In many medical institutions, standardized eGFR, which is based on generalized body surface area, is used for drug dosing evaluations. This can lead to overdosing. This study aimed to investigate the discrepancy between personalized eGFR and standardized eGFR and to identify indicators necessary for pharmacists to perform appropriate pharmacotherapy. From October 2022 to February 2023, the study targeted patients aged 18 years and older who continuously visited the Saera Pharmacy Kurashiki. Among the 347 participants, a significant discrepancy of 6.3 mL/min was observed between personalized eGFR and standardized eGFR (p < 0.001). Factors predicting an eGFR discrepancy of 10 mL/min or more were identified as follows: for males, a serum Na level below 140 mEq/L (area under the ROC curve: AUC = 0.656), and for females, a BUN level below 13 mg/dL (AUC = 0.647). Recognizing that patients with these factors are at risk of inaccurate renal function assessment, it is considered beneficial for pharmacists to prioritize obtaining height and weight measurements to ensure appropriate dosing of renally excreted drugs.