APA Dictionary of Psychology

any drug with enhancing effects at opioid receptors in the central nervous system. Opioid agonists may be complete (pure) or partial agonists. Morphine is a pure opioid agonist; other examples include codeine, heroin, methadone, meperidine, and LAAM. Opioid partial agonists (e.g., buprenorphine) have lower levels of activity than complete opioid agonists at the same receptors and consequently have less analgesic activity. Also called narcotic agonist.