Personalized medicine based on pharmacogenomics is being developed at the clinical stage. Pharmacogenomics involve the study of the role of genomic variation in individual variations in drug response, a phenotype that varies from potentially life-threatening adverse drug reactions to an equally serious lack of therapeutic efficacy. These developments in pharmacogenomics are strongly supported by marked innovations in genetic analyses. The molecular targets of clinical pharmacogenomics are the genetic and genomic factors of both the patient and the pathological organ. Molecular target drugs have become standard treatments in cancer therapy. The efficacy of these drugs can be predicted by characterization of the molecular target before administration. Such characterization for treatment guidance is called companion diagnostics. Companion diagnostics are tests that are intended to assist physicians in making treatment decisions for their patients based on the best response to therapy. Moreover, companion diagnostic co-development can highly change the drug development process and clinical trials by significantly yielding safer drugs with better therapeutic efficacy in a faster, more cost-effective way. However, many companies are interested in the development of companion diagnostics for targeted therapies in specific fields such as cancer. Thus, there are currently no potential companion diagnostics for common diseases. As common diseases are multifactorial, multiplex testing is required to predict therapeutic efficacy. The development of multiplex companion diagnostics is anticipated for the expansion of personalized medicine.