Fentanyl, Amphetamine-type Stimulants, Heroin, and xylazine in drug residues in two Northern border Mexican cities.

Illicitly manufactured fentanyl (IMF) and amphetamine-type stimulants (ATS) are driving the fourth wave of the opioid crisis in the United States and Canada. Accordingly, the aim of this paper was to determine the presence of heroin, fentanyl, ATS, xylazine, and other substances in Tijuana and Mexicali, Baja California, two cities on the US-Mexico border. We also tested the positive predictive value of fentanyl and ATS test strips. To do this, we randomly selected 300 drug residues from syringe plungers provided by two non-governmental organizations (NGOs) with harm reduction programs (one in each city) and from community samples obtained at shooting galleries and meeting places for people who inject drugs. We then analyzed them with GC-MS and BTNX test strips for amphetamine-type stimulants (ATS) and fentanyl and its analogs. Test strips showed a high positive predictive value of 81% when compared with GC-MS. Most residues had fentanyl, followed by methamphetamine, heroin, and xylazine. 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 our samples. In conclusion, in two key Mexican northern border cities, most heroin samples contained fentanyl. Opioid and ATS use was common in Tijuana and Mexicali, while xylazine emerged as a significant adulterant in Mexicali and was subsequently detected in Tijuana. The evolving drug market demands continuous drug testing while public health policies are required to address the challenges raised by this scenario.