This is a dose-response, double blind , placebo-controlled study. All patients are required to have 2 weeks washout period from prior medication. Subjects will discontinue current medications 2 weeks prior to randomization. Among those who meet the inclusion criteria, only subjects recording visual analogue scale(VAS) of higher than 40mm(VAS range from 0mm(no pain) to 100mm(unbearable pain)) are enrolled in this study.