How does illicit fentanyl reach the US, and what are precursor chemicals?

Illicit fentanyl reaches the United States through a supply chain that starts with chemicals, passes through a network of makers and brokers, and ends as powder or counterfeit pills. The central difficulty for anyone trying to stop it is a simple one: many of the starting chemicals, called precursors, have legitimate industrial uses.

Anti-narcotics officials in the US and Mexico describe the fentanyl supply chain in Mexico as a vast and elusive network that potentially involves hundreds of players, whose exact number is unknown. A major challenge in disrupting it lies in the global chemical industry. Many of the compounds involved are dual-use chemicals: they have legitimate industrial applications. These substances are often unregulated or only lightly controlled in key countries such as the United States, Mexico and China, and brokers can move between the legal chemical trade and the illicit drug market. Mexico and China are described as the primary source countries for fentanyl and related substances, with India emerging as a source too.

Governments have tried to close the gap. In May 2019 China regulated the entire class of fentanyl-type drugs and two fentanyl precursors, and in the months before February 2025 it began scheduling internationally banned precursors. According to the national archives and the US Drug Enforcement Administration (DEA), direct fentanyl shipments from China have stopped since 2022. The result, as the CDC describes it, is that illegally made fentanyl has largely saturated the illegal drug supply, in powder form or pressed into counterfeit pills, and can have other drugs mixed in.

Cities feel the end of that chain. Fentanyl-laced heroin has become a big problem for major cities including Philadelphia, Detroit and Chicago, where fentanyl was already linked to an outbreak of overdose deaths in 2006.