1 INTRODUCTION Drug overdose deaths are at an all-time high in North America, and over the last several years, the primary cause of opioid overdose has shifted from heroin to synthetic opioids.1 Increasingly, illicit and counterfeit drugs contain fentanyl,2 a potent synthetic opioid, which has led to an increase in fentanyl misuse and overdoses. Despite the increased presence of fentanyl in the drug supply, our understanding of how fentanyl alters the brain remains incomplete.
