A number of genetic polymorphisms have been reported to affect responsiveness to (pharmacokinetics and pharmacodynamics of) pharmaceuticals, and this study field, pharmacogenomics, is rapidly developing. Several genetic biomarkers associated with drug safety and efficacy have been already applied to diagnostic and prescribing purposes. In this review, we would like to outline the recent progress in pharmacogenomics, focusing on anti-cancer drugs tegafur (with reference to CYP2A6 polymorphisms), irinotecan (UGT1A1), and cetuximab (somatic mutations in KRAS), and an anti-platelet drug clopidogrel (CYP2C19). This review also addresses genetic markers for drug-induced severe cutaneous adverse reactions (HLA-B) and myopathy(SLCO1B1).