Acknowledgments: Supported by NIH Grants EB005803 and GM 068968.

Variation in the behavior of drugs between people, and variation in drug behavior in a given patient over time, have both presented us with challenging problems in the optimal description of such behavior as well as challenges of how best to act on such information. New methods of genetic testing and measurement of variations in gene expression over time in a single patient present us with issues of 1) how best to use all such information in the overall process of planning drug dosage regimens for individual patients, especially if the drug is potentially toxic; 2) how to further refine our knowledge about the patient during of the course of therapy with the drug; and 3) how best to adjust the dosage regimen to the new information we obtain about him/her as a unique individual. Human genetic variation, in the form of gene sequence or expression variability, provides us with important covariate information to help us further individualize our dosage regimen for a particular patient based on tha