Objective: Compare the analgesic ef cacy and tolerability of acetaminophen versus acetaminophen plus codeine in the relief of cancer-related pain.Methods: This 7-day, prospective, double-blind, randomized, parallel-group study was conducted with 36 patients with cancer pain.Outpatients were eligible for the study if they were aged ≥ 18 years and had moderate to severe cancer-related pain.Eligible patients were randomly assigned to receive acetaminophen or acetaminophen plus codeine.The primary end point was the proportion of patients who achieved pain relief; the second end point was time to achieve relief pain.Results: Of the 36 patients who participated, 20 received acetaminophen and 16 acetaminophen plus codeine.At baseline there were not differences between the groups.None of the between-group differences in response rates were signi cant (p > 0,05).The most common adverse effect in codeine group was constipation (p = 0,001).Conclusion: Ef cacy was comparable between acetaminophen and acetaminophen plus codeine over 7 days of treatment in this patients with moderate or severe cancer-related pain.Codeine was associated with signi cantly greater constipation.