The purpose of this study was to characterize the effect of potent CYP2D6 inhibition byparoxetine on atomoxetine disposition in extensive metabolizers. This was a single-blind, two-period, sequential studyin 22 healthy individuals. In period 1, 20 mg atomoxetine bid was administered to steady state. In period 2, 20 mg paroxetine was administered qd for 17 days. On days 12 through 17, 20 mg atomoxetine bid were coadministered. Plasma pharmacokinetics of atomoxetine, 4-hydroxyatomoxetine, and N-desmethylatomoxetine was determined at steady state in each treatment period. Plasma pharmacokinetics of paroxetine were determined after the 11th and 17th doses. Paroxetine increased C(ss,max), AUC0-12, and t1/2 of atomoxetine by approximately 3.5-, 6.5-, and 2.5-fold, respectively. After coadministration with paroxetine, increases in N-desmethylatomoxetine and decreases in 4-hydroxyatomoxetine concentrations were observed. No changes in paroxetine pharmacokinetics were observed after coadministration with atomoxetine. It was concluded that inhibition of CYP2D6 by paroxetine markedly affected atomoxetine disposition, resulting in pharmacokinetics similar to poor metabolizers of CYP2D6 substrates.