Investigators who nicknamed the street mixture have detected it or recorded overdoses blamed on it in Alabama, Georgia and Ohio. The drug looks like concrete mix and varies in consistency from a hard, chunky material to a fine powder. The substance is a combination of several opioids blamed for thousands of fatal overdoses nationally, including heroin, fentanyl, carfentanil — sometimes used to tranquilize large animals like elephants — and a synthetic opioid called U-47700. ... Last year, the U.S. Drug Enforcement Administration listed U-47700 in the category of the most dangerous drugs it regulates, saying it was associated with dozens...