Increased risk of bleeding w/ acenocoumarol, anisindione, dicumarol, phenindione, phenprocoumon, warfarin; hypoglycemia w/ glimepiride; cardiotoxicity w/ pimozide. May inhibit metabolism & increase plasma conc of terfenadine, astemizole & cisapride. May increase risk of cyclosporine, phenytoin, phosphenytoin & trimetrexate toxicity; effects of opioid (eg, fentanyl). Increase in bioavailability of tolterodine in people w/ weak CYP450 2D6 activity. May reduce metabolism of carbamazepine. Increased plasma conc w/ oxybutinin & oxycodone.