Aim Data on the adverse events of opioids for cancer-related pain in Sudanese patients are limited. We conducted this study to evaluate the pattern and severity of adverse events of immediate release morphine, and tramadol used in the treatment of cancer-related pain. A secondary aim was to determine the response rate to opioids for pain control in cancer patients treated at the National Cancer Institute-University of Gezira (NCI-UG), Sudan. Methods This descriptive cross-sectional study was conducted at NCI-UG between 12 March 2019 and 12 May 2019. A pre-designed questionnaire was used to collect the clinical data of cancer patients on oral opioids for pain control during the study periods. Chi square test was applied to determine whether there is a significant association between the adverse events and the demographic/clinical variables. p value < 0.05 was considered statistically significant in all analyses. Results One-hundred and thirteen patients were screened in the study. Of these, three suffered from cognitive dysfunction and ten patients declined to participate in the study. Thus, 100 patients met the criteria for inclusion in this study. Breast cancer was the most frequent diagnosis (17%) followed by colorectal cancer (16%). The majority (91%) of patients had advanced or metastatic disease. The most frequently reported opioids-related adverse events were dry mouth (68%), constipation (61%), sedation (49%), nausea (31%) and vomiting (24%). Most of these symptoms were graded as mild to moderate and no patient had grade IV or V adverse events. Constipation and dry mouth were more frequent among patients received morphine compared to patients received tramadol ( p value < 0.005). Pain was controlled in 36% of patients, improved in 53% and not controlled in 11% of them. Conclusion This study shows a high prevalence of opioids-related adverse events. The majority of the opioids-related adverse events were grade I or grade II. There seem to be differences in the prevalence of opioids-related adverse events between patients receiving tramadol and those treated with morphine. Moreover, suboptimal pain control adds to the burden on already limited health resources. Therefore, the adequacy of cancer pain management in our setting should be systematically evaluated and effective cancer pain management programmes should be developed.