• Change in Pain Scores on the Numerical Rating Scale [ Time Frame: Baseline and 12 weeks. ] [ Designated as safety issue: No ]
  

• Change in Health Related Quality of Life Scores on the Short Form 36 Health Survey [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
  
• Change in Pain Scores on the Brief Pain Inventory Short Form [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
  
• Sleep Questionnaire items to evaluate patients' quality of sleep [ Time Frame: 13 weeks ] [ Designated as safety issue: No ]
  

This is a randomized (study drug assigned by chance), multicenter, double-blind (neither physician nor patient knows the name of the assigned drug), placebo-control (patients are randomly assigned to a test treatment or to an identical-appearing treatment that does not contain the test drug) parallel-group (participants receive the same intervention throughout the protocol) comparison study in the patients with chronic pain due to osteoarthritis of the knee or low back pain. The study will be explained and informed consent will be obtained. Patients will be screened for study eligibility at Visit 1. Potential patients must satisfy all eligibility criteria to be enrolled in the study. Eligible candidates will proceed to the study treatment period. At the time of study entry, all prohibited medications will be discontinued and will be disallowed throughout the study. The study consists of 3 periods: a 1-week screening period during which patients are evaluated for study eligibility; a 12-week study treatment period (maximum of 6-week dose adjustment phase and minimum of 6-week of dose maintenance phase); and a one-week follow-up period. The sponsor will collect adverse events starting with the signing of the informed consent form until the follow up period. Adverse events include any occurrence that is new in onset or gets worse in severity or frequency from the baseline condition, or abnormal results of diagnostic procedures, including laboratory test abnormalities. Blood samples for serum chemistry and hematology, a urine sample for urinalysis, vital signs, 12-lead electrocardiography and clinical opioid withdrawal symptoms questionnaire will be also collected for safety evaluation. Numerical Rating Scale, Sleep questionnaire, Brief Pain Inventory Short Form, Western Ontario MacMaster Questionnaire (only for patients with osteoarthritis [OA] of the knee), Roland Morris Disability Questionnaire (only for patients with LBP), Short Form 36 Health Survey, Patient's global impression of change and Physician's global assessment will be collected for efficacy evaluation. Serum drug concentrations will also collected for pharmacokinetic evaluation. Patients will be randomized to one of the two following treatment groups: Tapentadol ER group and Placebo group. Tapentadol ER or placebo will be administered orally twice daily in the morning and evening. The dose ranges is between 50 and 500 mg/day. The maximum dose is 250 mg/intake (500 mg/day). The investigator determines (based on consultation at patient visits and the patient diary) whether or not the dose may be increased. Tapentadol ER or placebo will be administered orally twice daily in the morning and evening. The dose range is between 50 and 500 mg/day. The maximum dose is 250 mg/intake (500 mg/day). The 12-week treatment period consists of two phases: maximum of 6-week dose adjustment phase and minimum of 6-week dose maintenance phase.