Context: Inadequate pain relief is commonly experienced by patients on pain medications for chronic diseases. Tramadol, an opioid analgesic is metabolized by CYP2D6 to active metabolite, responsible for analgesic activity. The presence of non-functional CYP2D6*4 allele may affect pain relief. Hence the aim of our study is to investigate the influence of CYP2D6*4 on the analgesic effect of tramadol in acute osteoarthritic knee pain