Prolonged release morphine sulfate (I) microgranules, comprising neutral carrier particles coated with an active layer and a prolonged release layer, is new. The prolonged release layer contains (a) a methacrylic acid-methyl methacrylate copolymer in which the ratio of free carboxy groups to ester groups is 0.5； and (b) hydrophobic silica. Independent claims are included for: (A) the preparation of the microgranules, by applying the active layer and prolonged release layers to the carrier particles, by coating in aqueous solution； and (B) pharmaceutical compositions containing the microgranules.
