To administer optimal and safe pharmacotherapy, development of stratified and personalized therapy is imperative. Pharmacogenomics (PGx) is useful in elucidating factors causing individual differences in drug efficacy and the emergence of adverse effects. It also helps design accurate drug administration methods by evaluating the effects of patient-related factors, such as genetic factors, that influence pharmacokinetics (PK) and pharmacodynamics (PD). In addition, selection of appropriate therapeutic agents requires the implementation of precision medicine allowing accurate disease diagnosis. To establish precision medicine, it is necessary to uncover the association of pathophysiological factors, which are represented as endotype or genotype, with the pathology of several phenotypes. This review describes two aspects related to realization of individualized medicine, namely the effectiveness of PK/PD/PGx studies and the stratification of pathological conditions. First, we conducted a PK/PD/PGx study with the aim to individualize warfarin treatment. In this study, we elucidated the effect of CYP4F2 polymorphisms associated with vitamin K metabolism by measuring the blood concentrations of warfarin and vitamin K. Then, to develop precision medicine for asthma and chronic obstructive pulmonary disease (COPD), we analyzed not only clinical symptoms but also pathological biomarkers and genes associated with inflammation. The findings may contribute toward better understanding of the pathological conditions of asthma, COPD, and asthma-COPD overlap.