BACKGROUNDDespite the fact that numerous medications have been introduced to treat renal colic, none has been proven to relieve the pain rapidly and thoroughly. In this study, we aimed at comparing the effects of intravenous lidocaine versus intravenous morphine in patients suffering from renal colic.METHODSIn a prospective randomized double-blind clinical trial performed in the emergency department of Imam Reza educational hospital of Tabriz, Iran, we studied 240 patients, 18-65 years old, who were referred due to renal colic. Patients were divided into two groups. In group I (120 people) single-dose intravenous lidocaine (1.5 mg/kg) was administered and in group II (120 people) single-dose intravenous morphine (0.1 mg/kg) was administered slowly. Visual Analogue Pain Scale (VAS) was recorded while admission, 5, 10, 15 and 30 minutes after injection. Statistical data and results were studied using descriptive statistics as percentage and Mean ± SD. To compare the response to treatment, Mann-Whitney U-test was used in two groups. Consequently, the data were analyzed using the SPSS16 software.RESULTSPain score measured in two groups five minutes after the injection of lidocaine and morphine were 65 % and 53 % respectively (95% CI 0.60 - 0.69, CI 0.48 - 0.57, p = 0.0002).108 (90 %) patients (95 % CI 0.84 - 0.95) from group I and 84 (70%) patients (95 % CI 0.62 - 0.78) from group II responded appropriately at the end of the complete treatment. The difference was statistically significant (p = 0.0001).CONCLUSIONSChanging the smooth muscle tone and reducing the transmission of afferent sensory pathways, lidocaine causes a significant reduction in pain.TRIAL REGISTRATIONClinical Trials IRCT138901042496N3.