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Rationale Dependence can develop during chronic opioid use, and the emergence of withdrawal might promote drug taking. Objective This study examined how chronic morphine administration or withdrawal modified self administration of heroin or cocaine. Methods Four monkeys responded under a fixed ratio 10 schedule to receive i.v. infusions of heroin (0.56–560 µg/kg/infusion) or cocaine (1–100 μg/kg/infusion)...
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