Multivitamin [water & fat soluble] + Normal Saline contains folic acid, and folic acid can make your body break down some epilepsy medicines faster. Primidone is one of these, along with phenobarbital and phenytoin. If you take Multivitamin [water & fat soluble] + Normal Saline together with Primidone, the Primidone may not stay in your body long enough to work well.
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.