A Long-term Safety Study With Tapentadol ER and Oxycodone CR in Patients With Moderate to Severe Pain Due to Chronic, Painful Diabetic Peripheral Neuropathy (DPN)

Inclusion Criteria: * Man or woman aged 18 years or older * Patients with Type 1 or 2 diabetes mellitus must have a documented clinical diagnosis of painful diabetic peripheral neuropathy with symptoms and signs for at least 6 months, and pain present at the time of screening * Diagnosis must include pain plus reduction or absence of pin sensibility and/or vibration sensibility on Total Neuropathy Score - Nurse (TNSn) examination in lower and/or upper extremities at screening * The investigator considers the patient's blood glucose to be controlled by diet, or hypoglycemics, or insulin for at least 3 months prior to enrolling in the study (this control should be documented by figures of glycated hemoglobin (HbA1c) no greater than 11% at screening) * Patients have been taking analgesic medications for the condition for at least 3 months prior to screening (patients taking opioid analgesics must be dissatisfied with current treatment, and patients taking non-opioid analgesics must be dissatisfied with current analgesia) * Patients currently requiring opioid treatment must be taking daily doses of an opioid-based analgesic equivalent to \<=160mg of oral morphine * Patients with baseline score for average pain intensity in the previous 24 hours of =\>4 on the 11-point numerical rating scale (NRS) at the beginning of the titration period Exclusion Criteria: * Significant pulmonary, gastrointestinal, endocrine, metabolic (except diabetes mellitus), neurological, psychiatric disorders (resulting in disorientation, memory impairment or inability to report accurately as in schizophrenia, Alzheimer's disease) * History of moderate to severe hepatic impairment * Severely impaired renal function * Clinically significant laboratory abnormalities * Clinically significant cardiac disease * History of seizure disorder or epilepsy * History of any other clinically significant disease that in the investigator's opinion may affect efficacy or safety assessments or may compromise patient safety during study participation.