APA Dictionary of Psychology

n. any of a group of compounds that include the naturally occurring opiates (e.g., morphine) and their semisynthetic derivatives (e.g., heroin); the synthetic opioid agonists (e.g., meperidine, methadone), opioid antagonists (e.g., naloxone, naltrexone), and mixed agonist–antagonists (e.g., butorphanol); and the endogenous opioids. The effects of opioids include analgesia, drowsiness, euphoria or other mood changes, respiratory depression, and reduced gastrointestinal motility. Opioids are used clinically as pain relievers, anesthetics, cough suppressants, and antidiarrheal drugs, and many are subject to abuse and dependence.