When President Biden and Chinese President Xi Jinping pledged to revive a joint crackdown on illegal drug trafficking in November, it sent a brief shock wave through the vast network of Chinese suppliers fueling the production of fentanyl.

       Drug suppliers hit pause on international orders, as local Chinese officials conducted site inspections and began circulating fresh reminders of regulations. Beijing sent a warning notice to its pharmaceutical industry and shut down 25 Chinese companies selling fentanyl precursors — the chemical building blocks of a drug that accounts for the majority of more than 70,000 synthetic opioid overdose deaths a year in the United States.