PHENYTOIN AND SOLIFENACIN

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.