Careful consideration of biological phenotypes, as represented in population pharmacokinetics, turns a second line of attack on the problem of tailoring treatments to patients' specific needs. A key correlate of treatment response that has never been examined in bipolar disorder is consistent and adequate medication exposure. Essential to understanding variability in treatment response is being able to distinguish true non-responders from those who never received adequate exposure to drug. Consistency of drug exposure can be determined using a combination of electronic monitoring of drug-taking and population pharmacokinetic analysis.