The study will continue for 3 months to observe the safety and tolerability. After screening at Screening visit, patients begin the study by the first day visit (Day 1). Pain assessment, study related questionnaires are giving out to measure the baseline scale of each patient. Questionnaires including rate of quality of analgesia, EQ-5D and clinical opiate withdrawal scale (COWS). The prescription of the study drug will be recorded from Day 1. Patients will need to visit bi-weekly, including Wk 2 (Day 14±3), Wk 4 (Day 28±3), Wk 6 (Day 42±3), Wk 8 (Day 56±3), Wk 10 (Day 70±3) and Wk 12 (Day 84±3). Pain assessment will be assessed in each visit, drug accountability will be recorded accordingly. The rate of quality of Analgesia will be assessed in each visit; QoL (EQ-5D) will be recorded in Wk 4, Wk 8 and Wk 12. The COWS will be assessed again in Wk 12.