Designating or of a group of natural or synthetic biochemicals, esp. endorphins, that relieve pain in the same way that opiates do.
adjective
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Any such biochemical.
noun
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Any of various compounds that bind to specific receptors in the central nervous system and have analgesic and narcotic effects, including naturally occurring substances such as morphine; synthetic and semisynthetic drugs such as methadone and oxycodone; and certain peptides produced by the body, such as endorphins.