Objectives. To identify opioids associated with a spike in opioid-related mortality in Wayne County, Michigan, from July 2016 through February 2017.

Methods. We reviewed records from the Wayne County Medical Examiner’s Office of 645 people who died because of accidental nonmedically prescribed opioid overdoses from July 2015 through July 2017. We analyzed basic demographics, locations of death, and all opioid toxicology results. Decedents who died in hospitals were excluded.

Results. Of the 645 people who died because of nonmedically prescribed opioid overdoses, 65% were male and 63% were White, with an average age of 43 years. Carfentanil was detected in 129 (20%) cases. During the 8-month mortality spike, carfentanil was detected in 114 of 419 cases (average = 27.2%; range = 6.4%–45.2%). Substances most frequently detected with carfentanil included morphine (57%), 6-monoacetylmorphine (38%), fentanyl (43%), norfentanyl (33%), tetrahydrocannabinol (34%), and cocaine (29%).

Conclusions. The Wayne County spike in mortality temporally corresponded with the detection of carfentanil and a proportional increase in opioid overdose deaths with detectable carfentanil.

Public Health Implications. The abrupt decrease in carfentanil-detected mortality coincided with an announcement indicating an impending ban on fentanyl analogs from China, which suggests that source control is an effective countermeasure.