Disclosed is a prolonged release pharmaceutical dosage form comprising a plurality of coated beads, each comprising a granule, a first layer coated on the granule comprising hydromorphone, naloxone, an antioxidant compound and a chelatmg compound； and a second layer coated on the first layer comprising a prolonged release agent. The dosage form has improved stability and dissolution properties. Also disclosed is the use of a combination of an antioxidant, such as sodium metabisulfite, and a chelatmg agent, such as EDTA, to improve the stability and/or dissolution properties of a prolonged release dosage form comprising hydromorphone and naloxone.
