SIBO and IBS are two different explanations for very similar symptoms. IBS is a disorder of how the gut and brain interact, diagnosed from your symptom pattern once other conditions have been ruled out. SIBO stands for small intestinal bacterial overgrowth, and it means too many bacteria living in the small intestine, the part of the gut that digests and absorbs food. You can have both at the same time, and some people diagnosed with IBS test positive for SIBO. How many is disputed, because the tests used to look for SIBO give different answers depending on how they are done. This guide covers what SIBO is, why it is so hard to separate from IBS, how it is investigated and treated in the UK, and when symptoms need a GP appointment.
What is SIBO?
The small intestine normally holds far fewer bacteria than the large intestine. Stomach acid kills many of the bacteria that arrive with food, waves of muscle movement sweep the small intestine clean between meals, and a valve at its far end limits how much travels back up from the large intestine. SIBO happens when those controls fail and bacteria build up where they should be sparse. The excess bacteria ferment food before your body has absorbed it, producing gas inside a narrow stretch of gut, and they can interfere with how nutrients are taken in.
The result is bloating, wind, tummy pain and often diarrhoea, and in more severe cases weight loss or low vitamin levels. The American College of Gastroenterology's 2020 guideline defines SIBO as excessive bacteria in the small bowel causing symptoms like these. It is more likely in people who have had gut surgery, in conditions that slow the gut down, such as diabetes affecting the gut nerves, and in older age. In people with none of those risk factors, it is much less clear how common it is.
Why are SIBO and IBS so hard to tell apart?
The symptom lists overlap almost completely. Bloating, wind, pain that eases after a bowel movement, diarrhoea and constipation all appear in both. Neither condition shows up on standard blood tests, and both tend to flare after meals. IBS itself has a diagnostic pathway and a set of management options of its own, set out from start to finish in the complete guide to IBS.
The overlap runs deeper than symptoms. Researchers increasingly think gut bacteria contribute to IBS symptoms in at least some people, and there are ways to work out whether your gut microbiome is triggering your IBS symptoms. SIBO is one specific version of that idea, with the bacteria in the wrong place rather than just the wrong balance. That is why the two diagnoses keep being discussed together, and why some researchers argue that a share of what gets labelled IBS is really unrecognised SIBO. Other researchers argue the opposite, that SIBO is being over-diagnosed on the back of unreliable tests. Both camps have evidence to point at.
How many people with IBS actually have SIBO?
The studies agree on the direction and disagree on the size. A 2020 pooled analysis of 25 studies, covering around 3,200 people with IBS, found they were roughly four times more likely to test positive for SIBO than people without IBS. A 2026 pooled analysis reached a similar conclusion, with people with IBS around five times more likely to test positive, and the link strongest in IBS with diarrhoea.
The absolute numbers vary much more. Depending on which test a study used, the share of people with IBS who tested positive ranged from about one in seven to well over a third. The same 2020 analysis found that on breath testing, nearly 30 in every 100 people without IBS also tested positive, which is a very high figure for people with no gut complaint. Its authors rated the overall quality of the evidence as low, mainly because the tests themselves are so inconsistent. Put together, SIBO is more common in people with IBS than in people without it, a meaningful minority of people with an IBS diagnosis would meet SIBO criteria if tested, and no one can currently say whether that minority is one in seven or one in three.
How is SIBO tested, and why are the tests disputed?
The most direct method is to pass a tube into the small intestine during an endoscopy, draw out a sample of fluid and grow the bacteria in a lab. This is the reference test, and it is invasive, expensive and rarely done outside research or complex cases.
In practice, testing usually means a breath test. You drink a sugar solution, either glucose or lactulose, and breathe into a machine at intervals. Bacteria fermenting the sugar produce hydrogen and methane, which pass into your breath, so an early rise suggests bacteria are meeting the sugar high up in the gut. A 2017 North American consensus statement standardised the doses and cut-offs precisely because labs had been running the test in different ways and reading it differently.
The dispute is that breath tests struggle to tell overgrowth apart from a fast gut. If the sugar reaches the large intestine quickly, its resident bacteria produce the same gases and the test reads positive with nothing wrong in the small intestine. Lactulose-based tests are the most affected. Across the pooled IBS studies, lactulose-based tests flagged several times more people as positive than glucose tests did, in people with IBS and healthy people alike. A positive breath test is a clue, not a diagnosis, and the guideline bodies that recommend breath testing say so themselves.
In the UK, breath testing is a hospital test. Some NHS gastroenterology departments offer it after a GP referral, usually where there is a specific reason to suspect overgrowth, such as previous gut surgery, and availability varies by area. It is not part of the standard NHS work-up for IBS. The British Society of Gastroenterology's guideline on IBS, which sets out how the condition should be diagnosed and managed in the UK, builds its diagnostic advice around ruling out other conditions with blood and stool tests, not around SIBO breath testing.
Can a stool test diagnose SIBO?
No. A stool sample is made up overwhelmingly of bacteria from the large intestine, and SIBO is a problem of the small intestine, further up the gut. No stool test, from any company, can diagnose SIBO, and a result should never be read that way.
What a stool test can do is describe the community it does sample. An at-home gut microbiome test profiles the bacteria in a stool sample and shows the overall variety and balance of your large-bowel bacteria, including groups that studies have observed to differ in people with IBS. That is context for a conversation with your GP about your symptoms as a whole, not a SIBO answer and not a diagnosis of anything. Before deciding whether that is useful to you, look at what gut microbiome testing for IBS can and cannot tell you.
How is SIBO treated?
Where SIBO is confirmed, treatment is a short course of antibiotics chosen by the clinician, alongside looking for the reason the overgrowth happened, because without that it tends to come back. The antibiotic most studied is rifaximin, which stays in the gut rather than being absorbed into the body. In the two large trials that tested it in IBS without constipation, about 41 in 100 people had adequate relief of their symptoms after a two-week course, compared with about 32 in 100 given a placebo. That is a real difference and a modest one, and most people in the trials did not respond. The 2026 pooled analysis found rifaximin cleared the overgrowth in around 6 in 10 people with both IBS and a positive SIBO test.
In the UK, rifaximin is a prescription-only medicine, it is not licensed for IBS, and where it is used for suspected overgrowth the decision usually sits with a hospital specialist rather than a GP. What a GP can do is rule out other causes of your symptoms, treat the IBS picture in front of them, and refer to gastroenterology where something points beyond IBS. For most people whose symptoms fit IBS, working through the treatments for IBS with the strongest evidence behind them is where the gains are, whatever a breath test might have shown. Do not take antibiotics left over from another illness for suspected SIBO, and do not buy antibiotics online. The wrong antibiotic, or repeated courses, can make gut symptoms worse.
Why is SIBO diagnosed so often outside the NHS?
SIBO has become one of the most talked-about gut diagnoses on social media, and a market has grown around it. Private clinics and websites sell breath tests directly to the public, and some diagnose SIBO from stool tests or symptom questionnaires, which cannot detect it at all. The symptoms involved, bloating, wind and irregular bowels, are among the most common gut complaints there are, so a label that explains them will always find an audience.
Nearly 3 in 10 people with no gut symptoms test positive on breath testing, so a positive result in someone with bloating does not establish that overgrowth is the cause. If a private test has told you that you have SIBO, take the result to your GP rather than starting a treatment plan built on it. Some of those plans involve long exclusion diets or repeated antibiotic and herbal protocols, which carry costs and risks of their own, and a restrictive diet should only ever be attempted with support from a dietitian.
When should you see a GP?
Book a GP appointment for blood in your stool or black, tarry stools, a change in bowel habit lasting more than three weeks, unexplained weight loss, difficulty swallowing, persistent vomiting, unexplained tiredness that turns out to be low iron, or gut symptoms that start for the first time over the age of 50 or alongside a family history of bowel or ovarian cancer. These need investigation before any conversation about IBS or SIBO.
See your GP too if you are losing weight alongside diarrhoea, if symptoms began after abdominal surgery, or if you have a condition such as diabetes and your digestion has changed. Those are the situations where checking for overgrowth has a proper place, and your GP can refer you to a specialist who can arrange the right test.
This guide is for information only and does not replace medical advice. Speak to your GP or pharmacist about your own symptoms and treatment.
Sources
- Pimentel M, Saad RJ, Long MD, Rao SSC (2020). ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. American Journal of Gastroenterology. PubMed | DOI
- Shah A, Talley NJ, Jones M, et al. (2020). Small Intestinal Bacterial Overgrowth in Irritable Bowel Syndrome: A Systematic Review and Meta-Analysis of Case-Control Studies. American Journal of Gastroenterology. PubMed | DOI
- Lu H (2026). Relationship between small intestinal bacterial overgrowth and irritable bowel syndrome and the efficacy of rifaximin intervention: a systematic review and meta-analysis. Frontiers in Microbiology. PubMed | DOI
- Rezaie A, Buresi M, Lembo A, et al. (2017). Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. American Journal of Gastroenterology. PubMed | DOI
- Vasant DH, Paine PA, Black CJ, et al. (2021). British Society of Gastroenterology guidelines on the management of irritable bowel syndrome. Gut. PubMed | DOI
- Pimentel M, Lembo A, Chey WD, et al. (2011). Rifaximin therapy for patients with irritable bowel syndrome without constipation. New England Journal of Medicine. PubMed | DOI
