First Detection of Xylazine in Texas Wastewater and Its Association with Fentanyl Use

Abstract

Xylazine is a veterinary sedative increasingly found in the U.S. illicit drug supply and is a growing public health concern due to its association with overdose events. To investigate its spatial distribution and relationship with fentanyl use, we analyzed 124 wastewater samples from four treatment plants serving the City of El Paso, Texas, over 13 months (June 2023 to July 2024) using liquid chromatography tandem mass spectrometry. Xylazine detections varied across sewersheds, with detection rates of 29.0%, 12.9%, 9.7%, and 0% and peak loads of 110±40 μg/day/1000 people. Norfentanyl (a primary fentanyl metabolite) was detected in all samples (100%), reaching peak loads of 2400±300 μg/day/1000 people. Sewersheds with higher xylazine detection exhibited significantly higher norfentanyl loads (p<0.05), suggesting a community-level association between xylazine and fentanyl use. This study provides the first evidence of xylazine in Texas wastewater and demonstrates the utility of wastewater monitoring for tracking emerging drug trends. These findings could inform resource allocation and public health planning for harm reduction services, including xylazine awareness programs.

Graphical Abstract

1. Introduction

Xylazine is a veterinary sedative and an emerging threat in the drug overdose crisis^1,2. Its presence in overdose deaths has grown steadily since its first identification as an adulterant in Puerto Rico in the early 2000s³. Xylazine (known as “tranq” in street drugs) is often found mixed with fentanyl and other drugs^4–7. The addition of xylazine extends the effects of opioids, while also increasing the risk of fatal overdose^8,9. Xylazine is not an opioid. The treatment of Narcan/Naloxone against fentanyl is ineffective against xylazine^10,11. Recognizing this threat, the White House Office of National Drug Control Policy (ONDCP) declared xylazine as an emerging threat in April 2023¹².

There are many challenges in addressing the xylazine threat. First, there are limited data on its prevalence and distribution, as xylazine is not routinely included in standard toxicology screens^13–15. This leads to underreporting and an incomplete understanding of its true impact. Second, traditional surveillance methods, such as surveys and clinical reports, often suffer from delays of months or even years^16–18, which hinders rapid response. Third, the illicit use of xylazine makes it difficult to accurately assess consumption patterns through conventional approaches¹⁹. Finally, the varying legal status of xylazine across jurisdictions complicates coordinated monitoring and intervention efforts. Currently, only Florida has explicitly banned xylazine, while it remains unregulated in many other states^20–22. These challenges impede timely and effective public health responses to address the xylazine threat within the broader opioid crisis.

Wastewater-based epidemiology (WBE) offers a promising approach to overcoming many challenges in tracking drug use at the community level^23,24. Innovative methods have analyzed wastewater to detect and quantify drugs and their metabolites, providing near real-time data on drug use trends^25,26. Utilizing excretion rates and wastewater flow data, wastewater data can be used to estimate drug consumption levels in the community or ‘sewershed’. The noninvasive nature of wastewater sampling also allows for continuous monitoring without the delays associated with traditional surveillance methods²⁷. Researchers have successfully applied WBE to monitor the use of opioids, stimulants, psychoactive medications, and other drugs^28–36. For example, a two-year wastewater surveillance program in rural Massachusetts consistently detected ten opioids and stimulants and identified increases in drug use during the COVID-19 pandemic, which highlights the value of longitudinal wastewater testing for evolving drug use patterns²⁶. More recently, a study in Kentucky detected xylazine in wastewater samples across the state²⁵. These studies demonstrate the potential of WBE in providing timely insights into emerging drug trends and complementing existing health monitoring programs.

Our study focuses on El Paso, Texas, the sixth largest city in the state and a US-Mexico border city known for its major trade hub. El Paso County’s overdose death rate nearly doubled from 11.3 to 21.8 per 100,000 residents between 2018–2019 and 2022–2023, with 58% of these deaths involving fentanyl³⁷. The prevalence of xylazine use in El Paso is unclear, but the city is located within a region of concern based on Drug Enforcement Administration (DEA) seizure reports indicating that xylazine is increasingly found mixed into fentanyl^38,39. Furthermore, a DEA Joint Intelligence Report showed a 1127% increase in deaths involving xylazine across the southern United States from the year 2020 to 2021²¹. Given these concerning trends, this study aimed to detect the presence of xylazine and examine its spatial association with fentanyl use in El Paso’s wastewater over 14 months (June 2023 to July 2024). This investigation is critical for understanding the evolving drug trends in this important border community and is the first wastewater-based xylazine surveillance study in Texas.

2. Materials and Methods

2.1. Wastewater sample collection

The raw influent wastewater samples were collected from the four wastewater treatment plants (WWTPs) in the city of El Paso, Texas, from June 26, 2023, to July 15, 2024. The four WWTPs serve ~751,982 total customers in the city^40,41. The WWTPs were anonymized for this paper and were identified as WWTPs I, II, III, and IV. Composite samples were collected using HACH AS950 All-Weather Refrigerated Samplers programmed to collect aliquots every 30 minutes over a 24-hour period. Aliquots of 500 mL were shipped overnight on ice to the laboratory and stored at −20°C until analysis. Sample collection and shipment occurred consistently on Mondays throughout the study period based on agreement with the facility. A total of 124 samples were collected and analyzed in this study (31 from each WWTP, Table S1). Sampling frequency was monthly during 2023 (36 samples) and increased to weekly during 2024 (88 samples) due to improved resource availability.

2.2. Sample Preparation and Quantification with LC-MS/MS

Raw wastewater samples (35 mL) were centrifuged, transferred, and further filtered with 0.22 μm polyethersulfone membrane (Millipore Sigma, #SCGP00525) to remove remaining detritus. The filtrate (30 mL) was spiked with 100 ng/L isotopically-marked standards (xylazine-d6 and fentanyl-d5, Cayman Chemical, #38353 and #14721) and extracted using solid-phase extraction cartridges on an extraction manifold (Waters, #WAT200606). Specifically, samples were acidified to pH 2.5 using hydrochloric acid (HCl, Millipore Sigma, #320331) and then loaded onto an Oasis MCX cartridge (Waters, #186000255), which was pre-conditioned sequentially with HPLC-grade methanol (Millipore Sigma, #A456), HPLC-grade water (Millipore Sigma, #W64), and 0.1% HCl in HPLC water (v/v). A vacuum pump was applied. The sample bottles were sequentially rinsed with water and 0.1% v/v HCl, and the rinse solutions were also loaded onto the cartridge.

Methanol was used to wash the cartridges, and the vacuum was applied for 30 seconds to eliminate any residual wash solvent. The cartridge was then air-dried at room temperature. The analytes were eluted from the cartridge using a 2 mL mixture of MTBE (tert-butyl methyl ether/ isopropanol/ammonium hydroxide at a ratio of 78:20:2 (v/v/v, Fisher Scientific, #E127). The vacuum was applied for 30 seconds to flush out any residual elution buffer. The eluent was collected in 15 mL tubes, dried with a gentle nitrogen gas flow, and reconstituted with 150 μL of 50% aqueous methanol, which was further vortexed and centrifuged at 15,000 rcf for 15 minutes. This SPE process resulted in a 200-fold concentration (from 30 mL to 150 μL).

The supernatants were transferred to sample vials, and 5 μL was injected into the Thermo Vanquish UHPLC (Thermo Fisher Scientific) coupled with a Thermo TSQ Quantis tandem mass spectrometer (MS/MS) for analysis and quantification. The analytes and the internal standards (IS) were separated on an Agilent XDB-C18 column (4.6 mm × 50 mm, 3 μm) and eluted by a water-acetonitrile mobile phase system (both containing 0.1% formic acid, v/v) with a gradient from 15% to 95% organic phase within a 6-minute run. The flow rate was set at 0.3 mL/min, and the column temperature was set at 40 °C. Analytes and IS were monitored under the selected reaction monitoring (SRM) mode coupled with a positive electrospray ionization (ESI) source. The ion spray voltage was set at 3500 V. Ultra-high purity nitrogen was used as the sheath gas (50 arbitrary units), auxiliary gas (10 arbitrary units), and sweep gas (1 arbitrary unit), and ultra-high purity argon was used as the collision gas. The temperatures of the ion transfer tube and vaporizer were 325 °C and 350 °C, respectively. Signals from 1.5 min to 5.5 min were recorded by the mass spectrometer. The retention times of the analytes and IS along with quantification SRM ion transitions are listed in Table S2. The LC-MS/MS instrument parameters were also listed in Table S3.

Calibration standards at six concentration levels (0.2–10 ng/L for xylazine and 1–50 ng/L for norfentanyl) were prepared for each analytical batch by spiking HPLC-grade water with certified reference standards of xylazine and norfentanyl. These concentrations are the analyte levels in water before SPE extraction and the ranges were selected based on concentration ranges measured in our wastewater samples. HPLC-grade water blanks served as negative controls. Method validation included assessment of linearity, intra-day and inter-day precision & accuracy and recovery. Limits of detection (LOD) and lower limit of quantification (LLOQ) were determined using signal-to-noise ratios of 3:1 and 10:1⁴², respectively. The LOD and LLOQ are the raw concentrations spiked into water before SPE. Quality control samples at low, medium, and high concentrations were analyzed in triplicate within each batch to monitor method performance and ensure data quality throughout the study period. The SPE recoveries were calculated as follows: 1) peak area 1 (SPE-ed): Xylazine and norfentanyl of 3 QC levels were spiked into water and processed via SPE; 2) peak area 2 (Post SPE): Drug-free water was processed via SPE, and the samples were reconstituted with 50% methanol-water containing corresponding concentrations of xylazine and norfentanyl (adjusted for the 200-fold concentration factor). Recovery = peak area 1/peak area 2*100%. Isotope-labeled IS were added together with the analytes and the recoveries were calculated in the same method.

2.3. Calculation of xylazine and norfentanyl mass loads and per-capita consumption

The xylazine and norfentanyl mass loads were calculated by multiplying the measured concentrations (ng/L) by the daily influent flow volume (Equation 1)^24,32,43. Daily wastewater flow data (million gallons per day, MGD) were provided by the wastewater treatment plants. One MGD equals 3,785,412 liters. The per-capita consumption (per 1000 people) was then determined by dividing the mass load by the population size in each sewershed (Equation 2). This estimation does not account for the drug excretion rate into urine and stool, which is approximately 70% for xylazine based on studies conducted in rats⁴⁴, and 91% for norfentanyl in humans^43,45. To date, no corresponding human excretion data for xylazine has been reported. All the per-capita consumption values are reported with significant figures to account for the uncertainties associated with sampling^46–48.

$$\mathit{\text{Mass}}\mathit{\text{load}}\mathit{\text{of}}\mathit{\text{drug}}\left(\mathrm{\mu }\mathit{\text{g}}/\mathit{\text{day}}\right)=\left(\mathit{\text{measured}}\mathit{\text{concentration}}\left(\mathrm{\mu }\mathit{\text{g}}/\mathit{\text{L}}\right)*\mathit{\text{Daily}}\mathit{\text{Flow}}\left(\mathit{\text{liters}}\mathit{\text{per}}\mathit{\text{day}}\right)\right)$$ Equation 1:

$$\mathit{\text{Drug}}\mathit{\text{per}}\mathit{\text{capita}}\mathit{\text{consumption}}(\mathrm{\mu }\mathit{\text{g}}/\mathit{\text{day}}/1000\mathit{\text{people}})=\frac{\mathit{\text{mass}}\mathit{\text{load}}\mathit{\text{of}}\mathit{\text{drug}}}{\mathit{\text{Population}}\mathit{\text{size}}}*1000$$ Equation 2:

2.4. Contextual data collection about mortality-involving xylazine and local veterinary clinics

Xylazine-involved mortality data were obtained from the National Forensic Laboratory Information System (NFLIS)⁴⁹. The NFLIS data was queried to investigate reported xylazine-involved mortalities from 2019 to 2023 from the NFLIS Public Data Query System (DQS). The search term “xylazine” was applied for those years, and the numbers of related drug reports were collected at the state (Texas) and national levels for analysis.

To assess the potential relationship between wastewater detection patterns and veterinary xylazine use, we mapped the locations of registered veterinary clinics in El Paso to their corresponding sewersheds. Specifically, the addresses and service details of all veterinary clinics in the city of El Paso were obtained from the El Paso Veterinary Medical Association (EPVMA) website. These addresses were geocoded to a map with their longitudinal and latitudinal coordinates using Google Maps. Each clinic was then matched to its respective sewersheds based on its geographic location using the software ArcGIS Pro (version 3.6). To assess the potential spatial impact of hospitals on norfentanyl load in the wastewater, we mapped the locations of hospitals in El Paso to their corresponding sewersheds. This information was obtained through public records and geocoded through ArcGIS Pro in the same manner as the veterinary clinics.

2.5. Statistical analysis

The Welch two-sample t-test was employed to assess differences in norfentanyl per-capita consumption loads between sewersheds. This test was chosen for its robustness in comparing groups with potentially unequal variances. A significance level of α = 0.05 was used, with p-values below this threshold indicating statistically significant differences between compared sewersheds. All statistical analyses were performed using R software (version 4.3.1), with the ‘t.test()’ function for the Welch t-test.

3. Results

3.1. Method validation and analytical performance

To validate the analytical method, we validated the parameters including LOD, LLOQ and linearity, intra- & inter-day accuracy & precision, and recoveries for both target analytes according to the guidelines of FDA⁴² (Tables 1–3). Xylazine and norfentanyl peaks were not detected in blank matrix (HPLC water), showing good specificity of the method (Figures S1 and S2). LODs were determined as 0.1 ng/L for xylazine and 0.5 ng/L for norfentanyl, with corresponding LLOQs of 0.2 ng/L and 1 ng/L, respectively, by serial dilution. Linear calibration ranges were established at 0.2–10 ng/L for xylazine and 1–50 ng/L for norfentanyl according to their concentrations in the wastewater samples with good linearities (R²>0.99) (Figure S3). Low, medium, and high levels of QC (0.5, 2, and 8 ng/L for xylazine; 2, 10, and 40 ng/L for norfentanyl) were chosen across the calibration curve ranges and were used in the following method validation processes (Tables 2 and 3). We also measured the recovery of the solid-phase extraction process, which varied across concentration levels, ranging from 41.46% to 64.52% for xylazine, and 25.92% to 29.26% for norfentanyl. The average recoveries of xylazine-d6 and fentanyl-d5 were 43.76% and 59.10% (Table 1), respectively, similar to the corresponding analytes.

Table 1:

Method validation parameters and extraction recovery rates.

Targets	LOD (ng/L)	LLOQ (ng/L)	Linear range (ng/L)	Nominal concentration (ng/L)	Peak area		Recovery (%)
					SPE-ed	Post SPE
Xylazine	0.1	0.2	0.2–10	0.5	616	955	64.52
				2	2910	6139	47.40
				8	10491	25303	41.46
Norfentanyl	0.5	1	1–50	2	1687	5765	29.26
				10	7741	29866	25.92
				40	27865	102534	27.18
Fentanyl-d5	\	\	\	100	779952	1319733	59.10
Xylazine-d6	\	\	\	100	158873	363026	43.76

Table 3.

Inter-day precision and accuracy.

	Nominal concentration (ng/L)	Test 1 (ng/L)	Test 2 (ng/L)	Test 3 (ng/L)	Average (ng/L)	Precision (%)	Accuracy (%)
Xylazine	0.5	0.52	0.53	0.51	0.52	1.87	3.93
	2	2.09	1.88	2.16	2.04	7.14	2.02
	8	7.31	7.16	6.92	7.13	2.78	−10.90
Norfentanyl	2	2.10	2.01	2.24	2.12	5.36	5.79
	10	8.77	8.59	8.92	8.76	1.88	−12.39
	40	39.97	43.43	44.92	42.77	5.93	6.93

Table 2.

Intra-day precision and accuracy.

	Nominal concentration (ng/L)	Test 1 (ng/L)	Test 2 (ng/L)	Test 3 (ng/L)	Average (ng/L)	Precision (%)	Accuracy (%)
Xylazine	0.5	0.52	0.43	0.50	0.48	9.32	−3.37
	2	2.09	2.08	1.98	2.05	2.92	2.58
	8	7.31	8.31	7.95	7.86	6.43	−1.81
Norfentanyl	2	2.10	1.87	2.24	2.07	8.88	3.51
	10	8.77	9.70	9.50	9.33	5.24	−6.75
	40	39.97	41.53	41.83	41.11	2.43	2.78

We also evaluated the intra-day precision and accuracy using 3 QC levels prepared on the same day (Table 2), and inter-day precision and accuracy using 3 QC levels over three independent analytical runs on different days (Table 3). Within the same day, the precision (coefficient of variation) ranged from 2.92% to 9.32% for xylazine and 2.43% to 8.88% for norfentanyl; accuracy (deviation from nominal QC concentrations) ranged from −1.81% to 2.58% for xylazine and −6.75% to 3.51% for norfentanyl. The inter-day precision ranged from 1.87% to 7.14% for xylazine and 1.88% to 5.93% for norfentanyl; the inter-day accuracy ranged from 3.93% to −10.90% for xylazine and 6.93% to −12.39% for norfentanyl. Overall, the method demonstrated sufficient precision (within 15% for all levels) and accuracy (within ±15% for all levels) for reliable quantification of xylazine and norfentanyl in wastewater samples^42,50.

3.2. Xylazine detection and heterogeneous prevalence across sewersheds

With the established method, we analyzed a total of 124 wastewater samples for xylazine from four wastewater treatment plants serving the entire border city of El Paso, Texas, over a 13-month period from June 2023 to July 2024. To account for the varying flow rates and population sizes across sewersheds, xylazine data are presented as population-normalized mass loads (μg/day/1000 people) rather than raw concentrations. LC-MS/MS results revealed xylazine detection in three of the four WWTPs, with spatial heterogeneity in detection frequency and load across sewersheds (Fig. 1A).

Figure 1. Xylazine detection in wastewater and its association with fentanyl use.

(A) Temporal trends of xylazine load per capita across the four sewersheds in El Paso, Texas. (Points below the limit of detection for samples from WWTP-I, II, and III were omitted for visualization). (B) Positive detection rates of xylazine in wastewater from each sewershed. (C) Mean norfentanyl per-capita load across sewersheds. Xylazine and norfentanyl loads were calculated by multiplying the measured concentrations by the wastewater flow volume and normalized by the population size. Error bars represent standard error (n=31 samples for each WWTP). P-value was obtained via the Welch two sample t-test.

Specifically, detection rates varied substantially between sites: WWTP-II showed the highest detection frequency with 9 positive samples out of 31 (29.03%), followed by WWTP-I with 4 positive detections (12.90%), and WWTP-III with 3 positive detections (9.70%, Fig. 1B). WWTP-IV had no detectable xylazine throughout the entire monitoring period (0%). Among the positive detections, WWTP-II had the highest xylazine loads (110±40 μg/day/1000 people), with a peak of 410 μg/day/1000 people on February 19, 2024. WWTP-I reached its maximum load of 290 μg/day/1000 people on February 26, 2024. The sporadic positive detections at WWTP-III remained at relatively low levels (50–70 μg/day/1000 people). These results demonstrate positive detection of xylazine in wastewater and reveal spatial heterogeneity in usage patterns across different areas of El Paso.

3.3. Xylazine prevalence is associated with fentanyl consumption rate

Given that xylazine is frequently found co-formulated with fentanyl in street drug supplies^4–7, we investigated potential spatial correlations between xylazine detection and fentanyl consumption across El Paso’s sewersheds. We analyzed norfentanyl (a stable fentanyl metabolite) in all 124 samples to assess fentanyl consumption patterns across the four WWTPs. Norfentanyl was detected in all samples, with loads ranging from 160 to 4700 μg/day/1000 people. This ubiquitous presence of norfentanyl suggests a high prevalence of fentanyl use throughout the city of El Paso.

Comparison of average norfentanyl loads across sewersheds revealed a spatial pattern that closely paralleled xylazine detection frequencies (Fig. 1C). WWTP-II had the highest fentanyl consumption burden with norfentanyl loads at 2400±300 μg/day/1000 people, followed by WWTP-I (1300±140 μg/day/1000 people), WWTP-IV (990±90 μg/day/1000 people), and WWTP-III (730±90 μg/day/1000 people). Statistically significant differences were found between WWTP-II and each of the other three WWTPs. Notably, this spatial distribution of fentanyl consumption corresponds with xylazine detection patterns, where WWTP-II showed the highest xylazine detection rates (29.0%) and loads, while WWTP-III and WWTP-IV exhibited minimal or no xylazine detection. These findings demonstrate a spatial correlation between xylazine prevalence and fentanyl consumption and suggest co-distribution of these substances within El Paso’s illicit drug market.

4. Discussion

This study provides the first evidence of xylazine presence in wastewater samples from El Paso, Texas, a major city on the U.S.-Mexico border. We detected xylazine in three out of four wastewater treatment plants serving the city, with the highest prevalence in the WWTP-II sewershed. Notably, we observed an association between xylazine prevalence and higher fentanyl consumption rates in the same areas, suggesting a potential link between the use of these substances. These findings provide a population-level perspective on the emerging xylazine threat in a region already grappling with the opioid crisis.

The detection of xylazine in El Paso wastewater aligns with the rising trend of xylazine-involved mortalities reported across the United States, particularly in the South. According to the DEA, the Southern U.S. had the highest increase in xylazine identifications and xylazine-involving overdose mortalities nationwide²¹. This is consistent with the national trend of xylazine-involved mortalities based on data from the NFLIS (Fig. 2). In Texas, while xylazine-related deaths decreased starting in 2021, the total mortality in 2023 was approximately nine-fold higher than in 2019⁴⁹. Although public records have not yet indicated xylazine-related mortalities in El Paso, the DEA has reported seizing fentanyl and xylazine mixtures within the county³⁸. Our results provide direct evidence of xylazine’s presence in the city’s wastewater, potentially indicating a more widespread issue than previously recognized. The spatial heterogeneity observed in xylazine detection across sewersheds might reflect neighborhood-level drug market dynamics and socioeconomic factors^51,52. The co-occurrence of higher fentanyl consumption levels in areas with more frequent xylazine detection is particularly concerning, because the combination of these drugs is known to drastically increase the risk of fatal overdoses^53,54, and the standard opioid antagonists like naloxone are ineffective against its sedative effects^4,55.

Figure 2. Xylazine-related mortality trends and study sewershed mapping with veterinary clinics.

(A) Xylazine-involved mortality trends in Texas and the United States from 2019 to 2023. Data were obtained from the NFLIS data query system (y-axis: natural logarithm).

This study has four limitations that should be considered when interpreting the results. First, in estimating xylazine consumption rates, we did not account for the excretion rate (approximately 70% in rats, and is unknown in humans), and potential degradation / losses in the sewer system^44,56–58. Including these factors would likely increase our consumption rate estimates and allow for more accurate dose estimations per sewershed. Further investigation of the potential transformation pattern of xylazine in the sewer would also improve the reliability of consumption rate estimations^59,60.

Second, the detection of xylazine and norfentanyl in wastewater may originate from multiple sources. Xylazine is used as a veterinary sedative, primarily for large animals⁶¹, and fentanyl has medical applications in surgery, pain management, and childbirth⁶². To contextualize our findings, we mapped all registered veterinary clinics and hospitals in the city (Table 4)⁶³. The majority of veterinary clinics, including those serving large animals, are located in the WWTP-III and IV, and most hospitals are in WWTP-II and IV. Notably, no xylazine was detected in the sewershed IV, and only 9.7% detection was observed in the sewershed III (Fig. 1), despite these sewersheds having the most veterinary clinics. The spatial association between xylazine and norfentanyl detection in wastewater does not correspond directly to the distribution of these facilities, suggesting the observed association likely indicates human use patterns rather than facility-based sources alone. Further data collection on local veterinary xylazine use and xylazine-involved human cases would enable a more precise assessment of source contributions.

Table 4.

Hospitals and registered veterinary clinics in El Paso, Texas

WWTP	# of Hospitals	# of Veterinarian clinics	# of Veterinarian clinics serving large animals	# of Veterinarian clinics serving small animals
I	3	4	0	4
II	8	5	1	5
III	1	14	3	13
IV	5	17	4	17

Third, xylazine metabolites were not measured in this study. A recent study from Missouri demonstrated the detection of five xylazine metabolites in human samples via LC-MS/MS⁶⁴, which indicates that inclusion of metabolites in future wastewater analyses could provide additional resolution for source attribution and strengthen the interpretation of community-level exposure. Finally, wastewater data in this study may not fully capture localized use patterns due to population movement within the city and the daily influx of visitors. El Paso receives an average of 15,532 pedestrians, 35,712 passenger vehicles, and 2,937 commercial vehicles daily through its ports of entry⁶⁵. Furthermore, all samples were collected on Mondays, which enhances spatial comparisons across sewersheds, but may not reflect the day-of-week variability in substance use. Future studies employing stratified random sampling across multiple days^66,67 and integrating facility-level dispensing records and clinical surveillance data would be crucial for a more comprehensive assessment of temporal patterns and source contributions.

Despite these limitations, our study provides the first detection of xylazine in Texas wastewater using a validated analytical method and reveals spatial heterogeneity in xylazine prevalence across sewersheds and its association with elevated fentanyl use. The findings highlight the need for continued surveillance in El Paso and similar communities experiencing an elevated drug overdose burden. Proactive, data-driven strategies informed by wastewater analysis can help guide the allocation of resources and implementation of harm reduction measures. Moreover, this approach can serve as a tool to evaluate the effectiveness of policies and interventions aimed at addressing the xylazine and opioid crisis⁶¹. By tracking changes in drug use patterns over time, communities can assess the impact of their efforts and make data-driven adjustments to their strategies.

Synopsis:

Wastewater surveillance detected xylazine in Texas for the first time and revealed spatial association with fentanyl use to guide harm-reduction efforts.

Data and Code Availability

All data analyzed in this study are included within the article and its supplementary materials. The analysis scripts will be made publicly available on GitHub upon publication.