10. Female subjects must be post-menopausal, physically incapable of becoming pregnant, or using acceptable birth control method. Acceptable methods of birth control include hormonal contraceptives or double-barrier methods (condom or diaphragm with spermicidal agent or IUD). If practicing an acceptable method of birth control, confirmation of negative pregnancy urine test for female subjects is required at the Screening Visit. No special precautions are required for male subjects because levorphanol is an approved therapy for chronic pain.