Patients vary widely in their response to drugs. Having an understanding of the pharmacokinetic and pharmacodynamic properties of various medications is importantwhen assessing ethnic differences in drug response. Genetic factors can account for 20 to 95 percent of patient variability. Genetic polymorphisms for many drug-metabolizing enzymes and drug targets (e.g., receptors) have been identified. Although currently limited to a few pathways, pharmacogenetic testing may enable physicians to understand why patients react differently to various drugs and to make better decisions about therapy. Ultimately, this understanding may shift the medical paradigm to highly individualized therapeutic regimens.