Small intestinal bacterial overgrowth syndrome (SIBO) and intestinal methanogen overgrowth (IMO) are disorders involving excessive colonization of the small intestine by bacteria (in the case of SIBO) or methanogenic archaeons (in the case of IMO). These disorders are increasingly being diagnosed in the general population, and their symptoms, such as bloating, a feeling of fullness, abdominal pain, diarrhea or constipation, significantly reduce patients' quality of life. This study assesses the knowledge of patients diagnosed with SIBO or IMO after a hydrogen or hydrogen-methane test about nutrition and supplementation during and after antibiotic therapy. A survey conducted from February to May 2024 involved 210 people aged 19−68 years. Among respondents, the most common forms of SIBO were hydrogen-methane and hydrogen-methane and IMO (32.4%, 30.5% and 32.9% of respondents, respectively). The most commonly reported symptoms were bloating (97.1%), abdominal pain (71.9%) and excessive gas (55.7%). Irritable bowel syndrome (IBS) was the most common comorbidity in respondents (24.3%). The high percentage of respondents (77.6%) declaring current or past use of the low-FODMAP diet indicates its popularity in the nutritional management of people with SIBO/IMO. In the context of supplementation, respondents most often declared the use of probiotics (92.4%), sodium butyrate (80%), B vitamins (45.7%) and digestive enzymes (30.5%). Knowledge of natural substances that exhibit antimicrobial activity was also high, with oregano indicated by 64.3% of respondents and berberine by 61%. Statistical analysis showed significant differences in the level of knowledge - patients referred for diagnosis by a nutritionist showed higher nutritional awareness than those referred by physicians (p < 0.05). There was no correlation between time since diagnosis and patients' level of knowledge. The survey proved that there is a need to intensify nutrition education in this group of patients, especially during the period of active antibiotic therapy. It is reasonable to conduct organized educational activities and to include specialized diet therapy more widely in the standards of management of patients with SIBO/IMO. Keywords: Small intestinal bacterial over