Background: Renal colic is an acute flank pain which may radiate to the groin, lower abdomen, or external genitalia due to the passage of a urinary stones. Pain management is the most important task in emergency wards when a patient with renal colic attends. This study aims to compare intravenous acetaminophen plus a low dose of morphine with a full dose of morphine in renal colic. Methods: In present randomized clinical trial, 100 patients with confirmed renal colic were recruited from the emergency ward of Imam Reza Teaching Hospital affiliated to Tabriz University of Medical Sciences, Tabriz, Iran, during a one-year period. These patients randomly received either intravenous acetaminophen (Apotel, 1 g) plus a low dose of morphine (n = 50), or a high dose of morphine (5 mg) (n = 50). Visual analogue scale was used for reporting pain during 35 minutes. Side effects and rescue analgesic demand were recorded after 30 minutes. Findings: The two groups were matched for the patients' age and gender. Intra-group analysis showed significant gradual decreases in pain intensity after 35 minutes for both groups. Inter-group analysis, however, did not show a significant difference between the two groups in this regard. There was no significant difference between the two groups in terms of side effects. The rate of rescue analgesic demand was 36% in the first and 40% in the second group (P = 0.68). Conclusion: According to the results study, Apotel plus a low dose of morphine is at least as effective and safe as a full dose of morphine in patients with renal colic.