"Opioids are a class of drugs that include the illegal drug heroin, synthetic opioids such as fentanyl, and pain relievers available legally by prescription, such as oxycodone (OxyContin®), hydrocodone (Vicodin®), codeine, morphine, and many others."
NIDA. "Opioids." National Institute on Drug Abuse, , https://www.drugabuse.gov/drugs-abuse/opioids. Accessed 23 Sep. 2019.