Rationale: It remains unclear whether using N-acetylcysteine as an adjunctive treatment has any benefit for bipolar depression and major depressive disorder. Objectives: A systematic review and random-effect meta-analysis of double-blind, randomized placebo-controlled trials was conducted to explore the clinical question. Methods: Outcomes included improvement in depression scale scores (primary), Young Mania Rating Scale score, Hamilton Anxiety Rating Scale score, Clinical Global Impression-Severity Scale (CGI-S) score, Global Assessment of Functioning Scale score, Social and Occupational Functioning Assessment Scale score, Range of Impaired Functioning Tool score, Streamed Longitudinal Interval Clinical Evaluation for the Longitudinal Interview Follow-Up Evaluation score, quality of life scales scores, and the incidence of all-cause discontinuation and individual adverse events. Results: Seven studies (n = 728, 8–24 weeks, mean age = 46.81, % female = 58.45%) were included. N-acetylcysteine did not improve depressive symptoms compared with placebo (N = 7, n = 579, standardized mean difference (SMD) = − 0.12, 95% confidence interval (95% CI) = − 0.38, 0.14, p = 0.38, I2 = 52.74%). The meta-regression analysis detected an association between effect size and publication year (coefficient = 0.06, 95% CI = 0.00, 0.11, p = 0.04, I2 = 27.56%). Although N-acetylcysteine was superior to placebo in CGI-S score (N = 6, n = 563, SMD = − 0.28, 95% CI = − 0.47, − 0.10, p < 0.01, I2 = 14.88%), there was no significant difference in the other efficacy outcomes between the treatment groups. Although N-acetylcysteine was associated with a higher incidence of gastrointestinal adverse events compared with placebo (N = 4, n = 537, risk ratio = 1.79, 95% CI = 1.37, 2.32, p < 0.01, I2 = 0.00%, number needed to treat to harm = 7), there was no significant difference in all-cause discontinuation and other safety outcomes between the treatments. Conclusions: Although N-acetylcysteine decreased CGI-S score, no specific improvements in symptoms were identified.
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