The human gut microbiota creates a complex microbial ecosystem, characterized by its high population density, wide diversity, and complex interactions. Any imbalance of the intestinal microbiome, whether qualitative or quantitative, may have serious consequences for human health, including small intestinal bacterial overgrowth (SIBO). SIBO is defined as an increase in the number of bacteria (10³-10⁵ CFU/mL), an alteration in the bacterial composition, or both in the small intestine. The PubMed, Science Direct, Web of Science, EMBASE, and Medline databases were searched for studies on SIBO and related diseases. These diseases were divided into 12 groups: (1) gastrointestinal disorders; (2) autoimmune disease; (3) cardiovascular system disease; (4) metabolic disease; (5) endocrine disorders; (6) nephrological disorders; (7) dermatological diseases; (8) neurological diseases (9); developmental disorders; (10) mental disorders; (11) genetic diseases; and (12) gastrointestinal cancer. The purpose of this comprehensive review is to present the current state of knowledge on the relationships between SIBO and these 12 disease groups, taking into account risk factors and the causal context. This review fills the evidence gap on SIBO and presents a biological-medical approach to the problem, clearly showing the groups and diseases having a proven relationship with SIBO, as well as indicating groups within which research should continue to be expanded. These diseases were divided into 12 groups: (1) gastrointestinal disorders; (2) autoimmune disease; (3) cardiovascular system disease; (4) metabolic disease; (5) endocrine disorders; (6) nephrological disorders; (7) dermatological diseases; (8) neurological diseases (9); developmental disorders; (10) mental disorders; (11) genetic diseases; and (12) gastrointestinal cancer. The purpose of this comprehensive review is to present the current state of knowledge on the relationships between SIBO and these 12 disease groups, taking into account risk factors and the causal context. Even though most symptoms of SIBO may be limited to the gastrointestinal tract, an increasing body of evidence has emphasized the relationships between SIBO and other diseases. For this study, these associated diseases were divided into 12 groups: (1) gastrointestinal disorders; (2) autoimmune disease; (3) cardiovascular system disease; (4) metabolic disease; (5) endocrine disorders; (6) nephrological disorders; (7) dermatological diseases; (8) neurological diseases; (9) developmental disorders; (10) mental disorders; (11) genetic diseases; and (12) gastrointestinal cancer. This review fills the evidence gap on SIBO. [ 102 ], in a group of 35 patients with NAFLD, examined intestinal permeability and assessed the correlation between this phenomenon and the disease, the integrity of tight junctions in the small intestine, and the presence of SIBO. SIBO was diagnosed using GBT. NAFLD in humans is associated with increased intestinal permeability, and this abnormality is associated with an increased incidence of SIBO in these patients. For this reason, SIBO, low vitamin K2 status, or both could hypothetically pose an increased risk for the development of atherosclerotic disease [ 142 ]. It was confirmed through a study conducted in 2022 [ 141 ] that SIBO is associated with subclinical atheromatous plaques. Plaques were more common in the SIBO+ group than in the SIBO− group, and the mechanism of this association warrants further exploration [ 141 ]. Group 3 summary: In summary, it should be noted that knowledge regarding the relationship between SIBO and diseases in this group is flourishing, promoting continued expansion in this field of research. Undoubtedly, established facts such as the relationship between SIBO and CKI make it possible to observe patients for SIBO. Determination of the association between SIBO and AKI unquestionably requires further research in a representative cohort of patients. 5.7. Group 7. Dermatological Diseases 5.7.1. Acne Rosacea Acne rosacea is a chronic inflammatory After three months, no recurrence of psoriatic lesions in the oral cavity was observed [ 181 ]. The combination of rifaximin with a prebiotic that can benefit intestinal motility seems to be more useful in eradicating SIBO than rifaximin alone. Moreover, rifaximin is similar to rifampin, which is a biofilm-dispersing agent in addition to its antibacterial action. In other studies, Drago et al. [ 182 ] have shown that the prevalence of SIBO in patients with psoriasis is comparable to that in the control group. Therefore, further research is needed in order to explain the relationship between psoriasis and SIBO. Group 12: Gastrointestinal Cancer To date, the associations between SIBO and pancreatic cancer, cholangiocarcinoma [ 207 ], gastric and colorectal cancer (CRC) [ 208 , 209 ], esophagogastric cancer (OGC) [ 210 ], and hepatocellular carcinoma (HCC) [ 211 ] have been considered in patients. Liang et al. [ 208 ] have shown that in Chinese patients, SIBO is associated with gastrointestinal cancer. The preliminary study concluded that probiotic intervention combats SIBO in patients with this cancer and alleviates its symptoms. Deng et al. This observation requires further research Group 12 Gastrointestinal cancer SIBO is associated with gastrointestinal cancer Post-operative gastric and colorectal cancer patients are more likely to develop SIBO Probiotic intervention combats SIBO in patients with gastrointestinal cancer and alleviates its symptoms TLR4 protein expression in pancreatic carcinoma and cholangiocarcinoma patients was significantly higher in SIBO+ than SIBO− patients The expression levels of TLR2 and TLR4 and the incidence of SIBO in hepatocellular carcinoma (HCC) patients are significantly higher and may promote the development of HCC Finally, the limitations in studying the relationships between SIBO and various diseases should be mentioned.