Erythromycin almost doubles the AUC of
darifenacin by inhibiting its
metabolism by CYP3A4. Other
macrolides that inhibit CYP3A4 may interact similarly, see macrolide.
The US manufacturers suggests that no dosage adjustments are necessary with
erythromycin, 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.