May alter pharmacokinetics w/ CYP2D6 & CYP3A4 inhibitors. Avoid potent p-glycoprotein inhibitors eg, ciclosporin & verapamil. Increased in steady-state AUC by ketoconazole. Increased exposure in subjects who are poor metabolisers. Increased AUC & Cmax
when administered concurrently w/ erythromycin in subjects who are extensive metabolisers. Decreased plasma conc w/ CYP3A4 inducers eg, rifampicin, carbamazepine, barbiturates & St. John's wort. Higher mean Cmax
& AUC in the presence of cimetidine. Caution when used concomitantly w/ drugs that have narrow therapeutic window eg, flecainide, thioridazine or TCAs eg, imipramine. Modest increase in midazolam exposure. Monitor prothrombin time w/ warfarin. Small increase in digoxin exposure. May increase frequency & severity of antimuscarinic pharmacological effects.