There is described a prolonged release pharmaceutical dosage form comprising aplurality of coated beads, each of the coated beads comprising: (a) a granule；(b) a first layercoated on the granule, the first layer comprising: (i) hydromorphone or apharmaceuticallyacceptable salt thereof, (ii) naloxone or a pharmaceutically acceptable saltthereof, (iii) anantioxidant compound, and (iii) a chelating compound； and (c) a second layercoated on the firstlayer, the second layer comprising a prolonged release agent. The dosage formhas improvedstability and dissolution properties. Another aspect of the invention relatesto use of acombination of an antioxidant (such as sodium metabisulfite) and a chelatingagent (such asethylenedinitrotetraacetic acid disodium salt dihydrate) to improve thestability and/ordissolution properties of a prolonged release dosage form comprising (i)hydromorphone or apharmaceutically acceptable salt thereof and (ii) naloxone or apharmaceutically acceptable saltthereof.
