SIBO

Also called small intestinal bacterial overgrowth.

Dr Chad OkayFounder, Suna Health

4 min read. Updated .

SIBO stands for Small Intestinal Bacterial Overgrowth. It happens when bacteria that normally live in the colon take up residence higher up, in the small intestine. The result is fermentation in the wrong place. The most common symptom is bloating, often with excessive wind and stools that swing between loose and constipated.

Where SIBO happens and why it matters

The small intestine is meant to be relatively low in bacteria. It is where most digestion and nutrient absorption happens. The colon, downstream, hosts the dense bacterial community that ferments fibre. When bacteria migrate upwards into the small intestine, they ferment food before the body can absorb the nutrients, producing gas and disturbing the gut lining.

Common signs

  • Bloating that comes on within 30 to 60 minutes of eating.
  • Excessive wind, often described as worse than usual gut symptoms.
  • Loose stools or alternating loose and hard stools.
  • Feeling full quickly during meals.
  • Symptoms that have been labelled as IBS but do not respond to standard IBS interventions.

How it gets diagnosed

The standard non-invasive test is the lactulose or glucose hydrogen breath test, available privately in the UK and through some NHS gastroenterology clinics. The person drinks a sugar solution, then breathes into a collection device every 15 to 20 minutes for around three hours. A rise in hydrogen or methane gas earlier than expected suggests bacteria are present higher up than normal.

Breath testing is imperfect. False negatives and false positives both occur. Because of this, breath tests should be interpreted by a clinician alongside symptom history rather than used as a standalone diagnostic.

Why it happens

  • Slowed gut movement, a common contributing cause. Anything that slows transit (some medications, hypothyroidism, scleroderma, and possibly GLP-1 drugs) can encourage upward bacterial migration.
  • Anatomical changes after surgery or due to adhesions.
  • Reduced stomach acid, which normally kills incoming bacteria. Long-term proton pump inhibitor use is a known risk factor.
  • A failed migrating motor complex, the housekeeping wave that sweeps the small intestine clear between meals.

Treatment in the UK

The first-line treatment is a course of an antibiotic that stays largely in the gut, most often rifaximin. Some specialists pair it with a prokinetic to keep the gut moving afterwards. Diet changes, including a temporary low-FODMAP phase, can reduce symptoms but do not by themselves clear the overgrowth. SIBO recurs in roughly four out of ten cases within a year, so identifying the underlying cause matters more than the antibiotic round itself.

If you suspect SIBO, the right starting point is a referral from your doctor to gastroenterology. Self-prescribing rifaximin or repeated courses of broad-spectrum antibiotics can damage the wider microbiome and is strongly discouraged.

When it needs a doctor first

Bloating with weight going down on its own, anaemia, bleeding, or diarrhoea that wakes you at night is not a pattern to self-investigate: those need a doctor first. A changed bowel habit that has not settled deserves checking at any age (the FIT test has no minimum duration) before any breath testing, unexplained B12 deficiency alongside gut changes is worth raising, and overgrowth that keeps returning after treatment is a reason to look for the underlying cause with a specialist rather than repeat the treatment.

Common questions

Is SIBO the same as IBS?
No. SIBO is one possible cause of symptoms that look like IBS. Studies estimate that around one in three people with IBS-D have a positive breath test for SIBO. But IBS is a diagnosis of exclusion, and many people with IBS do not have SIBO.
Can SIBO go away on its own?
Mild cases sometimes settle if the underlying cause resolves, for example after stopping a slowing medication or finishing a course of PPIs. Most established cases need a targeted antibiotic course alongside addressing the cause. In more severe or structural cases, SIBO can contribute to nutrient deficiencies including B12, though these are often subtle or absent.
What is methane SIBO or IMO?
When the breath test shows raised methane rather than hydrogen, the condition is now classified as Intestinal Methanogen Overgrowth (IMO). It is associated with constipation rather than diarrhoea and is caused by archaea, not strictly bacteria. Treatment is similar but often pairs rifaximin with neomycin.
Can diet alone fix SIBO?
Diet, including low-FODMAP, reduces fermentable fuel for the bacteria and eases symptoms. It does not eradicate the overgrowth. Most clinicians use diet to manage symptoms during and after antibiotic treatment, not as a standalone cure.

Sources

Written by Dr Chad Okay. Contributor: Dr Parisah Hussain, MBChB, BSc (Hons), DRCOG, MRCGP. Last updated 17 September 2026. From Digests.

Digests explains how the body works in general, everyday terms. It is not medical advice. It is not a substitute for someone who can examine you and knows your history, and we do not tell you what dose of any medicine to take.

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