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o·pi·oid an·tag·o·nist(ō'pē-oyd an-tag'ŏ-nist)
Examples include naloxone and naltrexone that have high affinity for opiate receptors but do not activate these receptors. These drugs block the effects of exogenously administered opioids (e.g., morphine, heroin, meperidine, and methadone), or of endogenously released endorphins and enkephalins.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012