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Objective: We examined nonsystematic adverse events (AEs) in Part 2 (of 2) of a study describing the assessment and reporting AEs in clinical trials. Study Design and Setting: We examined 21 trials of gabapentin for neuropathic pain (52 sources) and seven trials of quetiapine for bipolar depression (80 sources) using data from the Multiple Data Sources study. We extracted and compared information about nonsystematic AEs (i.e., AEs that were not assessed for every participant), including AEs categorized as “serious.” We recorded whether AEs were grouped by anatomic or physiological system. Results: Trials of the same drug reported information about different AEs. Information in public sources was inadequate for decision-making. No public source reported all AEs, or all serious AEs, identified in nonpublic sources about the same trial. Of trials with only public sources, 2/15 (13%) gabapentin and 0/3 (0%) quetiapine trials grouped AEs by anatomic or physiological system. Conclusion: Public sources contained little information about nonsystematic AEs, including serious AEs. Grouping might make nonsystematic AEs easier to detect; however, most public sources did not report grouped AEs. Standards are needed to improve the collection and reporting of nonsystematic AEs so that stakeholders can use trials to assess the balance of potential benefits and harms.