Modified release gamma-hydroxybutyrate formulations having improved pharmacokinetics

A pharmaceutical composition comprising an immediate release portion and a modified release portion, wherein the immediate release portion and the modified release portion each comprise one or more pharmaceutically acceptable salts of gamma-hydroxybutyric acid, including their hydrate, solvate, complex or tautomer forms in an amount equivalent to 3.0 to 12.0 grams of sodium oxybate, wherein the composition is designed to be administered only once per day to a human subject, and wherein the composition achieves a relative bioavailability (RBA) of greater than 80% relative to a comparable amount of an immediate release liquid solution of sodium oxybate administered at to and to in equally divided doses, when administered approximately two hours after a standardized evening meal.