Abstract: A delayed, sustained-release pharmaceutical preparation is provided in which a water-soluble drug core is surrounded by a hydratable diffusion barrier which delays drug release for about 2-10 hours. The hydratable diffusion barrier comprises a film-forming polymer such as an acrylic resin or ethyl cellulose or mixtures thereof and an additive which controls the rate of hydration and permeability of the hydratable diffusion barrier selected from the group consisting of fully esterified acrylic resins containing quaternary amine side chains, lubricants, anionic surfactants, plasticizers, inert water-soluble materials, and mixtures thereof. In the preferred sustained-release pharmaceutical preparation, the film-forming polymer is combined with the additive and coated onto core drug granules to produce a diffusion barrier surrounding the core drug and form microparticles which may then be admixed with an immediate release drug.