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nalorphine (năl´ərfēn), derivative of morphine that acts to reverse the effects of morphine and other narcotics. It counteracts narcotic-induced nervous system and respiratory system depression but is not effective against depression induced by other sedatives such as barbiturates. Nalorphine and other narcotic antagonists are useful in reversing the effects of narcotic overdoses. Because nalorphine causes withdrawal symptoms in addicts, it is administered to apparent ex-addicts to determine if they have returned to drug use. Nalorphine is marketed under the trade name Nalline.