Opana ER is a prescription painkiller for individuals suffering from chronic arthritis, chronic low back pain, and cancer pain. Buy Opana ER online. Normal pain cannot be treated with this medication. It may be taken alone or in conjunction with other medications. As directed by your primary care physician, take this medication orally on an empty stomach (approximately one hour before or two hours after meals), typically every four to six hours. If you feel queasy, ask for tips on how to reduce nausea, such as getting one to two hours of rest with minimal head movement.