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The objectives of the study were to determine the impact of nocebo effects on adverse events (AEs) in drug trials in fibromyalgia syndrome (FMS) and painful diabetic peripheral neuropathy (DPN).MEDLINE, CENTRAL, SCOPUS, and the databases of the U.S. National Institutes of Health and the Pharmaceutical Research and Manufacturers of America were searched until December 31, 2010. Randomized controlled trials with a parallel design of any drug therapy compared with pharmacological placebo in patients with FMS and DPN were included. Pooled estimates of nocebo effects (number of patients with at least 1 AE and dropping out due AEs) were calculated for placebo and true drug groups by a random effects model.Fifty-eight FMS (62 DPN) trials included a total of 5065 (5095) patients in placebo groups. The quality of reporting the assessment strategy of AEs was poor in most trials. The pooled estimate of the event rate drop out rate due to AEs in placebo groups was 9.6 [95% confidence control (CI): 8.6-10.7] in placebo and 16.3 (95% CI: 14.1-31.2) in true drug groups of FMS trials and was 5.8 (95% CI: 5.1-6.6) in placebo and 13.2 (95% CI: 10.7-16.2) in true drug groups of DPN trials. Nocebo effects accounted for 72.0% (44.9) of the drop outs in true drug groups in FMS (DPN).Nocebo effects substantially accounted for AEs in drug trials of FMS and DPN. Standards to assess and report AEs should be defined by regulatory agencies. Strategies to minimize nocebo effects in both clinical trials and clinical practice should be developed.