Propoxyphene is a very popular pain reliever, widely known as "Darvon", that is a centrally acting narcotic analgesic. Commercially, propoxyphene is usually combined with aspirin or some other simple pain killer in a single tablet or capsule. This drug, which is structurally similar to methadone, has been widely abused and is tightly controlled. Side effects can include physical addiction, confusion, convulsions, dizziness, low blood pressure, and weakness.