-García et al. 2014 [Ref — animal/in vitro study, PMID 25114107, not yet in registry]). The clinical relevance for human illicit use is not established.

### Pharmacokinetics (Human Data)

Human PK data for illicitly acquired xylazine are limited but emerging:

- **Half-life:** 23–50 minutes in humans (StatPearls [Ref 48]; Lin et al. 2025 [Ref 20])
- **Distribution:** High lipophilicity; rapid CNS penetration; concentrates in CNS and kidneys [Ref 48]
- **Metabolism:** Hepatic; primary metabolites are 2,6-dimethylaniline (DMA) and 4-hydroxyxylazine, detected in urine. In a Turkish forensic cohort, 4-hydroxyxylazine appeared in 78% of positive cases and DMA in 25% (Erol Öztürk et al. 2025 [Ref 45])
- **Routes:** IV (predominant in illicit use), IM, SC, oral, inhalation (sniffing/smoking) [Ref 42]
- **Onset:** Rapid across routes — case reports note effects within 2 hours of ocular exposure (Velez et al. 2006 [Ref 8]) and effects within minutes of IV administration
- **Detection window:** Urinary detection possible at clinically relevant concentrations; mass spectrometry-based methods developed (Troike & McShane 2025 [Ref 22]); point-of-care immunoassay strips available with variable performance (Pattillo et al. 2025 [Ref — PMID 41405371])
- **Clinical PK in fentanyl/xylazine patients:** Lin et al. (2025 [Ref 20]) characterized PK in real-world patients testing positive for both substances; Bird et al. (2025 [Ref 21]) reported individual-level urine PK in OUD patients

**⚠️ FLAG:** No published controlled human PK/PD dose-response studies exist. All human PK data are from naturalistic or forensic settings. Binding affinity (Ki/EC50) data at human alpha-2A/C receptors and KOR for xylazine were not found in the published literature reviewed.

---

## History

### Synthesis and Veterinary Development
Xylazine was first synthesized in **1962** by Farbenfabriken Bayer AG in Leverkusen, West Germany, originally as a candidate antihypertensive drug (web-retrieved [Ref — see synthesis history WebSearch results]). It was investigated in humans for blood pressure reduction but was rejected for clinical development due to **profound hypotension and excessive central nervous system depression** at therapeutic doses [Ref 48]. The compound was subsequently developed for veterinary use and introduced to market in the late 1960s under the brand name Rompun. It became a foundational veterinary sedative-analgesic, widely used in large and small animals. In 1981, the alpha-2 adrenergic mechanism of sedation was described, contextualizing xylazine within the class of alpha-2 agonists alongside clonidine.

### Emergence in Human Drug Use
The first documented large-scale human use of xylazine as a drug adulterant was reported in **Puerto Rico in the early 2000s**, where it appeared as a heroin adulterant or complement among injection drug users (IDUs). Torruella (2011 [Ref 40]) documented this in a semi-structured interview study, noting severe physical deterioration associated with chronic use and describing xylazine as causing "unprecedented scale and depth" of harm on the island. This paper served as an early warning about the potential for geographic spread.

### Spread to Continental US and Canada
Xylazine was detected in Philadelphia's drug supply during the mid-2010s and its prevalence grew dramatically: from <2% of heroin/fentanyl overdose deaths in 2010–2015 to **31% of 858 fatal heroin/fentanyl overdose cases in Philadelphia in 2019** (Johnson et al. 2021 [Ref 30]). Friedman et al. (2022 [Ref 15]) documented national spread to 10 jurisdictions across all four US Census regions, rising from 0.36% of overdose deaths in 2015 to 6.7% in 2020. The CDC's SUDORS data (Kariisa et al. 2023 [Ref 36]) showed a 276% monthly increase in IMF-involved deaths with xylazine detected across 21 jurisdictions between January 2019 and June 2022.

**Toronto, Canada:** First detected in September 2020, representing 7.2% of fentanyl samples in the city's drug checking service (Bowles et al. 2021 [Ref 41]).

By 2021, xylazine was present in 91% of Philadelphia's local heroin/fentanyl samples, making it the most prevalent adulterant in that supply [Ref 48]. DEA NFLIS data document a surge from 9,330 reports in 2021 to 25,047 in 2024, with over 90% of all-time reports occurring in the 4 years 2021–2024, and 52.9% co-reported with fentanyl (Zhu et al. 2025 [Ref 24]).

The spread to the West Coast was initially slow but accelerating: Devincenzi & Rodda (2026 [Ref 29]) documented emergence in San Francisco across overdose deaths, impaired drivers, and other forensic contexts.

### Regulatory History
- **US:** Xylazine has never been a federally controlled substance. TRANQ Research Act enacted December 2023 (requires research coordination). Florida was the first state to schedule xylazine as Schedule I in 2018. By 2023, 26+ states had considered xylazine legislation. The **Combating Illicit Xylazine Act** (proposed) would make it Schedule III federally [Ref 49].
- **UK:** Following an ACMD review in February 2024, xylazine was classified as **Class C under the Misuse of Drugs Act 1971** and added to **Schedule 4 Part 1 of the Misuse of Drugs Regulations 2001**, effective **January 15, 2025**, preserving legitimate veterinary access [Ref 50].
- **EU:** Limited detection data. Xylazine-containing drug samples seized in Estonia; co-detected in 13% of used syringes in Riga, Latvia [Ref 50]. No EU-wide scheduling.
- **Canada:** No federal scheduling identified. Detection documented in Toronto from 2020 [Ref 41].
- **Australia:** No scheduling information found in literature reviewed.

**⚠️ FLAG:** Australia and most EU country-specific legal status data not found via literature search. Human review recommended.

---

## Subjective Effects

No controlled human pharmacology studies have characterized the subjective effects of xylazine. Available data come from case reports, forensic records, user surveys, and ethnographic accounts. All findings below should be labeled with this caveat.

### At Typical Illicit Use Doses (Combined with Fentanyl)
User accounts from Philadelphia-area ethnographic fieldwork and Reddit survey data (Spadaro et al. 2023 [Ref 34]) describe xylazine as:
- **Desired effects (from user perspective):** Extending the short duration of fentanyl's euphoric effects; xylazine described as "prolonging the high" and improving sedation quality [Ref 32, 15]
- **Unwanted effects:** Prolonged sedation beyond intended (81% of Reddit survey respondents reported this as a concern) [Ref 34]; increased skin wounds (43%) [Ref 34]; xylazine generally perceived as an **unwanted adulterant** by most users surveyed [Ref 34]
- **Behavioral effects:** Extreme sedation; slurred speech; ataxia; disorientation; dysarthria; miosis (constricted pupils); dysmetria [Ref 33, 48]
- **Duration:** Sedation can last significantly longer than fentanyl alone; xylazine's alpha-2 effects persist beyond the opioid component due to differential half-lives

### Effects Documented in Overdose/Toxicology Cases
- **CNS:** Sedation grading to coma; miosis; disorientation; dysmetria; delirium rare
- **Cardiovascular:** Initial brief hypertension → sustained hypotension; bradycardia; sinus bradycardia 40–50 bpm documented; AV conduction disturbances; premature ventricular contractions [Ref 8, 33]
- **Respiratory:** Respiratory depression; apnea (documented in pediatric inhalation case [Ref 10])
- **Temperature:** Hypothermia documented (Capraro et al. 2001 [Ref 10])
- **Autonomic:** Dry mouth; salivation noted in some animal studies
- **Prolonged sedation** from fentanyl/xylazine combination is clinically established (Nguyen et al. 2026 [Ref — PMID 41916094])

### Effect Profile Mapping (Psychedex taxonomy)
Based on documented human effects, the following taxonomy slugs are supported by evidence:

**Body:** sedation, respiratory-depression, abnormal-heartbeat, motor-control-impairment, pupil-constriction, pain-suppression, muscle-relaxation, nausea (reported in some cases), dry-mouth, temperature-dysregulation, vasoconstriction, bodily-heaviness, dizziness

**Thinking:** cognitive-impairment, confusion, amnesia (likely, given depth of sedation), information-processing-suppression

**Feeling:** anxiety-suppression (sedative effect), dysphoria (withdrawal), cognitive-dysphoria (post-sedation disorientation)

**Self:** depersonalization (during deep sedation), disinhibition

**⚠️ FLAG:** Effect profile is based on case reports and animal studies. No controlled human subjective experience data. Duration data for human use are not established by controlled study — only naturalistic reports and forensic data.

### Duration Data

From StatPearls [Ref 48] and case reports:
- **Half-life:** 23–50 minutes; sedation outlasts half-life due to receptor binding kinetics
- **Ocular exposure case:** Symptoms resolved ~25 hours after exposure [Ref 8]
- **Veterinary duration at clinical doses** (not directly applicable to human illicit use): sedation 30–90 minutes depending on species and dose

**⚠️ FLAG:** No controlled human duration data exist. Duration proposals for the Psychedex article should be based on case reports and use appropriate uncertainty framing.

---

## Safety & Toxicity

### Overdose Profile
The clinical toxidrome of xylazine overdose (alpha-2 toxidrome) consists of: **coma, miosis, apnea, bradycardia, hypotension, and hypothermia** (Capraro et al. 2001 [Ref 10]; Velez et al. 2006 [Ref 8]). This presentation overlaps with opioid overdose but is **not reversed by naloxone** for the xylazine component — naloxone reverses only concurrent opioid activity [Ref 18, 36].

Key dose data from the Ayub et al. (2023) systematic review [Ref 42]:
- **Dose range** in reported cases (all routes, primarily IV): **40 mg to 4,300 mg**
- **Average dose in fatal cases:** ~1,200 mg
- **Average dose in non-fatal cases:** ~525 mg
- **Significant overlap** between fatal and non-fatal dose ranges — individual susceptibility highly variable

From StatPearls [Ref 48]:
- **Lethal dose range:** 40–2,400 mg
- **Lethal plasma concentration range:** trace to 16 mg/L — extreme variability reflecting polydrug context

**Fatal outcome rate in case series:** 35.6% of 59 cases (21 of 59); 28.6% of fatal cases involved only xylazine via IV route; most deaths involved co-administration with opioids [Ref 42].

### Cardiovascular Toxicity
Bradycardia (40–50 bpm) and sustained hypotension are the cardinal cardiovascular effects. AV block and ventricular arrhythmias are documented. The fentanyl/xylazine combination produces **additive cardiorespiratory depression**, with SpO₂ decreases at 1:1 ratio being subadditive — a more complex interaction than simple addition (Puzovic et al. 2025 [Ref 38]).

### Tissue Necrosis and Wounds
Chronic xylazine use is associated with **deep, necrotic skin ulcers** that are pathognomonic in clinical context:
- Lesions occur **at and beyond** injection sites — systemic mechanism, not just local injection reaction
- Mechanism: peripheral alpha-2 mediated vasoconstriction → impaired skin perfusion → necrosis, impaired wound healing, secondary infection [Ref 12, 14]
- Additionally: kappa opioid receptor activation on epithelial cells directly disrupts skin homeostasis via keratinocyte extrusion and tissue contraction (Robertson et al. 2026 [Ref 1])
- Severe cases: tibial osteomyelitis, bacteremia (Staph aureus MRSA/MSSA, Strep pyogenes, E. coli, Enterococcus, Proteus) [Ref 14]
- Risk of limb amputation [Ref 13]
- Scoping review confirms characteristic clinical presentation: deep, punched-out appearance with surrounding erythema, extending to subcutaneous tissue [Ref 11]

### Hyperglycemia
Alpha-2 mediated inhibition of insulin release causes hyperglycemia in animal models and anecdotally in human users [Ref 46, 47, 31]. Clinical significance in context of chronic illicit use is uncertain; not well-characterized in controlled human studies.

### Pregnancy
A case report (Kasemodel et al. 2026 [Ref 27]) documented placental transfer of xylazine confirmed via umbilical cord assay. Animal models show dose-dependent uterine vasoconstriction, reduced uteroplacental blood flow, fetal growth restriction, and pregnancy loss. Neonatal sedation and contribution to neonatal abstinence syndrome are possible. No controlled human pregnancy safety data exist.

### LD50
No published human LD50 data. Animal LD50 data not found in literature reviewed. **⚠️ FLAG:** LD50 is unestablished and should not be inferred.

---

## Harm Reduction

### Overdose Response
1. **Naloxone remains critical** — reverses the opioid component of fentanyl/xylazine overdose, treating the immediately life-threatening respiratory depression. Higher doses may be needed [Ref 6, 18]. Naloxone does NOT reverse the xylazine component.
2. **Recovery position** — critical for unconscious patients to prevent aspiration; airway management priority [Ref 18]
3. **Atipamezole** (alpha-2 antagonist, veterinary use only): Reverses xylazine sedation in animals; preclinical rat data support combined naloxone + atipamezole for fentanyl/xylazine overdose (Choi et al. 2024 [Ref 37]; Puzovic et al. 2025 [Ref 38]). NOT approved for human use; not available outside veterinary settings.
4. **Hospitalization and supportive care:** IV fluids, vasopressor support, supplemental oxygen, cardiac monitoring [Ref 48]
5. **No FDA-approved antidote for xylazine itself**

### Drug Checking
Xylazine test strips (immunoassay-based) are available and can detect xylazine in drug supply samples. Performance data suggest variable sensitivity/specificity depending on strip type (Pattillo et al. 2025). FTIR spectroscopy can detect xylazine in drug checking programs at 5% concentration or higher (Rhode Island data [PMID 40977066]).

### Wound Care
- Early recognition critical — xylazine wounds progress rapidly
- Wound staging systems have been developed for clinical management (Retrouvey et al. 2024 [Ref 13])
- Low-barrier nurse-led wound care clinics have demonstrated feasibility in Philadelphia (McFadden et al. 2023 [Ref 43])
- Standard wound care + broad-spectrum antibiotics for secondary infection
- Harm reduction engagement for ongoing use counseling

### Contraindications
- **Combining with opioids:** Dramatically increases overdose risk through additive/synergistic respiratory and cardiovascular depression
- **Combining with other CNS depressants** (benzodiazepines, alcohol, other sedatives): Additive CNS depression
- **Diabetes:** Hyperglycemia risk through insulin inhibition [Ref 46]
- **Cardiovascular disease:** Bradycardia, AV block, hypotension particularly dangerous
- **Pregnancy:** Documented placental transfer; fetal risk [Ref 27]

---

## Tolerance & Dependence

### Physical Dependence
Xylazine produces physical dependence with a documented **withdrawal syndrome** distinct from opioid withdrawal. Clinical evidence includes:
- Case reports and survey data documenting withdrawal in chronic users (Strickland et al. 2026 [Ref 26]; Dowton et al. [Ref 51])
- Reddit survey: 53% of respondents reported experiencing xylazine withdrawal (Spadaro et al. 2023 [Ref 34])
- NIDA CTN consensus: xylazine "may increase the nature and severity of opioid withdrawal" (Perrone et al. 2024 [Ref 18])
- Xylazine exposure levels are positively associated with early physiological and withdrawal symptoms in OUD patients (Strickland et al. 2026 [Ref 26])

### Animal Models
Khatri et al. (2024 [Ref 17]) established a rat model of xylazine/fentanyl co-self-administration:
- Xylazine produced a **sex-specific withdrawal syndrome** — females showed delayed onset and enhanced sensitivity to motor-suppressing effects
- Withdrawal was **not altered by naloxone** (0.1 mg/kg)
- Xylazine suppressed fentanyl consumption in a dose-dependent manner when co-self-administered

**Naloxone-precipitated withdrawal:** Bedard et al. (2024 [Ref 19]) showed that naloxone precipitates withdrawal from xylazine in mice (consistent with the KOR agonism finding), with **enhanced sensitivity in females**. This is clinically relevant — administering high-dose naloxone to xylazine-dependent patients may precipitate severe withdrawal.

### Treatment Gaps
Standard medications for opioid use disorder (buprenorphine, methadone) are **not effective for xylazine withdrawal** [Ref 48, 18]. No FDA-approved medication exists for xylazine withdrawal management. Research agenda includes developing alpha-2 agonist taper protocols as potential management strategies. The Lancet Psychiatry (Dunn & Strain 2024 [Ref 54]) identifies xylazine co-exposure as a key understudied area in opioid withdrawal research.

### Tolerance
Cross-tolerance between xylazine and mu and delta opioid receptor agonists at supraspinal levels has been documented (Ware & Paul 2000 [PMID 10688982]), suggesting shared analgesic circuitry modulation. Chronic xylazine use may modify opioid tolerance development; fentanyl tolerance was prevented by alpha-2 agonist co-administration in rats (Yildiz Pehlivan et al. 2024 [PMID 37865211]).

---

## Interactions

### Fentanyl and Opioids (Primary Interaction of Public Health Concern)
- **Additive CNS and respiratory depression** — principal mechanism for increased overdose severity [Ref 2, 38]
- **Cardiorespiratory effects additive** for HR decrease, SpO₂ decrease, and respiratory rate reduction at most fixed ratios; subadditive for SpO₂ at 1:1 ratio [Ref 38]
- **Naloxone reverses opioid component only** — xylazine effects persist after naloxone administration, causing prolonged unconsciousness even after apparent "partial reversal"
- **Xylazine potentiates fentanyl/heroin-induced brain hypoxia** and eliminates the rebound oxygen recovery effect (Choi et al. 2024 [Ref 37])
- Fentanyl with higher xylazine proportion: fentanyl potency significantly reduced, and naloxone alone insufficient to reverse effects [Ref 2]

### Alpha-2 Antagonists (Reversal Agents)
- **Atipamezole** (veterinary): Selective alpha-2 antagonist that reverses xylazine; combined with naloxone is more effective than either alone for fentanyl/xylazine overdose in animal models [Ref 37, 44]. Cardiovascular instability (rebound tachycardia → sustained bradycardia) reported with the combination [Ref 38].
- **Yohimbine:** Non-selective alpha-2 antagonist; blocks xylazine antinociception [Ref 2]; increases or decreases fentanyl potency depending on dose
- **Idazoxan:** Selective alpha-2 antagonist; blocked xylazine antinociception [Ref 2]

### Oxycodone and other opioids
In vitro drug-drug interactions between oxycodone and xylazine in rat and human liver microsomes document metabolic interactions relevant to polydrug toxicity (Gu & Tyndale 2026 [PMID 41992412]).

### CNS Depressants
Additive depression expected with benzodiazepines, alcohol, barbiturates, and other sedatives — documented polydrug combinations in overdose deaths (Potoukian et al. 2023 [Ref 7]: 87% of deaths involved polydrug toxicity).

### Insulin/Diabetes Medications
Alpha-2 agonism inhibits insulin secretion; clinically relevant interactions with insulin, sulfonylureas, and other hypoglycemic agents are theoretically important [Ref 46, 47].

### Haloperidol
In rodent studies, haloperidol (D2/σ1 antagonist) significantly altered potency of both fentanyl and xylazine (Cakir et al. 2025 [Ref 2]) — clinical significance unknown.

**⚠️ FLAG:** No human controlled drug-interaction studies for xylazine exist. All interaction data extrapolated from animal models or inferred from case reports of polydrug overdose.

---

## Legal Status

| Jurisdiction | Status | Notes |
|---|---|---|
| **United States (Federal)** | **Not a controlled substance** | Regulated under FD&C Act only; TRANQ Research Act enacted Dec 2023; Combating Illicit Xylazine Act proposed (Schedule III) |
| **Florida (US state)** | **Schedule I** | First state to schedule, 2018 |
| **Ohio (US state)** | **Schedule III** | Governor executive order March 2023 |
| **26+ US states** | Various | Have considered xylazine legislation or regulations |
| **United Kingdom** | **Class C, Schedule 4 Part 1** | Effective January 15, 2025; Misuse of Drugs Act 1971; preserves veterinary use |
| **EU (general)** | **Not scheduled** | Detected in Estonia, Latvia; limited data on other countries |
| **Canada** | **Not scheduled federally** | Veterinary use approved; first detected in illicit supply 2020 |
| **Australia** | **Unknown** | No data found in reviewed literature |

---

## Clinical Trials

**NCT06949605** — "Quantitative Xylazine Measurement in ED" — Completed, Phase: N/A — Condition: Opioid Overdose / Substance Use Disorders — Sponsor: Icahn School of Medicine at Mount Sinai. This is the most directly xylazine-specific clinical trial identified.

No other trials specifically investigating xylazine treatment, reversal, or pharmacology in humans were identified in ClinicalTrials.gov searches. The NIDA CTN convening (Perrone et al. 2024 [Ref 18]) explicitly identifies research gaps and the absence of clinical trials as a critical deficiency.

---

## Community Feedback Analysis

No community feedback was submitted in Part A2 of the brief.

*No community feedback submitted.*

---

## Gaps & Flags

### Critical Safety-Critical Flags

1. **⚠️ No controlled human pharmacology studies exist** for xylazine. All human data come from case reports, forensic analyses, and naturalistic/observational studies. Dose, duration, and effect profile must be framed with this caveat.

2. **⚠️ LD50 not established in humans.** The dose range for fatal cases (40–2,400 mg based on case series [Ref 42, 48]) overlaps substantially with non-fatal doses and is confounded by polydrug use. This value must NOT be presented as a reliable LD50.

3. **⚠️ Kappa-OR agonism conflict:** The 2013 Romero study [Ref 4] reported xylazine antinociception involves mu-OR (not kappa-OR), while the 2024 Bedard and 2026 Huang studies [Ref 19, 25] confirm xylazine is a full KOR agonist. The 2013 finding may reflect indirect mu-OR effects via endogenous opioid release rather than direct mu-OR agonism. The generation agent should present the KOR finding as the current mechanistic consensus while noting the earlier conflicting data.

4. **⚠️ Naloxone + atipamezole combination risk:** Animal data suggest this combination is more effective than naloxone alone, but causes cardiovascular instability (rebound tachycardia → bradycardia) [Ref 38]. This is not yet studied in humans and atipamezole is not approved for human use. The article should not recommend this combination without appropriate clinical caveats.

5. **⚠️ Withdrawal treatment gap:** No FDA-approved treatment for xylazine withdrawal exists. Standard MOUD is ineffective for the xylazine component [Ref 48, 18]. Naloxone can **precipitate** withdrawal in xylazine-dependent subjects [Ref 19]. This is a critical harm reduction/clinical flag.

### Data Gaps

- **SMILES string:** Retrieved as CC1=C(C(=CC=C1)C)NC2=NCCCS2 via PubChem REST API. CAS 7361-61-7 web-retrieved; not returned by API.
- **Binding affinity data (Ki/EC50):** No published receptor binding affinity data (Ki at alpha-2A, alpha-2C, KOR) found for xylazine in the peer-reviewed literature reviewed. **FLAG for generation agent: do not fabricate binding constants.**
- **Dose/duration proposals:** Human illicit dosing ranges are only available from case series, not controlled studies. Best available estimate: non-fatal doses 40–525 mg (typical range), fatal doses 1,200+ mg average, but massive inter-individual variability. Duration of sedation is 30–120 minutes based on veterinary dosing and case reports; may be significantly prolonged with concurrent opioid use.
- **Australia legal status:** Not found. Human review recommended.
- **Specific EU country scheduling:** Not comprehensively identified; only UK confirmed.
- **Oral bioavailability in humans:** Not established.
- **CYP enzyme involvement in xylazine metabolism:** Not specifically characterized in reviewed literature.
- **Physical properties (melting point, solubility):** Not found in reviewed literature.
- **Therapeutic index:** Not calculable from available human data.
- **Cross-tolerance in humans:** Only animal data available.
- **Xylazine alone vs. fentanyl-xylazine combination duration/effects:** Much of available data conflates the two; isolated xylazine effects in human illicit use are poorly characterized.