Background Our ethnographic team has conducted longitudinal research focused on illicit drug markets in Northern Mexico since 2018. In 2021-2022, study participants described the arrival of new, unusually potent tablets sold as ostensibly controlled substances, without a prescription, directly from pharmacies that cater to US tourists. Concurrently, fentanyl- and methamphetamine-based counterfeit prescription drugs have driven escalating overdose death rates in the US, however their presence in Mexico has not been assessed. Aims To characterize the availability of counterfeit and authentic controlled substances at pharmacies in Northern Mexico available to English-speaking tourists without a prescription. Methods We employed an iterative, exploratory, mixed methods design. Longitudinal ethnographic data were used to characterize tourist-oriented micro-neighborhoods and guide the selection of n=40 pharmacies in n=4 cities in Northern Mexico. In each pharmacy, samples of 'oxycodone', 'Xanax', and 'Adderall' were sought as single pills, during English-language encounters, after which detailed ethnographic accounts were recorded. We employed immunoassay-based testing strips to check each pill for the presence of fentanyls, benzodiazepines, amphetamines, and methamphetamines. We used Fourier-Transform Infrared Spectroscopy to further characterize drug contents. Results Of 40 pharmacies, these controlled substances could be obtained in any form with no prescription at 68.3% and as single pills at 46.3%. Counterfeit pills were obtained at n=11 (26.8%) of pharmacies. Of n=45 samples sold as one-off controlled substances, n=20 were counterfeit including 9 of 11 (81.8%) of samples sold as 'Adderall' that contained methamphetamine, and 8 of 27 (29.6%) of samples sold as 'Oxycodone' that contained fentanyl, and n=3 'Oxycodone' samples containing heroin. Pharmacies providing counterfeit drugs were uniformly located in tourist-serving micro-neighborhoods, and generally featured English-language advertisements for erectile dysfunction medications and 'painkillers'. Pharmacy employees occasionally expressed concern about overdose risk and provided harm reduction guidance. Discussion The availability of fentanyl-, heroin-, and methamphetamine-based counterfeit medications in Northern Mexico represents a public health risk, and occurs in the context of 1) the normalization of medical tourism as a response to rising unaffordability of healthcare in the US, 2) plummeting rates of opioid prescription in the US, affecting both chronic pain patients and the availability of legitimate pharmaceuticals on the unregulated market, 3) the rise of fentanyl-based counterfeit opioids as a key driver of the fourth, and deadliest-to-date, wave of the opioid crisis. It is not possible to distinguish counterfeit medications based on appearance, because identically-appearing authentic and counterfeit versions are often sold in close geographic proximity. Nevertheless, US tourist drug consumers may be more trusting of controlled substances purchased directly from pharmacies. Due to Mexico's limited opioid overdose surveillance infrastructure, the current death rate from these substances remains unknown.