IBS Patients With SIBO Are 38% More Likely to Develop Anxiety, Large Cohort Study Finds

Roughly one in three people with irritable bowel syndrome also harbors an overgrowth of bacteria in their small intestine, and a new study suggests that combination carries a measurable psychiatric cost. Researchers analyzing electronic health records found that IBS patients with small intestinal bacterial overgrowth (SIBO) were significantly more likely to develop generalized anxiety disorder (GAD) over a 15-year window than IBS patients without SIBO. The findings, published in June 2026 on Research Square by Womack and colleagues, add real-world weight to the idea that the gut microbiome can directly influence psychiatric outcomes.

SIBO and GAD Incidence: What the Numbers Actually Show

The study drew on de-identified records from TriNetX, a global health research network, identifying two matched cohorts of 7,716 patients each, all between 18 and 50 years old and all diagnosed with IBS. Cohort 1 had a concurrent SIBO diagnosis; Cohort 2 did not. Patients with pre-existing generalized anxiety disorder, panic disorder, or depressive episodes before their IBS onset were excluded to prevent confounding. The cohorts were propensity-matched for sex, race, ethnicity, gastroesophageal reflux disease (GERD), non-infective enteritis, and proton pump inhibitor use.

Over the 15-year follow-up, GAD developed in 6.36% of the SIBO group versus 4.71% of the non-SIBO group. That 1.66 percentage point absolute difference (95% CI: 0.938% to 2.38%, p<0.0001) translated into an odds ratio of 1.377 (95% CI: 1.197 to 1.583) and a hazard ratio of 1.301 (95% CI: 1.136 to 1.491, p=0.0008). In plain terms: having SIBO alongside IBS was associated with roughly 30% to 38% elevated risk of receiving a GAD diagnosis, depending on the statistical model used. The authors describe the effect size as modest but clinically meaningful.

Why the Gut-Brain Axis Makes This Plausible

IBS has long been understood as a disorder of the gut-brain-microbiome, driven by bidirectional signaling between the central nervous system and the gastrointestinal tract. What SIBO may do, the authors argue, is amplify that dysfunction. When bacteria proliferate abnormally in the small intestine, they can trigger pro-inflammatory cytokine release, increase intestinal permeability, and disrupt the production of neurotransmitters including serotonin, much of which is synthesized in the gut wall. That triple mechanism, inflammation, leaky gut, and scrambled chemical signaling, provides a plausible biological route from an intestinal bacterial problem to an anxiety disorder.

Prior research had already flagged both components of this picture independently. A 2020 meta-analysis of 25 case-control studies found SIBO prevalence in IBS patients was around 31%, with an odds ratio of 3.7 compared to healthy controls. Separate work has documented associations between SIBO and psychiatric disorders including GAD, panic disorder, and major depressive disorder, suggesting the gut microbiome's influence on brain function may not be incidental. Broader gut-brain axis research has increasingly positioned the microbiome as a central mediator of mood and cognition, not merely a passenger.

The study cohort was predominantly female (76%), white (75%), and non-Hispanic (74%), reflecting the known demographic skew of IBS, which affects women and people under 50 at disproportionately higher rates in the US, UK, and Canada.

What the Retrospective Design Cannot Settle

The authors are careful about what their data can and cannot prove. Because this was a retrospective analysis using ICD-10 diagnostic codes, the study cannot confirm how SIBO was diagnosed in each case, whether by breath test, endoscopic culture, or clinical impression. Diagnostic method matters because SIBO diagnosis itself remains technically challenging, with considerable variability in testing criteria and no universal gold standard.

Severity also goes unmeasured: patients with more aggressive IBS may be both more likely to undergo SIBO testing and more prone to psychiatric comorbidity, which could inflate the apparent association. Variables that almost certainly matter, including dietary patterns, recent antibiotic use, and socioeconomic status, were not captured in the matched design. And because this is an observational study, it cannot establish whether SIBO causes anxiety, whether anxiety worsens gut permeability and drives SIBO, or whether a third factor drives both. The hazard ratio tells us that SIBO-positive IBS patients receive GAD diagnoses at a higher rate; it does not tell us why the biological arrow points in that direction.

Toward Integrated Care for IBS With Psychiatric Symptoms

Despite those caveats, the clinical implication the authors draw is straightforward: gastroenterologists treating IBS patients with persistent or treatment-resistant symptoms should consider evaluating for SIBO, particularly when anxiety symptoms are present. Recognizing the overlap earlier could allow for more timely psychiatric referral and a coordinated treatment approach that addresses both the gut and the brain simultaneously.

The researchers call for prospective randomized controlled trials to test whether treating SIBO in IBS patients reduces the incidence or severity of anxiety. That would be the cleaner test: assign patients to SIBO treatment versus placebo, then track psychiatric outcomes. Future research into gut-brain-microbiome interactions will need to incorporate microbial metabolite profiling, environmental covariates, and longer follow-up windows to disentangle the causal threads.

For now, the Womack et al. study adds a concrete, population-scale data point to a picture that has been building for years: what happens in the small intestine does not stay in the small intestine. In a cohort of more than 15,000 people tracked for up to 15 years, having an abnormal bacterial community in the gut was measurably linked to developing an anxiety disorder. The effect is not dramatic, but it is consistent, statistically robust, and increasingly hard to dismiss as coincidence.

Evidence rating

EMERGING — 75/100

Caveats:

• Retrospective observational design — this study shows association, not causation; SIBO may not directly cause anxiety.
• Diagnoses were based on ICD-10 billing codes, with no information on SIBO diagnostic method or disease severity.
• Unaccounted confounders include diet, recent antibiotic use, and socioeconomic status.
• The absolute risk difference is modest (1.66%), meaning the elevated risk, while statistically significant, is small in practical terms.
• Funding source was not disclosed by the authors.
• This is a preprint and has not yet completed peer review.

Retrospective propensity-matched cohort study using de-identified EHR data (n=7,716 per cohort). Demonstrates a statistically significant association between SIBO comorbidity in IBS patients and subsequent generalized anxiety disorder incidence over 15 years, but cannot establish causation due to its observational design. Findings are consistent with prior gut-brain axis literature but have not been independently replicated, and this work remains a preprint pending peer review.

References

1. Judah Womack, William Kohman, Jesse Devlin, Ryan Nader, Eduardo Espiridion (2026). Role of Small Intestinal Bacteria Overgrowth in Irritable Bowel Syndrome and Anxiety: A Retrospective Electronic Health Record (EHR)- Based Cohort Study Using TriNetX. https://doi.org/10.21203/rs.3.rs-9917138/v1
2. Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome – An Update – Frontiers. frontiersin.org. https://www.frontiersin.org/articles/10.3389/fmicb.2021.670878/full
3. Bartosz Bogielski, Katarzyna Michalczyk, Piotr Głodek, Bartosz Tempka, Wojciech Gębski, Dominika Stygar (2024). Association between small intestine bacterial overgrowth and psychiatric disorders. Frontiers in Endocrinology. https://doi.org/10.3389/fendo.2024.1438066
4. Priyanka Mehta (2024). Microbiome and mind: current insights and future directions in gut-brain research. World Journal of Biology Pharmacy and Health Sciences. https://doi.org/10.30574/wjbphs.2024.19.1.0396
5. Bruno César da SILVA, Gabriela Piovezani RAMOS, Luisa Leite BARROS, et al. (2025). DIAGNOSIS AND TREATMENT OF SMALL INTESTINAL BACTERIAL OVERGROWTH: AN OFFICIAL POSITION PAPER FROM THE BRAZILIAN FEDERATION OF GASTROENTEROLOGY. Arquivos de Gastroenterologia. https://doi.org/10.1590/s0004-2803.24612024-107
6. Tien S. Dong, Emeran Mayer (2024). Advances in Brain–Gut–Microbiome Interactions: A Comprehensive Update on Signaling Mechanisms, Disorders, and Therapeutic Implications. Cellular and Molecular Gastroenterology and Hepatology. https://doi.org/10.1016/j.jcmgh.2024.01.024