Gut Bacteria Overgrowth Linked to Higher Anxiety Risk in IBS Patients

If you have irritable bowel syndrome (IBS) and have also noticed your anxiety creeping up, a new study suggests your gut bacteria might be part of the story. Researchers tracking over 15,000 patients found that people with IBS who also had a bacterial overgrowth condition called SIBO were meaningfully more likely to develop an anxiety disorder than IBS patients without it. The link held up even after researchers controlled for age, sex, and several other gut conditions, adding to evidence that what happens in the gut doesn't stay in the gut.

Key takeaways:

• IBS patients with small intestinal bacterial overgrowth (SIBO) had a 6.36% rate of new generalized anxiety disorder (GAD) diagnoses over 15 years, versus 4.71% in IBS patients without SIBO.
• That works out to 38% higher odds of developing GAD (odds ratio 1.38) and a 30% higher hazard of developing it over time (hazard ratio 1.30) for the SIBO group.
• The study used real-world health records from over 15,432 matched patients, not a small lab sample, but it's retrospective and can't prove SIBO causes anxiety.
• SIBO wasn't confirmed with a breath test in this data, so some diagnoses may be less certain than in a dedicated SIBO study.

SIBO Diagnosis Tied to Higher Odds of Anxiety in IBS Patients

IBS patients who were also diagnosed with small intestinal bacterial overgrowth (SIBO, an excess buildup of bacteria in the small intestine) developed generalized anxiety disorder (GAD) at meaningfully higher rates than IBS patients without SIBO, according to a retrospective cohort study published in BMC Gastroenterology. Researchers from Drexel University and Reading Hospital pulled records from TriNetX, a health data network that aggregates de-identified electronic health records from more than 100 million patients across multiple healthcare systems. Over a 15-year follow-up window, 6.36% of IBS patients with SIBO developed GAD, compared with 4.71% of IBS patients without SIBO, a difference that translated to 38% higher odds (odds ratio 1.38, 95% CI 1.20-1.58) and a 30% higher hazard of developing the condition over time (hazard ratio 1.30, 95% CI 1.14-1.49).

Why This Matters for the Millions Living With IBS

IBS already carries a heavy psychological toll. A separate meta-analysis of anxiety and depression in IBS patients found anxiety disorders in roughly 39% of IBS patients, compared with about 23% of healthy controls, so clinicians already know these two conditions travel together often. What this new study adds is a possible reason some IBS patients seem to carry a heavier anxiety burden than others: their gut bacteria. SIBO is itself common in IBS, with a prior meta-analysis of SIBO in IBS patients estimating that as many as 35.5% of IBS patients test positive for it. If SIBO really does amplify anxiety risk on top of IBS, that's a meaningful chunk of the IBS population carrying extra psychiatric risk that current care may not be screening for.

How Researchers Built a 15,432-Patient Comparison

To isolate SIBO's effect from IBS itself, the researchers built two matched groups of 7,716 patients each, all aged 18 to 50: one group had both IBS and a SIBO diagnosis, the other had IBS alone. Anyone with a prior diagnosis of GAD, panic disorder, or depression before their IBS onset was excluded, so the study was only picking up anxiety that developed after (not before) their gut symptoms. The two groups were statistically matched for sex, race, ethnicity, acid reflux, other bowel inflammation, and use of common acid-suppressing medications, so the size of the study and the matching process are what let the researchers isolate SIBO's specific contribution rather than just IBS in general.

What This Means if You Have IBS

If you have IBS and you've noticed your anxiety has gotten worse, this study is a reason to mention it to your doctor rather than assume it's unrelated. Persistent or hard-to-manage IBS symptoms, especially bloating, gas, or symptoms that don't respond to standard IBS treatment, are sometimes signs worth asking about SIBO testing for. The authors frame this as an argument for a more coordinated approach: treating the gut and the mind together, not as two separate problems. That said, this study does not establish that treating SIBO will reduce anxiety, and it isn't a reason to self-diagnose SIBO or start any particular treatment. A 2023 review in Nature Reviews Gastroenterology & Hepatology on IBS and mental health likewise emphasizes multidisciplinary management, pairing gut-focused and psychiatric care, rather than treating either symptom set in isolation.

What This Study Can't Tell Us

This is a retrospective, correlational study, meaning it can show that SIBO and later anxiety diagnoses tend to occur together, not that SIBO causes anxiety. The researchers also relied on standardized diagnosis codes to identify SIBO cases in the health records, rather than requiring a breath test (the standard way SIBO is confirmed) for every patient, so some SIBO diagnoses in this dataset may be less rigorously confirmed than in a study designed specifically around SIBO testing. The data also came from a single database, TriNetX, so it hasn't yet been replicated in an independent dataset. Other likely contributors to anxiety risk, like diet, recent antibiotic use, and socioeconomic factors, weren't accounted for either.

What Comes Next

The study's authors call for prospective, randomized trials that would test the missing piece directly: does treating SIBO with antibiotics or other interventions actually reduce anxiety symptoms in IBS patients, or is the association just a marker of more severe underlying gut-brain dysfunction? Until that work is done, the honest takeaway is that SIBO looks like one more piece of the IBS-anxiety puzzle, not a proven trigger or a treatment target on its own.

Common Questions

Does this mean SIBO causes anxiety? No. The study found an association, not proof of cause and effect. It's a retrospective study, so it can't rule out that some other shared factor is driving both conditions.

How common is SIBO in people with IBS? Estimates vary, but a large meta-analysis found SIBO in roughly a third of IBS patients (35.5%), making it one of the more common comorbidities researchers look for when IBS treatment isn't working as expected.

Should I get tested for SIBO if I have IBS and anxiety? This study doesn't establish SIBO testing as a required step for every IBS patient with anxiety, but if your GI symptoms are persistent, unusual, or not responding to standard treatment, it may be worth raising with your doctor, especially alongside anxiety symptoms.

What is generalized anxiety disorder (GAD)? GAD is a diagnosable anxiety disorder marked by excessive, hard-to-control worry that persists most days for at least six months, often alongside physical symptoms like restlessness or fatigue.

References

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. BMC Gastroenterology. https://doi.org/10.1186/s12876-026-05255-w