The synthetic opioid fentanyl is 50 to 100 times more powerful than the painkiller morphine. Fentanyl has overtaken heroin and cocaine as the driving force behind the epidemic of deaths from opioid overdoses. But not because drug users seek it out. Often users are unaware that fentanyl--or an even stronger tranquilizer for large animals, called carfentanil--has been added to the drugs they buy. Some say knowing fentanyl is present would change how they use drugs.