Opana ER 40 mg contains oxymorphone extended-release, a strong opioid analgesic used for the long-term treatment of severe chronic pain when other options are inadequate. The “ER” formulation releases the medication slowly over many hours to provide continuous pain control. Opana 40 mg, which contains the opioid oxymorphone, is a prescription pain medication used to manage moderate to severe pain when other treatments are insufficient.