Studies that focus on multidrug interactions in natural settings are sparse. In this investigation, data from therapeutic drug monitoring (TDM) were used to study the impact of multiple cytochrome P450 enzyme (CYP) 2D6 substrates and inhibitors on plasma risperidone levels. CYP2D6 catalyzes the conversion of risperidone to the active metabolite 9-OH-risperidone. The question whether CYP2D6 activity is important for the level of the "active moiety" (ie, the sum of risperidone and 9-OH-risperidone) is controversial. Concentration-to-dose (C:D) ratios of risperidone and 9-OH-risperidone in 218 patients were associated with the number of concomitantly used substrates or inhibitors of CYP2D6. The C:D ratios of risperidone in patients with 0, 1, and >1 numbers of CYP2D6 inhibitors were 2.6, 8.5, and 17 nmol L mg, respectively. Differences between the groups were highly significant (P < 0.001). All patients with >1 CYP2D6 inhibitors were administered at least 1 potent CYP2D6 inhibitor, that is fluoxetine, paroxetine, thioridazine, and/or levomepromazine. The C:D ratios of the active moiety (risperidone + 9-OH-risperidone) in patients with 0, 1, and >1 numbers of concomitant CYP2D6 inhibitors were 17, 24, and 30 nmol L mg, respectively (P = 0.001), which was explained by higher levels of risperidone without any change in the levels of 9-OH-risperidone. Concomitant use of 1 or several drugs recognized as substrates for CYP2D6, without any proven inhibitory effect, had no apparent influence on the levels of risperidone or 9-OH-risperidone, suggesting that the risk of drug-drug interactions between different substrates of CYP2D6 is low when used in therapeutic doses. In conclusion, the results suggest that an increase in the number of concomitant inhibitors may be associated with a lower CYP2D6 activity, although the type of inhibitor is probably more important. Drug-dependent inhibition of CYP2D6 increases the active moiety of risperidone. An indication for risperidone TDM should therefore include concomitant medication with established CYP inhibitors.