Carfentanil, a synthetic opioid far more potent than fentanyl, is resurfacing in the U.S. drug supply and causing overdose deaths among users who don't know what they're taking. AP documented at least one death—Michael Nalewaja in November 2025—and reported the chemical is spreading as dealers mix it into street drugs. The article is well-sourced AP reporting, though the excerpt provided doesn't include the specific national death toll or geographic scope data that would fully quantify 'hundreds' of overdoses.