Multiple variables influence the selection and optimization of drug therapy for each individual patient. Pharmacogenomics, the study of the influence of individual genetic variations on drug response in patients, may yield additional information to further enhance safe and effective medication use. Whereas the field originally focused on the effects of specific variants within individual genes on drug response (i.e., pharmacogenetics), efforts increasingly examine the role of multiple variants across the genome and their potential relationship to drug therapy outcomes. As our understanding of pharmacogenomics and the biological relevance of specific genetic variants in individuals’ drug metabolizing enzymes, drug transporter proteins, and drug target receptors to certain drug responses has increased, we have learned that multiple variations across the genome can contribute to significant and relatively predictable treatment outcomes. Virtually every therapeutic area involving medication use includes a drug for which documented genetic variability has the potential to affect drug response. Some of this information is included in the FDA-approved package insert prescribing information. For some agents, the suitability of a specific drug or the determination of an appropriate initial dose for an individual patient based on pharmacogenetic information has been incorporated into dosing algorithms and patient care. As such, it is essential that health care professionals can interpret and utilize this information to facilitate safer and more effective use of medications for individual patients.