Opana ER is a prescription medication used to treat severe pain that requires around-the-clock, long-term opioid treatment. It contains the active ingredient oxymorphone, which belongs to a class of drugs known as opioid analgesics. Opana ER is designed to be taken orally and is formulated as an extended-release tablet, meaning it releases the medication slowly over an extended period of time to provide long-lasting pain relief.