APA Dictionary of Psychology

an agent that acts as an antagonist at opioid receptors. Generally, opioid antagonists are synthetic derivatives of morphine that, as a result of structural changes in the molecule, bind to opioid receptors but do not produce the effects of euphoria, respiratory depression, or analgesia that are observed with opioid agonists. Opioid antagonists may be complete (pure) or mixed. Complete antagonists, such as naloxone, naltrexone, nalmefene (U.S. trade name: Revex), and nalorphine, are generally used to reverse the effects of opiate overdose (notably respiratory depression). Mixed agonist–antagonist opioids, such as butorphanol and pentazocine (U.S. trade name: Talwin), were developed in attempts to produce opioid analgesics that did not possess the abuse potential of opioid agonists. Also called narcotic antagonist.