During this period, addiction research increasingly treated substance use disorders as medical conditions requiring pharmacological management and structured clinical care. Across animal models, research detailed morphine pharmacology, tolerance, dependence, and withdrawal, while longitudinal studies linked hospitalization outcomes to relapse risk, mortality, and progressive recovery trajectories. The emergence of standardized inventories such as the Addiction Research Center Inventory enabled cross-drug comparisons of subjective drug effects, supporting mechanistic and clinical differentiation among substances. Early psychometric profiling and disease-focused framing advanced diagnosis, treatment, and policy perspectives, setting the stage for a chronic-care paradigm. Historical Significance: The period culminated in transformative theoretical and practical shifts: the disease concept of alcoholism reframed addiction as a medical condition, influencing policy and reducing stigma; formal substitution therapies for opioid dependence demonstrated pharmacological viability; the ARCI and other tools provided rigorous measures for clinical trials; neurochemical insights into dopamine established a biochemical basis for reward and addiction that guided future research and therapeutic strategies.