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Unlabelled: Data on the tolerability of opioids in patients with cancer-related pain are limited. Here, we report a systematic review that includes all published prospective studies reporting adverse events (AEs) of morphine, oxycodone, fentanyl, methadone, or hydromorphone for cancer-related pain in patients naive for these opioids. We included 25 studies describing 31 treatment cohorts, made an overview of study characteristics, and reported rates of AEs per type of opioid. The frequency of the most commonly reported AEs varied widely: nausea from 3 to 85%, vomiting from 4 to 50%, constipation from 5 to 97%, drowsiness from 3 to 88%, and dry mouth from 1 to 94%. There was a large heterogeneity among included studies, especially regarding the assessment and reporting of AEs. We describe how differences in assessment and reporting influence outcome rates. Although AEs are an important issue in daily clinical practice, realistic incidence rates of AEs per type of opioid are unknown because of the immense heterogeneity among studies., Perspective: Although opioid-related adverse events are an important issue when treating cancer-related pain, realistic rates of adverse events per type of opioid are unknown because of immense heterogeneity among studies and lack of systematic assessment and reporting. There is an urgent need for studies with standardized outcome measures and reporting., (Copyright © 2015 American Pain Society. Published by Elsevier Inc. All rights reserved.)