Increased systemic exposure w/ CYP3A4 inhibitors (eg, ketoconazole, itraconazole, ritonavir, nelfinavir, clarithromycin & nefazodone). Concomitant use w/ drugs predominantly metabolized by CYP2D6 (eg, flecainide, thioridazine & TCAs); warfarin. Increased frequency &/or severity of dry mouth, constipation, blurred vision & other anticholinergic pharmacological effects w/ other anticholinergic agents.