1. A method for the treatment of a patient suffering from cataplexy in narcolepsy or excessive daytime sleepiness, the method comprising: administering a reduced daily dosage amount of gamma-hydroxybutyrate (GHB), a prodrug, or a salt thereof, to the patient; wherein the patient is concomitantly administered divalproex sodium; wherein the daily dosage amount of GHB, prodrug, or salt thereof, is reduced to compensate for pharmacokinetic (PK) and/or pharmacodynamic (PD) changes caused by the concomitant administration of divalproex sodium.