SIBO became a popular explanation for IBS after studies using breath tests suggested that many people with IBS had it. Later work showed that part of that link may come from the test itself: in one study, when a lactulose breath test turned positive, some of the test meal had already reached the large intestine in 22 of 25 cases, so the hydrogen could have come from normal colon bacteria rather than overgrowth.[1] Experts still disagree about how SIBO should be defined and what role it plays in IBS symptoms.[2]
What is SIBO, and how is it different from IBS?
Small intestinal bacterial overgrowth (SIBO) means there are too many bacteria in the small intestine, causing gut symptoms.[3] It has long been recognized in people whose small bowel empties slowly or abnormally, for example after small-bowel surgery or in diseases such as scleroderma.[4] IBS, by contrast, is a disorder of gut–brain interaction, diagnosed from its pattern of symptoms.[5]
| IBS | SIBO | |
|---|---|---|
| What it is | Disorder of gut–brain interaction[5] | Excess bacteria in the small intestine causing symptoms; the definition is still debated[3][2] |
| Typical symptoms | Recurrent belly pain linked to bowel movements, with changes in stool frequency or form[6] | Bloating, diarrhea, belly pain or discomfort; fatty stools (steatorrhea) in severe cases[2] |
| Lab clues | Simple blood and stool tests are normal[5] | Sometimes high folate, or low vitamin B12 or other nutrients[2] |
| How it's diagnosed | Symptom criteria (Rome IV) plus a few basic tests[6][5] | Culture of fluid sampled from the small intestine, or a glucose or lactulose breath test[4] |
| Main treatments | Diet, soluble fiber, psychological therapies and symptom-targeted medicines[5] | Correcting the underlying cause and any nutritional deficiencies, and antibiotics[2] |
Do I have IBS or SIBO? Why symptoms can't settle it
The main symptoms overlap, and the research linking the two conditions is shakier than it looks. A 2020 meta-analysis of 25 studies found positive SIBO tests more often in people with IBS than in controls (odds ratio 3.7), but rated the overall quality of evidence as low, largely because the studies used different selection criteria and imperfect tests.[7] The details show why:
- With breath tests, about 36% of people with IBS tested positive — but so did about 30% of people without IBS.[7]
- Lactulose breath tests found SIBO 3.6 times as often as glucose breath tests in people with IBS, and 7.6 times as often in controls.[7]
- Studies that cultured small-intestine fluid also found SIBO more often in IBS, with rates that varied widely depending on the cutoff used.[7] British guidelines, however, point to aspiration studies that found no increase in people with suspected IBS.[5]
So the honest answer to "is it SIBO or IBS?" is that symptoms can't tell you, and a positive breath test on its own doesn't prove that bacteria are the cause.
How reliable are SIBO breath tests?
A breath test measures hydrogen and methane after you drink a sugar solution, either glucose or lactulose. Microbes that ferment the sugar release these gases, which you breathe out. Under a North American consensus, a rise in hydrogen of at least 20 parts per million within 90 minutes counts as positive for SIBO.[8] The catch:
- Fast transit can look like SIBO. Lactulose isn't absorbed, so if it reaches the colon quickly, normal colon bacteria produce hydrogen early.[1] British guidelines warn that breath tests may be falsely positive in IBS because of rapid transit, and that the lactulose test agrees poorly with small-intestine sampling.[5]
- Accuracy varies widely. In 11 validation studies cited by the consensus, glucose breath tests detected between 20% and 93% of SIBO cases and lactulose tests between 31% and 68%, with false-positive rates just as variable.[8]
- A positive result doesn't predict treatment response. British guidelines note that a positive breath test doesn't predict who will respond to antibiotics.[5]
- Methane is a separate finding. The American College of Gastroenterology (ACG) classes excess methane as intestinal methanogen overgrowth and suggests methane testing for people with constipation.[3] Methane-positive tests are more common in IBS with constipation than in IBS with diarrhea.[7]
What do guidelines say about SIBO testing in IBS?
| Guideline | Position on breath testing in IBS |
|---|---|
| ACG guideline on SIBO (2020) | Suggests glucose or lactulose hydrogen breath testing to diagnose SIBO in people with IBS — a conditional recommendation based on very low-quality evidence[3] |
| ACG guideline on IBS (2021) | Makes no recommendation for or against breath testing.[4] The tests it highlights are celiac blood tests and fecal calprotectin for people with diarrhea[9] |
| British Society of Gastroenterology (2021) | No role for hydrogen breath testing to rule out SIBO in people with typical IBS symptoms — a strong recommendation, though based on weak evidence[5] |
| American Gastroenterological Association (AGA) update (2020) | The definition of SIBO "lacks precision," and its role in IBS symptoms remains controversial[2] |
So whether you're offered a breath test can depend on where you live and which guideline your doctor follows. If one is suggested, it's reasonable to ask how the result would change your treatment.
Does rifaximin treat IBS, SIBO or both?
Rifaximin is an antibiotic that is barely absorbed, so it acts mainly inside the gut. In two large trials, TARGET 1 and 2, of people with IBS without constipation, 40.7% had adequate relief of their IBS symptoms in the month after a two-week course, compared with 31.7% on placebo.[10] Those trials enrolled people on the basis of their IBS symptoms; only 98 of the 1,260 participants had a breath test.[4]
- The ACG recommends rifaximin for overall symptoms of IBS with diarrhea.[9]
- British guidelines call it an effective second-line drug for IBS with diarrhea in specialist care, with a limited effect on pain, and note that it isn't available for this use in many countries.[5]
- The AGA notes that IBS responds to a poorly absorbed antibiotic, but says whether SIBO explains that response still needs confirming in trials.[2]
So feeling better on rifaximin doesn't prove you had SIBO. There are also concerns about C. difficile infection and antibiotic resistance with repeated courses, which is one reason the decision belongs with your doctor.[5]
When to see a doctor
See a doctor rather than self-testing or self-treating, and promptly if you notice any of these:[11][5]
- Blood in your stool, unintentional weight loss or anemia
- Fever, or symptoms that wake you at night
- Symptoms that started after about age 50
- A family history of bowel cancer, inflammatory bowel disease or celiac disease
- Pale, greasy stools or low vitamin B12, which can be signs of malabsorption[2]
- Previous small-bowel surgery, or a condition such as scleroderma that slows the gut, which makes true SIBO more likely[4]
Two weeks of notes on your symptoms, meals and stool type make that appointment more useful. IBS Clue, an iPhone app, can turn these into a PDF report for your doctor. Other conditions that can look like IBS are covered in IBS vs IBD and IBS or celiac disease.