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Self-administration of fentanyl, cocaine, and methamphetamine in rats responding under a progressive ratio (PR) schedule of reinforcement. Data represent the mean (±SEM) for the total number of infusions earned for fentanyl (0.0032–0.032 mg/kg/inf), cocaine (0.32 mg/kg/inf), and methamphetamine (0.032 mg/kg/inf) (left ordinate), as well as the corresponding final ratio completed (right ordinate). Each point represents six to eight rats.
