Was the fluoxetine level truly high in this case?
Equation 2 shows that the dosing rate is only one of the two variables that determine the concentration of a drug. The dose of fluoxetine in this case was also likely raised based on clinical assessment of response (i.e., inadequate efficacy and no apparent problems with adverse effects). Ironically, like aripiprazole, fluoxetine at doses of 20-60 mg/day was shown to have a flat dose-response curve in the registration trials that led to its approval as an antidepressant. One possible explanation for the increase in the fluoxetine dose is that this patient needed a higher than usual dose (due to unusually rapid ability to clear fluoxetine) to reach the levels usually achieved by the patients who received 20 mg/day during the registration trials with this drug. To assess this possible explanation for the high dose, a fluoxetine level was ordered: the levels of fluoxetine, norfluoxetine, and the combination of the two were 482 ng/ml, 214 ng/ml, and 696 ng/ml, respectively. Many readers of
