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Effect of increasing the dose of continuous d-amphetamine on self-administration of 1.5 mg/kg/inj cocaine under a PR schedule. Points represent the mean (±SEM) breakpoints. Shaded portions represent the d-amphetamine treatment period (7 days of 5 mg/kg/day and 7 days of 7.5 mg/kg/day). The final ratio values corresponding to breakpoints are represented on the right y axis.
