Evaluating Fentanyl and Stimulant Test Strips Against GC–MS in Drug Residues from Used Syringes in Northern Mexico

Illicitly manufactured fentanyl (IMF) and amphetamine-type stimulants (ATS) are a major driver in the fourth wave of the opioid crisis in the United States and Canada. Because of this growing problem, the research team explored a new method to determine the presence of heroin, fentanyl, methamphetamine, xylazine, and other substances in samples of paraphernalia (used syringes) in Tijuana and Mexicali, Baja California. The results obtained were then compared with using fentanyl and ATS test strips and gas chromatography–mass spectrometry (GC–MS) to analyze the positive predictive value of these strips. The study’s findings were published in Harm Reduction Journal (1).

A major public health problem for many countries, the increasing production, distribution, and consumption of IMF have led to a previously undetected rise in overdose deaths, especially in the United States and Canada (2–5). Additional serious health risks include blood-borne infections and skin lesions in people who inject drugs (PWID), particularly opioids adulterated with xylazine (6). Because of its high profitability, low cost, and high potency, IMF is gradually replacing heroin in Canada and the United States (7,8).

The use of fentanyl use has played a major role in the opioid crisis in the United States, with over 80,000 deaths reported in 2024 (9). IMF was identified in over 75,000 samples collected from 2012 to 2022 in Canada; results revealed that, up to 2019, IMF was combined mainly with heroin, cocaine, and methamphetamine, but, from 2020 onwards, there was a noticeable local expansion of synthetic drugs and an increase in fentanyl mixed with sedatives (10).

The researchers report that the test strips showed a high positive predictive value of 81% when compared with GC–MS. Most residues contained fentanyl, followed by methamphetamine, heroin, and xylazine; in fact, fentanyl and methamphetamine co-occurrence were the norm rather than the exception in drug residues. Xylazine was more prevalent in fentanyl-containing samples in Mexicali, whereas the combination of fentanyl and methamphetamine was more common in Tijuana. Benzodiazepines were not detected in any of the samples analyzed (1).

“The evolving drug market demands continuous drug testing,” conclude the authors of the study (1), “while public health policies are required to address the challenges raised by this scenario.”

However, the researchers acknowledged that the drug-residue analysis of paraphernalia has its limitations compared to the analysis of biological samples (such as urine, hair, and blood). They referred to studies that documented cross-contamination when analyzing waste, false positives because of the type of rapid test used, or detection problems arising from the sample preparation process (11). Aware of this limitation, but also of the context surrounding the drug-using population in this border region, the research team proposed working with doses from syringes and not associating any samples with individuals, for ethical and biosafety reasons (1).

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References

1. Bautista, C. F.; Medina-Mora, M. E.; Cruz, S. L. et al. Fentanyl, Amphetamine-Type Stimulants, Heroin, and Xylazine in Drug Residues in Two Northern Border Mexican Cities. Harm Reduct J. 2025. DOI: 10.1186/s12954-025-01355-0
2. Fleiz, C.; Arredondo, J.; Chávez, A. et al. Fentanyl is Used in Mexico's Northern Border: Current Challenges for Drug Health Policies.Addiction 2020, 115 (4), 778-781. DOI: 10.1111/add.14934
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9. Ahmad, F.; Cisewski, J.; Rossen, L. et al. Provisional Drug Overdose Death Counts.2025. (accessed 2025-03-06). https://www.cdc.gov/nchs/nvss/vsrr/drug-overdose-data.html
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