Ketoconazole increases
solifenacin levels 2- to 3-fold by inhibiting CYP3A4. The manufacturers predict that potent CYP3A4 inducers (e.g.
phenytoin, and therefore probably fosphenytoin) will decrease
solifenacin levels.
Be alert for a reduction in the
efficacy of
solifenacin in patients taking
phenytoin.