Prevalence and drug use correlates of inadvertent fentanyl exposure among individuals misusing drugs in seven U.S. states.

Fentanyl has emerged as the leading cause of fatal drug overdoses in the U.S. Individuals misusing drugs may not always be aware of exposure to fentanyl. To determine the prevalence of fentanyl use and extent of awareness of fentanyl exposure among a national sample of treatment-seeking individuals with opioid use disorder ( = 1098). Participants provided oral fluid and urine specimens, which were tested for drugs by liquid chromatography/tandem mass spectrometry. Participants also provided self-reports of fentanyl use. 49.5% tested positive for fentanyl in oral fluid, urine, or both. Of those testing positive for fentanyl, 29.8% were unaware that they had been exposed to fentanyl. Participants testing positive for opioids methadone, and specifically 6-monoacetylmorphine (6-MAM), a unique metabolite of heroin, were significantly more likely to be unaware of fentanyl exposure than participants testing negative for these substances, with a similar trend for oxycodone and tramadol. These findings may be due to fentanyl's effect being difficult to distinguish from that of other opioids, whereas when other types of drugs are adulterated with fentanyl, the differences in effects are likely to be readily discernable. These results support the importance of expanded drug-checking services.