Opana ER is a long-acting semi-synthetic opioid analgesic that appears as the hydrochloride salt. It is available for oral administration as 10 mg, 15 mg, 20 mg, 30 mg, 40 mg, and 50mg sustained-release tablets or 5 mg/2 mL and 10 mg/2 mL concentrate for solution for injection. The onset of action of Opana ER occurs following oral administration. The peak plasma concentration usually occurs within 60 min following an oral dose and between 4 to 12 hours after an intramuscular dose.Opana ER (oxymorphone hydrochloride) is a prescription pain medicine used to treat moderate to severe pain. It is also used before surgery to cause you to be sleepy and relaxed. Opana ER belongs to a class of drugs known as opioid (narcotic) analgesics, or sometimes just simply opioids. These are medications that act on chemicals in the brain and nerves to relieve pain or other symptoms.