Is it IBS or SIBO? How to tell the difference
Being told you have IBS while you’re still bloated, gassy and unwell is the exact situation this comparison is for. The honest answer: IBS and SIBO can look identical from the outside - the difference is that IBS names your symptoms, while SIBO names a cause that can be tested for. Roughly a third of people diagnosed with IBS turn out to have a positive SIBO breath test, so for a meaningful minority the two questions are really one. Below you’ll find a side-by-side comparison, a symptom checklist, and a clear sense of when testing is actually worth it.
In this guide
The crucial difference
This is the heart of it. IBS is a diagnosis of exclusion. Under the Rome IV criteria, it is defined as recurrent abdominal pain linked to bowel habit, once serious disease has been ruled out.1 It describes a pattern; it does not tell you why your gut behaves that way. The standard NHS pathway for IBS, set out by NICE, is built to exclude the dangerous causes and then manage symptoms - it does not include a SIBO breath test.2
SIBO is a mechanism. Small intestinal bacterial overgrowth means too many bacteria in the small intestine, fermenting your food and producing gas. It is something you can test for and treat. So the two labels aren’t really rivals: SIBO is one of the specific things that can be sitting underneath an “IBS” diagnosis. A 2020 meta-analysis found positive SIBO breath tests in around a third of IBS patients - three to five times more often than in healthy people.4
The reframe that helps: “Is it IBS or SIBO?” is often better asked as “Is my IBS being driven by SIBO?” The first sounds like a contradiction; the second is a testable question with a possible treatment attached.
IBS vs SIBO, side by side
| IBS | SIBO | |
|---|---|---|
| What it is | A symptom-based diagnosis (a pattern) | A measurable cause (too many bacteria in the small bowel) |
| Core symptoms | Abdominal pain, bloating, altered bowels | Bloating, wind, pain, altered bowels - near-identical |
| How it’s diagnosed | Clinically, after excluding other conditions | Hydrogen/methane breath test, read with symptoms |
| Typical timing clue | Often lifelong or stress-linked | Often began after food poisoning, surgery, or antibiotics |
| On the NHS | Recognised; clear NICE pathway | Not part of the IBS pathway; testing usually private |
| Treatment angle | Diet, symptom relief, gut-brain therapies | Reduce overgrowth (e.g. rifaximin), then prevent relapse |
The symptom checklist
No checklist can diagnose SIBO - only a test and a clinician can - but certain features make overgrowth more likely to be part of the picture. The more of these that fit, the more sense testing makes:
- Bloating is your dominant symptom, often worsening through the day and after carbohydrate-rich meals.
- Your symptoms began after a clear trigger - a bout of food poisoning or gastroenteritis, abdominal surgery, or a course of antibiotics.
- You have visible distension - a flatter stomach in the morning that swells as the day goes on.
- Diarrhoea or constipation predominates rather than a comfortable middle ground (constipation can point to the methane/IMO subtype).
- First-line IBS measures haven’t worked - fibre changes, antispasmodics, and standard dietary advice have made little difference.
- You take, or recently stopped, long-term acid suppression (proton pump inhibitors), which can encourage overgrowth.
The single strongest clue is a clear onset after a gut infection. “My stomach has never been right since that holiday” is one of the most common stories in post-infectious SIBO, where the original infection damages the gut’s natural cleaning wave.
When testing is worth it
A test earns its place only if the result would change what you do. On that basis, a SIBO breath test is most worthwhile when your IBS is bloating- or diarrhoea-predominant, began after an identifiable trigger, or hasn’t responded to first-line care - the situations where a positive result would genuinely redirect treatment.
It’s worth knowing the test’s limits before you pay for one. The breath test measures gas, not bacteria directly, and the lactulose version in particular can read positive simply because the test sugar reached your colon quickly - false-positive rates above 50% have been reported.3 That doesn’t make it useless; it makes interpretation matter. A result should always be read alongside your symptoms, never in isolation. The mechanics of testing are covered in the testing guide, and the full treatment picture - including what a positive result leads to - in the SIBO pillar.
What to ask your GP
- My IBS symptoms are mainly bloating/diarrhoea and started after [food poisoning / antibiotics / surgery] - could SIBO be contributing?
- Can I be referred to gastroenterology for a hydrogen/methane breath test, or is that something I’d need to arrange privately?
- Have we definitely excluded coeliac disease and inflammatory bowel disease (e.g. with blood tests and a faecal calprotectin)?
- If testing isn’t available on the NHS here, what would you suggest as the next step?
- Are any of my medications - especially acid suppressants - worth reviewing?
References
- Rome Foundation. Rome IV Criteria (IBS). theromefoundation.org, 2016.
- NICE. Irritable bowel syndrome in adults: diagnosis and management (CG61). nice.org.uk/guidance/cg61, 2008 (updated 2017).
- Rezaie A, et al. Hydrogen and Methane-Based Breath Testing in GI Disorders: The North American Consensus. Am J Gastroenterol. PMID 28323273, 2017.
- Shah SC, et al. Small Intestinal Bacterial Overgrowth in IBS: systematic review and meta-analysis. Am J Gastroenterol. journals.lww.com, 2020.
This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.