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Effects of morphine and clonidine on withdrawal-associated increases in heroin choice (abscissa: dose morphine or clonidine in mg/kg/h; ordinate: percent of choices allocated to heroin across all heroin doses). Morphine dose-dependently blocked withdrawal-associated increases in heroin choice, whereas clonidine did not up the doses that produced overt sedation. (From Negus and Rice 2009; reprinted, with permission, from the authors.)
