The United States is witnessing a troubling resurgence of carfentanil, a highly potent chemical linked to increased overdose deaths. Once briefly curtailed after China's bans on certain precursors, traffickers are now reportedly using carfentanil as a powerful substitute to enhance fentanyl's effects, especially amid a decline in fentanyl seizures and overdose rates. Authorities warn that even minuscule amounts of carfentanil can be fatal, posing a significant threat to drug users and law enforcement alike.