Objective: Combining different drugs increases the potential for drug-drug interactions en-hancing the risk of adverse drug reactions. We aimed to unravel potential pharma-cokinetic interactions between aripiprazole and duloxetine. Methods: Plasma concentrations of aripiprazole of two groups of 78 patients each, receiving aripiprazole as a monotherapy, or combined with duloxetine, were compared. A po-tential impact of duloxetine on the metabolism of aripiprazole was expected in high-er plasma concentrations of aripiprazole and higher dose-adjusted plasma concen-trations. Results: Patients co-medicated with duloxetine showed significantly higher plasma concen-trations of aripiprazole (p=0.019) by 54.2%. Dose-adjusted plasma concentrations were 45.6% higher (p=0.001). 65.4 % of these patients exhibited aripiprazole plasma concentrations above the upper limit of the therapeutic reference range, in the con-trol group this was only the case for 43.6% of the patients (p=0.006). A positive rela-tionship was found between the daily dose of duloxetine and dose-adjusted plasma concentrations of aripiprazole (p=0.034). Conclusions: Combining duloxetine and aripiprazole leads to significantly higher drug concentra-tions of aripiprazole, most likely via an inhibition of cytochrome P450 CYP2D6 and to a lesser extent of CYP3A4 by duloxetine. Clinicians have to consider increasing aripiprazole concentrations when adding duloxetine to a treatment regimen with ari-piprazole.