AIM: Dosage adjustment is essential in older individuals because they are prone to experience a decline in liver function and changes in body composition. However, quantitative tests or equations for evaluating the activity of hepatic drug metabolism have not yet been clearly established. We examined hepatic drug metabolism activities in older individuals, focusing on changes in body composition parameters. METHODS: Lansoprazole and nifedipine, substrates of the metabolic enzymes cytochrome P450 (CYP) 2C19 and 3A4, respectively, were selected to study hepatic drug metabolism. Residual samples from blood test for older patients were evaluated to determine drug metabolism. The body composition of relevant patients was determined by analyzing characteristic parameters of skeletal muscle mass index (SMI), handgrip strength (HGS) and hepatic steatosis index (HSI). The differences in hepatic drug metabolism were studied statistically among categories in terms of the cut-off value of these parameters. RESULTS: Older male patients receiving lansoprazole and nifedipine in the low SMI (<7.0 kg/m2 ) category showed an 85-90% reduction in respective CYP2C19 and CYP3A4 metabolic activities compared with the normal SMI category. For the female patients, CYP2C19 and CYP3A4 metabolic activities showed no significant correlation with SMI and HGS. Fatty liver disease (HSI ≥36) was found to reduce CYP2C19 metabolic activity particularly in older female patients. CONCLUSIONS: Low CYP2C19 metabolic activity was statistically correlated with low SMI in male patients and high HSI in female patients, whereas low CYP3A4 metabolic activity was statistically correlated with low HGS in male patients. Geriatr Gerontol Int 2022; 22: 449-454.