Darifenacin increases the levels of CYP2D6 substrates such as the tricyclics (imipramine AUC increased by 70%). The manufacturers therefore advise caution with other CYP2D6 substrates, and specifically name
flecainide.
The
clinical importance of this potential interaction does not appear to have been assessed, but be alert for the need to reduce the
flecainide dosage if
darifenacin is added.