Study Design. Prospective, single-blinded, randomized clinical trial.Objective. This study evaluates the added benefit of promethazine administration as an anxiolytic adjunct to morphine analgesia in reducing acute low back pain (LBP) compared to morphine alone.Introduction. Acute low back pain (LBP) is one of the most common reasons for emergency department (ED) visits. The optimal analgesic treatment for acute LBP remains controversial. Anxiety relief has been shown to improve pain management in ED setting. We hypothesized that administration of the antihistamine promethazine as an anxiolytic adjunct to morphine analgesia will improve LBP management compared to morphine alone.Methods. Fifty-nine adults, who were treated in our ED for severe acute LBP (visual analogue scale [VAS] ≥70mm), were randomly enrolled in the study. Thirty patients received slow infusion of intravenous morphine 0.1mg/kg in normal saline and 29 patients received an analgesic regimen of intravenous morphine 0.1mg/kg with promethazine 25mg administered similarly. Pain and anxiety levels were subjectively assessed by the patients on a 100mm VAS scale before and after treatment. Adverse event related to analgesia were recorded in real time.Results. Following analgesia administration patients' pain rating decreased by 43mm in the morphine group and by 39mm in the morphine/promethazine group (p = 0.26). Similarly, patients' anxiety decreased by 19mm in the morphine group and by 13mm in the morphine/promethazine group (p = 0.37). The average ED stay was 78 minutes longer in the morphine/promethazine group (p = 0.01), due to the strong sedative effect of promethazine. Patients' satisfaction and the rate of adverse events were similar in both groups.Conclusion. Intravenous administration of morphine/promethazine regimen for pain and anxiety relief associated with acute LBP showed no advantage over intravenous morphine alone and significantly lengthened the overall ED stay. Thus, we believe that promethazine has no place in acute LBP management in the adult ED setting.