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Background and Objectives: Multi-regional clinical trials (MRCTs) are an efficient drug development strategy for eliminating drug lag in East Asian countries. In planning MRCTs according to the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use E17 guideline, it is expected that East Asian populations with relatively similar ethnicity can be pooled as one population. However, evidence supporting this assumption is limited. This study aimed to investigate population/regional differences considering influencing factors among East Asian regions using MRCT data as a research model., Methods: A retrospective analysis was conducted to determine the efficacy of two drugs, asenapine, a schizophrenia drug, and tadalafil, a dysuria drug for benign prostatic hyperplasia, using MRCT data from Japan, Korea, and Taiwan. First, predictive factors and effect modifiers were evaluated. Then, population/regional differences were evaluated using multivariate regression models, with the interaction term Region-by-Treatment group and adjustment for influencing intrinsic/extrinsic factors., Results: Among the 4 outcomes for the two drugs, no significant population/regional differences were detected (P > 0.05) by the adjusted regression models. The effect modifiers, such as pretreatment drug status or concurrent diseases, were common among countries., Conclusions: No significant population/regional efficacy differences were found for the two drugs among the three regions. This finding supported the possible applicability of the region pooling strategy for MRCTs in East Asia, emphasizing the benefits of exploring ethnic difference/influencing factors at an early stage to design further confirmatory studies. However, further evidence for various drugs should be accumulated.