A systematic review was conducted following PRISMA 2020 guidelines across PubMed, Scopus, CENTRAL, and PsycINFO. Studies were eligible if psychiatric diagnoses were DSM-V–based and IM composition was characterized at phylum, family, or genus level. Observational and interventional studies were included. Risk of bias was assessed using NIH Quality Assessment Tools.

Eighty studies involving 2,691 participants met criteria. Consistent alterations were observed in Firmicutes, Bacteroidetes, and Actinobacteria, with both disorder-specific and overlapping patterns. ASD showed reduced Firmicutes (↓4.8%) and Bacteroidetes (↓3.3%), but elevated Bifidobacteriaceae (↑5.9%) and Eggerthellaceae (↑5.5%). Mood disorders, including depression and bipolar disorder, were associated with increased Christensenellaceae (↑18.1%) and decreased Ruminococcaceae (↓2.0%). Schizophrenia displayed enrichment of Lachnospiraceae, Christensenellaceae, and Enterobacteriaceae (↑11–28%) alongside reductions in Akkermansia and Turicibacteraceae (↓9–28%). Eating disorders showed profound dysbiosis, with complete loss of Akkermansia (100%) and reductions in Lactobacillus (↓48.5%). ADHD was marked by Firmicutes/Bacteroidetes imbalance (↑49.8%, ↓56.6%).