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Aim: To investigate the effect of duloxetine on the pharmacokinetics and tolerability of tolterodine and its active 5-hydroxymethyl metabolite (5-HM)., Methods: Sixteen healthy subjects received two 5-day treatment regimens in a randomized, double-blinded, crossover fashion: tolterodine (2 mg, BID) + duloxetine (40 mg, BID), tolterodine (2 mg, BID) + duloxetine placebo (BID). Plasma concentrations of tolterodine and 5-HM were measured on day 5. Adverse events, clinical safety laboratory data and vital signs were assessed during the study., Results: Duloxetine increased the AUC(tau,ss) of tolterodine by 71%[geometric mean, 95% confidence interval (CI) 31, 123], and its C(max,ss) by 64% (CI 30, 106), and prolonged its t(1/2) by 14% (CI 1, 28). Duloxetine did not affect the plasma concentrations or t(1/2) of 5-HM. Laboratory data and vital signs did not reveal any clinically significant changes or abnormalities., Conclusions: Duloxetine exhibited minor inhibitory effects on the pharmacokinetics of tolterodine but not 5-HM. Coadministration of these drugs was well tolerated and demonstrated no significant safety findings in the studied population. These findings suggest that there should not be a need for routine adjustment of tolterodine dosage in the presence of duloxetine.