The presence of fentanyl metabolites in this population was almost entirely among women who reported using heroin or opioid pain medications and was not associated with stimulant use in adjusted analysis. These data therefore do not support the hypothesis that fentanyl is being routinely added to stimulants as an adulterant in this region.

The different findings in self-reported vs. metabolite data for benzodiazepines in adjusted analyses may be explained by the longer detection period for benzodiazepines, although illicitly produced benzodiazepines may also be contaminated with fentanyl. Methamphetamine metabolite presence and self-reported use were associated with fentanyl presence in unadjusted analyses, but not in analyses adjusted for other substances. Given the potential confounding effects of multiple concurrent substance use, these findings emphasize the importance of adjusted analyses in future surveillance.

Two studies, based in British Columbia and Canada, have found evidence suggesting occasional fentanyl contamination of methamphetamine. Amlani et al. found that reported use of both fentanyl and methamphetamine was independently associated with fentanyl detection among harm reduction services participants, even after adjusting for concurrent opioid use [13]. In a drug-checking program in British Columbia, 90.6% of samples expected to be heroin and 5.9% of samples expected to be methamphetamine tested positive for fentanyl [14]. Exactly where in the supply chain [18,19,20] that fentanyl contamination of stimulants is occurring is unclear, but at the end of the supply chain is one possibility that is consistent with recent surveillance in Ohio. Examination of weight patterns in stimulant seizures from Ohio found that the rare instances of fentanyl contamination appear to occur in cocaine seizures of under 1 gram, and less often in methamphetamine seizures, suggesting that contamination of cocaine occurs at the end of the supply chain in that region [21].

The finding that only 0.3% of cases where women who did not report opioid use tested positive for fentanyl suggests that intentional contamination of stimulants with fentanyl is likely rare. Nevertheless, accidental exposure to fentanyl consumption with resultant overdose has been reported in San Francisco [6, 7]. The strong associations between opioid pain medications or heroin use and fentanyl detection echo prior research with opioid-using populations that reinforces the importance of community-based naloxone distribution programs [22, 23], fentanyl test strips [24, 25], drug-checking services [26], and supervised consumption or overdose prevention sites [27, 28], where overdose prevention services can be accessed. Although many people who use drugs are now aware of the possibility of fentanyl contamination, the regular application of overdose prevention measures is often inconsistent due to structural factors such as poverty and homelessness [29].

Study limitations include sampling from a single city, which limits generalizability to other locations, and a longer self-reported use period than drug detection periods. There was also no direct assessment of known fentanyl use as distinct from other opioid pain medication [30] for this secondary data analysis. Study strengths include longitudinal comparisons of both self-reported and mass-spectrometry toxicology-confirmed substance use, a focus on stimulant use, and a focus on women.

The current study found insufficient evidence for the contamination of stimulants with fentanyl, yet a strong relationship between fentanyl exposure and heroin and opioid pill use.