Fentanyl and other synthetic opioids have overtaken prescription opioids as the most common drug involved in fatal drug overdoses in the United States, according to a new report.

Government researchers found the percentage of fatal overdoses involving synthetic opioids rose from 14 percent in 2010 to 46 percent in 2016, according to NBC News. Of the 42,249 opioid-related overdose deaths recorded in 2016, the researchers found 19,413 involved synthetic opioids. Their findings are published in the Journal of the American Medical Association.

Synthetic opioids are increasingly found in illicit drug supplies of heroin, cocaine, methamphetamine and counterfeit pills, the researchers noted. “A lack of awareness about the potency of synthetic opioids, their availability and the increasing adulteration of the illicit drug supply poses substantial risks to individual and public health,” they wrote.