There is described a prolonged release pharmaceutical dosage form comprising a plurality of coated beads, each of the coated beads comprising: (a) a granule； (b) a first layer coated on the granule, the first layer comprising: (i) hydromorphone or a pharmaceutically acceptable salt thereof, (ii) naloxone or a pharmaceutically acceptable salt thereof, (iii) an antioxidant compound, and (iii) a chelatmg compound； and (c) a second layer coated on the first layer, the second layer comprising a prolonged release agent. The dosage form has improved stability and dissolution properties. Another aspect of the invention relates to use of a combination of an antioxidant (such as sodium metabisulfite) and a chelatmg agent (such as ethylenedinitrotetraacetic acid disodium salt dihydrate) to improve the stability and/or dissolution properties of a prolonged release dosage form comprising (i) hydromorphone or a pharmaceutically acceptable salt thereof and (ii) naloxone or a pharmaceutically acceptable salt thereof.
