The result of this research indicated that diabetes could significantly reduce the activity of CYP2C19 (by ~50%) since the mean metabolic ratios of omeprazole was changed from 0.091±0.005 in the control group to 0.054±0.005 in the untreated diabetic rats (P-value=0.003) (Figure 3). The gene discussed is CYP2C19; the disease is diabetes mellitus.