Do I Have SIBO, or Is It IBS?

They overlap so much that the question is harder than it sounds. In one review of 17 studies, methane-positive small intestinal bacterial overgrowth was found in about a quarter of people with irritable bowel syndrome, but it was no more common than in people without it. A test result on its own does not separate the two.

If you have spent an evening reading about bloating, you have probably met the idea that SIBO is the hidden cause behind it, and that a breath test will finally explain everything. That idea is popular, and the research behind it is thinner and more interesting than the confident version suggests.

Key takeaways

• SIBO means too many bacteria in the small intestine. IBS is a disorder of how the gut and brain communicate. Their symptoms look very similar from the outside.
• In a review of 17 studies, methane-positive SIBO was found in 25% of people with IBS, but it was not more common than in people without IBS.
• It was significantly more common in the constipation type of IBS than the diarrhoea type, with an odds ratio of 3.1. The authors rated the evidence quality as low.
• SIBO is worth considering when there are specific risk factors and the result would change what happens next, not as a default explanation for bloating.
• IBS is diagnosed positively, from your symptom pattern, not by ruling everything else out one test at a time.

What is SIBO, and what is IBS?

They are two different kinds of answer to the same complaint.

SIBO, short for small intestinal bacterial overgrowth, describes an excess of bacteria in the small intestine, where numbers are normally much lower than in the colon. A related pattern involves methane-producing organisms, sometimes described as intestinal methanogen overgrowth. [1]

IBS is something else. The Rome Foundation reframed it, along with related conditions, as a disorder of gut-brain interaction: a disturbance in the two-way communication between the digestive tract and the nervous system. [2] That framing matters, because it explains how symptoms can be entirely real while endoscopy, scans and routine blood tests all come back normal.

The trouble is that the two produce a similar list of complaints: bloating, abdominal discomfort, wind, and changes in bowel habit. You cannot tell them apart by symptoms alone, and that is exactly why a test feels so appealing.

Can a test tell them apart?

Less cleanly than you would hope, and this is the most useful thing in this article.

A systematic review and meta-analysis looked at methane-positive SIBO across 17 independent studies in people with IBS. It found a prevalence of 25.0%. But it also found that this was not increased compared with control groups overall. [1]

Read that twice, because it is the part that usually gets left out. A quarter of people with IBS tested positive, and so did a comparable share of people without IBS. If a finding is about as common in people who have the condition as in people who do not, then on its own it does not explain the condition.

The review did find one real signal. Methane-positive SIBO was significantly more common in constipation-predominant IBS than in diarrhoea-predominant IBS, with an odds ratio of 3.1. The authors concluded that methane positivity is associated with the constipation type, while rating the overall evidence quality as low and noting that cause and effect remain uncertain. [1]

So the honest position is narrower than the internet's: there is a genuine association with one particular pattern, and there is not a general explanation for bloating.

When is it worth considering SIBO?

When something in your history points at it, and when the answer would change what happens next.

The research describes SIBO and methane overgrowth as one possible pattern among several, relevant in selected people rather than assumed in everyone with a bloated stomach. [1] The features that make it worth a conversation are reasonably specific:

• constipation, hard stools or a slow transit pattern
• a sense of incomplete evacuation
• a constipation-predominant IBS pattern
• previous gastrointestinal surgery
• a condition or medication affecting gut motility
• symptoms that have not responded to appropriate first-line care, where a result would genuinely change the plan

If none of those apply, a positive result is difficult to interpret and a negative one rarely settles anything. Testing decisions belong with a doctor who knows your history, and this article is not a reason to request or skip one.

How is IBS actually diagnosed?

Positively, which surprises most people.

IBS is not a diagnosis of last resort arrived at by eliminating everything else. Clinical guidance describes it as a positive clinical diagnosis: made when the symptom pattern is typical, recognised diagnostic criteria are met, and alarm features are absent. [3, 4] Part of that is identifying which pattern you have, because it changes what tends to help: constipation-predominant, diarrhoea-predominant, mixed, or unclassified. [4]

This is also why a long series of tests is not automatically better care. A confident diagnosis still requires a safe frame, which means the alarm features below have to be checked first. But once they have been, more testing does not usually produce more clarity.

The same principle applies to bloating in general. European consensus guidance treats functional bloating and distension as conditions with several possible mechanisms, diagnosed from the clinical picture rather than from a single test result. [5]

Where Probitec Intrinsic Bowel Support fits

Probitec IBS - Intrinsic Bowel Support is a multi-strain probiotic providing 20 billion CFU per daily serving across eight named strains, including Lactobacillus rhamnosus GG, Lactobacillus paracasei Lpc-37 and Bifidobacterium lactis HN019, in a capsule-in-capsule design intended to help protect the bacteria until they reach the intestine. It may assist with digestive comfort, bowel regularity, gut function and microbiome balance as part of a normal diet and lifestyle.

Two things have to be said plainly. No study described in this article tested Probitec Intrinsic Bowel Support or any Probitec product. And the research this article draws on is explicit that probiotics should not be presented as a single answer for bloating, because the strain, the pattern of symptoms and the person all change what is reasonable to expect. Nothing here should be read as a claim that a probiotic treats SIBO or IBS.

When to speak to a healthcare professional

Bloating and changes in bowel habit are common and are usually not caused by serious disease. Some features need checking before anything else, though. See a doctor promptly if yours comes with:

• unexplained weight loss
• blood in the stool or rectal bleeding
• diarrhoea that wakes you at night
• fever, or feeling generally unwell
• persistent vomiting
• symptoms that keep getting worse
• symptoms that started for the first time later in life
• a family history of bowel cancer, coeliac disease or inflammatory bowel disease
• swelling that is there most of the day and night rather than coming and going, particularly in women over 50
• eating less and less, or cutting out more and more foods, to control symptoms

A probiotic or a change of diet is not a substitute for that assessment, and you should seek medical care if symptoms persist or worsen.

Practical takeaway

Before chasing a label, describe the pattern. Is your bowel habit mainly constipated, mainly loose, or alternating? Does the bloating build after meals and settle overnight? Have you had gut surgery, or anything affecting how quickly things move through? Take that description to a doctor or a registered dietitian. If your pattern is constipation-predominant and first-line measures have not helped, ask whether testing would actually change the plan. If the answer is no, the test is unlikely to be the thing you need.

SIBO and IBS frequently asked questions

Can you have SIBO and IBS at the same time?

Yes, and the overlap is much of the difficulty. In a review of 17 studies, about a quarter of people with IBS tested positive for methane-positive SIBO, though so did a comparable proportion of people without IBS. The two are not mutually exclusive, and a positive test does not cancel an IBS diagnosis.

Does a positive breath test mean SIBO is causing my symptoms?

Not on its own. The same review found methane-positive SIBO was not more common in people with IBS than in people without it overall, and rated the evidence quality as low, with cause and effect still uncertain. A result is interpreted alongside your symptom pattern and history, not instead of them.

Is SIBO more likely if I am constipated?

The research points that way for one specific type. Methane-positive SIBO was significantly more common in constipation-predominant IBS than in the diarrhoea-predominant type, with an odds ratio of 3.1. That is an association in one pattern, not proof that methane causes constipation.

Do I need lots of tests to be diagnosed with IBS?

Usually not. Clinical guidance describes IBS as a positive diagnosis, made when symptoms are typical, criteria are met and alarm features are absent. Checking for those alarm features matters. Beyond that, more tests do not generally produce more certainty.

Will a probiotic fix SIBO?

That is not a claim this article or the research behind it makes. The research is explicit that probiotics should not be treated as a single solution for bloating, and that what is reasonable to expect depends on the strain, the symptom pattern and the person. Anything involving suspected overgrowth is a conversation for a clinician.

Related SIBO and IBS reading

• Understanding IBS
• Why does my stomach swell up during the day?
• Why do certain foods make me bloated?
• Understanding constipation

SIBO and IBS references

1. Gandhi A, Shah A, Jones MP, Koloski N, Talley NJ, Morrison M, Holtmann G. Methane positive small intestinal bacterial overgrowth in inflammatory bowel disease and irritable bowel syndrome: a systematic review and meta-analysis. Gut Microbes. 2021;13(1):1933313.
2. Drossman DA, Hasler WL. Rome IV: Functional GI disorders: disorders of gut-brain interaction. Gastroenterology. 2016;150(6):1257-1261.
3. Lacy BE, Pimentel M, Brenner DM, Chey WD, Keefer LA, Long MD, Moshiree B. ACG Clinical Guideline: management of irritable bowel syndrome. Am J Gastroenterol. 2021;116(1):17-44.
4. Vasant DH, Paine PA, Black CJ, Houghton LA, Everitt HA, Corsetti M, et al. British Society of Gastroenterology guidelines on the management of irritable bowel syndrome. Gut. 2021;70(7):1214-1240.
5. Melchior C, Hammer H, Bor S, Barba E, Horvat IB, Celebi A, et al. European Consensus on Functional Bloating and Abdominal Distension: An ESNM/UEG Recommendations for Clinical Management. United European Gastroenterol J. 2025;13(9):1613-1651.

SIBO and IBS evidence note

The findings discussed in this article come from clinical research and guideline documents on bloating, small intestinal bacterial overgrowth and irritable bowel syndrome. They are for general education and do not diagnose, treat, prevent or cure disease. No study described here tested a Probitec product.