Pharmacogenetics is a growing field of research that focuses on the interaction between genetics and drug therapy. Relationships between genetic variation and drug effect have been observed for a growing number of commonly used drugs. Validation studies may soon define the use of these relationships in clinical practice, moving the field toward routine application. Currently, there are only a few pharmacogenetic diagnostic tests available, and clinical guidelines for pharmacogenetically tailored therapy are lacking. It is likely that guidelines for pharmacogenetic dosing of certain commonly used drugs such as warfarin, codeine, and inhaled beta agonists will become available within the next few years.