Pharmacogenetics arose with studies of single genes, which had major effects on the action of particular drugs. It turned into pharmacogenomics through realization that the controls of most drug responses are multifactorial. Then, variable gene expression posed new problems, for example what do drugs do to genes, or how useful is any genetic pretesting of a person? A common disease may be caused by different groups of genes in different people, who therefore require different drugs for treatment. Personalized medicine is currently represented by a physician's attention to a patients age, sex, or ethnic background, that is groups showing smaller genetic variation than is typical for general humanity. Occasionally, there is also the use of single-gene pretesting of a patient before drug administration. Over time, improvements in multigenic testing promise to increase the role of personalized medicine. However, the many pharmacogenomic complexities, and particularly time-dependent changes of gene expression, will never allow personalized medicine to become an error-free entity.