SIBO vs. IBS: The Overlap, Post-Infectious Autoimmunity & Rome IV

In short: irritable bowel syndrome (IBS) is diagnosed from a pattern of symptoms; small intestinal bacterial overgrowth (SIBO) is diagnosed with a test. The two overlap: a 2020 meta-analysis found that people with IBS had 3.7 times the odds of SIBO compared with people without IBS, but the rates varied widely with the test used and the evidence was rated low quality³. SIBO may explain symptoms in a subset of people with IBS, especially those with diarrhea or a history of food poisoning, but it is not the cause of most IBS.

This guide explains how SIBO and IBS differ, where they overlap, and what the overlap means for testing and treatment.

What is IBS?

IBS is a disorder of gut-brain interaction defined by recurring abdominal pain linked to changes in bowel habits, under the Rome IV criteria. It is common: in a 2021 survey of more than 73,000 adults in 33 countries, 4.1% of internet respondents met the Rome IV criteria, and 10.1% met the older Rome III criteria².

The 2021 US guideline on IBS recommends making a positive diagnosis based on symptoms, rather than treating IBS only as what is left after excluding everything else. It suggests a blood test for celiac disease and a stool calprotectin test to rule out inflammatory bowel disease in people with diarrhea symptoms¹.

IBS is grouped by the main bowel pattern: IBS with diarrhea (IBS-D), IBS with constipation (IBS-C) and mixed IBS.

How SIBO and IBS differ

How often do people with IBS have SIBO?

Estimates range widely because studies used different tests and cut-offs. The best summary is a 2020 meta-analysis of 25 case-control studies with more than 3,000 people with IBS³:

• People with IBS had 3.7 times the odds of SIBO compared with controls.
• With breath tests, 35.5% of people with IBS tested positive, compared with 29.7% of controls.
• With fluid culture at the 1,000 CFUColony-forming units (CFU) A count of live microbes in a sample. SIBO is defined as more than 1,000 CFU per milliliter of small-intestinal fluid (older definitions used 100,000)./mL threshold, 33.5% of people with IBS had SIBO, compared with 8.2% of controls.
• LactuloseLactulose A synthetic sugar the body cannot digest or absorb. As a breath-test substrate it travels the whole small intestine, which also makes false positives more likely. breath tests found SIBO 3.6 times more often than glucose tests in people with IBS, and 7.6 times more often in controls.
• SIBO was more common in IBS-D, and methane-positive tests were more common in IBS-C.

The large difference between lactulose and glucose results is one reason for caution. In a 2011 imaging study of 40 people with IBS, the lactulose test meal had already reached the colon when hydrogen rose in 22 of 25 positive tests, suggesting many positive lactulose results reflect fast transit rather than SIBO¹².

Food poisoning, IBS and SIBO

A bout of gastroenteritis is one of the clearest triggers for IBS. A 2017 meta-analysis of 45 studies with more than 21,000 people found that about 10% developed IBS within a year of infectious enteritis, and the risk was 4.2 times higher than in people without a recent infection⁶. After Campylobacter infection specifically, the risk of IBS is 2.7 to 5.6 times higher, and in animal studies the same infection leads to SIBO⁸.

One proposed mechanism involves a bacterial toxin called cytolethal distending toxin BCytolethal distending toxin B (CdtB) A toxin released by food-poisoning bacteria such as Campylobacter jejuni, E. coli and Salmonella. Antibodies against it can cross-react with vinculin and damage the gut nerves that run the migrating motor complex. (CdtB), made by Campylobacter, E. coli, Salmonella and other food-poisoning bacteria. In animal studies, antibodies against CdtB cross-react with vinculinVinculin A protein found in gut nerve cells. Anti-vinculin antibodies, which can form after food poisoning, are a marker of post-infectious IBS., a protein in gut nerve cells, producing an IBS-like condition. In a 2015 study of more than 2,000 people with IBS-D, anti-CdtB and anti-vinculin antibodies were higher than in people with inflammatory bowel disease, celiac disease or healthy controls⁷. These antibodies are the basis of a commercial blood test for post-infectious IBSPost-infectious IBS Irritable bowel syndrome that develops after a bout of food poisoning. It is linked to anti-CdtB and anti-vinculin antibodies that impair gut motility.; see our blood tests guide.

Damage to gut nerves can weaken the migrating motor complexMigrating motor complex (MMC) A wave of muscle contractions that sweeps the stomach and small intestine between meals, roughly every 90 to 120 minutes, pushing leftover food and bacteria into the colon. Eating interrupts it., the wave of contractions that sweeps bacteria out of the small intestine between meals. Absence of this wave is associated with SIBO¹⁴, which offers a possible link between food poisoning, IBS and SIBO.

Gas types and IBS subtypes

Breath gases line up with IBS subtypes:

• Methane and IBS-C. Methane slows the gut; in a 2006 animal study it slowed small-intestinal transit by 59%¹¹. People with IBS-C have higher breath methane and more Methanobrevibacter in stool¹⁰. See our methane SIBO guide.
• Hydrogen and hydrogen sulfide and IBS-D. People with IBS-D have higher breath hydrogen and hydrogen sulfide, with more hydrogen- and sulfide-producing bacteria in stool¹⁰. See our hydrogen sulfide SIBO guide.

A 2024 study that cultured and sequenced fluid from the upper small intestine of 483 people found that, at 1,000 CFU/mL or more, microbial diversity fell and a few strains of E. coli and Klebsiella made up about 40% of all bacteria. These changes were linked to the severity of abdominal pain, diarrhea and bloating¹³.

Pain, bloating and the gut-brain axis

Not all IBS symptoms come from bacteria. Many people with IBS have visceral hypersensitivityVisceral hypersensitivity Heightened sensitivity of the gut's nerves, so that normal amounts of gas or stretching are felt as pain., meaning normal amounts of gas or stretching are felt as pain¹⁵. Some also have an abnormal reflex after meals in which the diaphragm contracts and the abdominal wall relaxes, causing visible distension without much extra gas¹⁶. These mechanisms help explain why treating SIBO does not resolve every case of IBS, and why the US IBS guideline also recommends gut-directed psychotherapy¹.

What this means for treatment

• RifaximinRifaximin An antibiotic that is barely absorbed into the bloodstream, so it acts mainly inside the gut. It is a common prescription treatment for SIBO. is approved for IBS-D. In two large 2011 trials of people with IBS without constipation, 2 weeks of rifaximin 550 mg three times a day relieved global IBS symptoms in 40.7% of people, compared with 31.7% on placebo⁹. The US IBS guideline recommends it for IBS-D¹.
• Methane and constipation. For people with IBS and methane on a breath test, rifaximin combined with neomycin cleared methane in 87% in one study¹⁷. See our antibiotics guide.
• Low-FODMAPFODMAPs Fermentable oligo-, di- and monosaccharides and polyols: short-chain carbohydrates that are poorly absorbed and quickly fermented by gut microbes. diet. The US IBS guideline recommends a limited trial of the low-FODMAP diet¹; see our low-FODMAP guide.
• Relapse prevention. If SIBO is found and treated, a prevention plan addressing gut motility is usually needed.

Should people with IBS be tested for SIBO?

The US SIBO guideline suggests that breath testing can be useful for diagnosing SIBO in people with IBS⁴. Given the risk of false positives with lactulose¹², results should be interpreted alongside symptoms and risk factors. Our diagnosis guide explains which test to use, and our symptom checker can help you prepare for the conversation with your doctor.

Common questions

Is it SIBO or IBS?

It can be both. IBS describes a pattern of symptoms; SIBO is a specific, testable finding that is present in some people with IBS³. A breath test or fluid culture is needed to tell whether SIBO is part of the picture.

Can SIBO cause IBS?

SIBO can produce IBS-like symptoms, and treating it can improve them. Food poisoning is a shared risk factor for both⁶⁸. But most people with IBS do not have SIBO on the more specific tests³.

Why does rifaximin help IBS?

It reduces gut bacteria, which may help when SIBO or other changes in gut bacteria contribute to symptoms. In the 2011 trials, it helped about 9 more people in every 100 than placebo⁹.