May 9 (UPI) -- The number of Americans using illicit opioids, including fentanyl, is 20 times higher than previously estimated, according to a study published Friday by researchers from the RAND Corp. and the University of Southern California. Researchers said government counts may significantly underestimate illicit drug use, and they suggest new methods are needed to better track the nation's opioid crisis, which transitioned from heroin in the early 2010s to illicitly manufactured fentanyl a