STUDY OBJECTIVE This randomized, placebo-controlled trial evaluates the analgesic efficacy and safety of intravenous single-dose paracetamol and morphine for the treatment of renal colic. METHODS We conducted a randomized, double-blind, placebo-controlled clinical trial comparing single intravenous doses of paracetamol (1 g), morphine (0.1 mg/kg), and placebo (normal saline solution) for patients presenting to the emergency department (ED) with suspected renal colic. Subjects with inadequate pain relief at 30 minutes received rescue fentanyl (0.75 microg/kg). We compared changes in pain intensity 30 minutes after treatment among the 3 arms, as well as the need for rescue medication and the presence of adverse effects. RESULTS Six hundred forty-five consecutive patients were screened for study and 165 were entered. Eight subjects were subsequently excluded from analysis because of protocol violations and 11 were excluded because of uncertain diagnoses, leaving 146 subjects available for analysis. The mean reduction in visual analogue scale pain intensity scores at 30 minutes was 43 mm for paracetamol (95% confidence interval [CI] 35 to 51 mm), 40 mm for morphine (95% CI 29 to 52 mm), and 27 mm for placebo (95% CI 19 to 34 mm). Statistically significant mean differences in pain intensity reductions compared with those for placebo were observed for paracetamol (16; 95% CI 5 to 27; P=.005) and morphine (14; 95% CI 0.4 to 27; P=.05); however, no difference was found between paracetamol and morphine (2; 95% CI -13 to 16; P=.74). Rescue analgesics at 30 minutes were required by 21 subjects (45%) receiving paracetamol, 24 subjects (49%) receiving morphine, and 34 subjects (67%) receiving placebo (P=.08). At least 1 adverse effect was experienced by 11 (24%) receiving paracetamol, 16 (33%) receiving morphine, and 8 (16%) in the placebo group (P=.14). There were no serious adverse events. CONCLUSION Intravenous paracetamol is an efficacious and safe treatment for ED patients with renal colic.