The folic acid in Multivitamin [water & fat soluble] + Normal Saline can make the body break down some epilepsy medicines faster. Phenytoin is one of them, along with phenobarbital and primidone. Because of this, phenytoin may work less well while you are getting this multivitamin.
These medicines are often taken together with some care. Your doctor may change a dose, space them apart or check on you more often. Make sure your doctor and pharmacist know you take both.