The data show that the fatalities associated with fentanyl used together with either cocaine or opiates/opioids had a marked geographic residential clustering of the victims in Massachusetts, compared with the random dispersion of those using the drug legally. Illicit fentanyl use occurred in younger people with higher fentanyl and lower morphine postmortem blood concentrations and more frequent cocaine co-intoxication, compared with fentanyl deaths linked to its legal use. There was a wide range of blood concentrations in postmortem samples with detectable fentanyl. This may reflect the various opioid tolerances of drug users, the dose and route of administration, and co-intoxication with other central nervous system depressant drugs such as ethanol and benzodiazepines, or cases of polydrug toxicity. Cases in which fentanyl was determined by a medical examiner to be the exclusive cause of death had a minimum fentanyl concentration of 8.6 ng/ml; however, this value is limited by the fact that it was not a single-agent fentanyl death. While recognizing that this study does not claim to prove causation regarding cause of death, the authors associate a postmortem blood fentanyl concentration of approximately 7 ng/ml or greater with fatality in the illicit use of fentanyl in polydrug cases. By this measure, the study concludes that fatal fentanyl co-administration with opiates/opioids and/or cocaine is prevalent in Massachusetts. 3 tables, 3 figures, and 19 references