Erythromycin almost doubles the AUC of
darifenacin by inhibiting its
metabolism by CYP3A4. Other moderate inhibitors of CYP3A4, such as
diltiazem and
verapamil are expected to interact similarly.
The US manufacturers suggests that no dosage adjustments are necessary with these
calcium-channel blockers, whereas the UK manufacturer suggests increasing the
darifenacin dose with caution. Bear in mind the possibility of an interaction if antimuscarinic effects (dry mouth,
constipation, drowsiness) are increased.