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Introduction Elevated breath methane is used to diagnose intestinal methanogen overgrowth (IMO). Breath methane is strongly associated with constipation, and reducing methane may improve intestinal transit. However, treatment often requires dual antibiotic therapy. We investigated the prevalence of IMO in healthy adults compared to patients with gastrointestinal symptoms and explored microbiome correlates of methane production.Methods We studied 53 healthy adults participating in a clinical trial ( NCT04705662) and 731 patients who completed a lactulose breath test and questionnaire at a single UK centre (NCT05122520). All participants followed a low-fermentable diet and an overnight fast prior to breath testing. A baseline breath sample was collected, then the participants ingested 10 g of lactulose in 200 ml of water. Nine subsequent breath samples were collected every 15-minutes. IMO was defined as breath methane ≥10 ppm at any time during the test. Breath samples were collected in Exetainer vials (Labco, UK), analysed by gas chromatography (Agilent, CA), and corrected using a carbon dioxide factor. Statistical analysis included Spearman correlation, logistic regression and t-tests.Results The prevalence of IMO was 28.3% in healthy adults versus 31.7% in patients (p = 0.617). In healthy adults, methane levels correlated with stool methanogen abundance (rs = 0.744, p <0.001), and methane producers exhibited higher microbiome diversity (Shannon index p < 0.001; Simpson index p = 0.032) than non-producers. In the patient cohort, elevated methane was positively associated with constipation (p < 0.001) and negatively associated with diarrhoea (p = 0.002).Conclusion IMO occurs at similar rates in healthy adults and symptomatic patients. These findings suggest that a single breath methane cutoff may not be sufficient to guide antibiotic treatment, and therapeutic decisions should consider symptom correlation. Further research is needed to elucidate the mechanisms by which elevated methane contributes to constipation in certain individuals.