SIBO treatment in the UK: the complete guide
Treating SIBO in the UK usually means a 14-day course of the gut-selective antibiotic rifaximin, often followed by a prokinetic to prevent relapse - but because rifaximin is not licensed for SIBO here, most people access it off-label and privately, and breath testing sits outside the NHS’s standard IBS pathway. This guide explains how SIBO is tested for, the treatments that have evidence behind them, and how to navigate UK access without wasting money on the parts that don’t work.
The short version
- What it is: too many bacteria in the small intestine, where there should be relatively few.
- How it’s found: a hydrogen/methane breath test - not part of NHS IBS care, so usually private.
- First-line treatment: rifaximin (off-label in the UK), with herbal antimicrobials and an elemental diet as alternatives.
- The catch: relapse is common unless the underlying cause - usually impaired gut motility - is addressed.
In this guide
What SIBO actually is
Small intestinal bacterial overgrowth (SIBO) means an excessive number of bacteria in the small intestine - the long, winding section between the stomach and the colon where most digestion and absorption happen. The small bowel is meant to be relatively sparse in bacteria; the dense microbial population belongs further down, in the colon. When bacteria colonise the small intestine in large numbers, they ferment carbohydrates before you can absorb them, producing gas, drawing in fluid, and irritating the gut wall.
That fermentation is the source of the classic picture: bloating that worsens through the day, excessive wind, and bowel habits that swing between loose and sluggish. Because the bacteria are competing for your food, SIBO can also cause genuine malabsorption - of fats, B12, and fat-soluble vitamins - in more severe or long-standing cases.
Hydrogen, methane (IMO) and hydrogen sulfide
SIBO is not one thing. It is categorised by the gas the overgrowing microbes produce, and the gas tends to track with the bowel pattern.
| Subtype | Produced by | Typical bowel pattern | Notes |
|---|---|---|---|
| Hydrogen-dominant | Fermenting bacteria | More often diarrhoea | The classic, most-studied form |
| Methane - now “IMO” | Archaea (e.g. Methanobrevibacter smithii), not bacteria | More often constipation | Renamed intestinal methanogen overgrowth because the culprits aren’t bacteria and can overgrow beyond the small bowel5 |
| Hydrogen sulfide | Sulfate-reducing microbes | Often diarrhoea, sometimes a “rotten egg” smell | Newest and least validated; needs a three-gas breath test to detect |
The distinction is not academic. Methane-positive overgrowth (IMO) responds poorly to a single antibiotic and usually needs combination treatment, so knowing your subtype changes the plan.
Symptoms - and why they look like IBS
SIBO’s symptoms are almost indistinguishable from irritable bowel syndrome: bloating and visible distension, abdominal pain or discomfort, excessive wind, and altered bowel habits. That overlap is the whole problem. IBS, under the Rome IV criteria, is defined by recurrent abdominal pain linked to bowel habit, with serious disease excluded1 - it is a description, not a cause. SIBO is one of the causes that can sit underneath it.
How often? Pooled data tell the story. A 2020 meta-analysis found a positive SIBO breath test in around a third of people with IBS, and roughly three to five times more often than in healthy controls.4 So while most IBS is not SIBO, a substantial minority is - enough that the question is worth asking, especially if your symptoms are bloating- or diarrhoea-predominant. For a side-by-side comparison, see Is it IBS or SIBO?
The NHS gap
The NICE guideline for IBS (CG61) does two things well: it excludes dangerous conditions, and it manages symptoms.2 What it does not do is test for SIBO - hydrogen/methane breath testing is not part of the pathway. This is not an oversight so much as a design choice: NICE concluded the evidence base for routine SIBO testing in IBS was not strong enough to recommend it.
The practical consequence is that if SIBO is driving your symptoms, the standard NHS route is unlikely to find it. Most people who get tested do so privately, and most who treat it do so off-label. None of that means SIBO is fringe - it is the subject of major gastroenterology research - but it does mean you may have to be the one who raises it.
Key insight: “Your tests are normal” usually means the tests that were done were normal. The standard IBS work-up does not include a breath test, so a normal result there says nothing about whether you have SIBO.
Breath testing: what the numbers mean
A breath test works on a simple principle. You drink a sugar solution (usually glucose or lactulose); if bacteria in your small intestine ferment it, they release hydrogen and methane that pass into your blood, reach your lungs, and show up in your breath. You blow into a collection device at intervals over two to three hours.
The widely used interpretation comes from the 2017 North American Consensus.3 A rise in hydrogen of at least 20 parts per million above baseline within 90 minutes is considered positive for SIBO; a methane level of at least 10 ppm at any point indicates methane overgrowth (IMO).3 These cut-offs give the test consistency, but the underlying evidence is graded as low quality - the numbers are a sensible convention, not a perfect truth.
| Glucose | Lactulose | |
|---|---|---|
| Absorbed? | Yes, in the upper small bowel | No - travels the whole small intestine |
| Strength | More specific (fewer false positives) | More sensitive to overgrowth further down |
| Weakness | Can miss distal overgrowth | Often reflects how fast the sugar reaches the colon, not true SIBO |
This is the most important caveat in the whole topic: a lactulose breath test can read “positive” simply because the sugar reached your colon quickly, where normal colonic bacteria ferment it. Studies using imaging alongside breath testing found the tracer had often already reached the colon before the breath test “turned positive,” with false-positive rates above 50% in some series.3 A positive test is a clue to interpret alongside your symptoms - not a verdict. How to weigh testing is covered further in the testing guide.
Root causes: why you got it
SIBO is almost always a downstream consequence of something else. Treating the bacteria without addressing the cause is why people relapse. The main drivers:
Impaired motility (the big one). Between meals, a wave of muscular activity called the migrating motor complex sweeps residue and bacteria out of the small intestine - a kind of housekeeping. When that wave is weak, bacteria are not cleared and they accumulate. This is the central mechanism in most SIBO.
Post-infectious SIBO. A bout of food poisoning can trigger lasting motility damage. The leading hypothesis: a bacterial toxin (CdtB) provokes antibodies that cross-react with vinculin, a protein the gut’s nerve cells rely on, impairing the motor complex.13 This is why “my IBS started after a stomach bug abroad” is such a common story.
Low stomach acid. Stomach acid is a first line of defence that sterilises much of what you swallow. When it is suppressed - often by long-term proton pump inhibitors - more bacteria survive into the small bowel. A meta-analysis linked PPI use to roughly double the odds of SIBO.12
Structural and medical causes. Adhesions from surgery, a removed or incompetent ileocaecal valve (which lets colonic bacteria reflux upward), and conditions that slow the gut - scleroderma, diabetes, hypothyroidism - all predispose to overgrowth.
Treatment: what works
There are three evidence-supported ways to reduce the overgrowth. They are not mutually exclusive, and the right choice depends on subtype, cost, and tolerance.
Rifaximin
Rifaximin is an antibiotic that stays almost entirely in the gut (under about 0.4% is absorbed), which makes it well tolerated.14 A meta-analysis of SIBO studies put breath-test normalisation at roughly 60-70%.8 Its evidence in IBS-without-constipation comes from the large TARGET 1 and 2 trials, where a 14-day course gave durable symptom relief in significantly more people than placebo (about 41% versus 32%),6 and repeat courses were shown to be safe and effective for those who relapse.7
The UK catch: rifaximin is not licensed for SIBO or IBS here - its UK licences are for hepatic encephalopathy (the 550 mg product, branded Targaxan) and travellers’ diarrhoea (the 200 mg product, Xifaxanta).14 Using it for SIBO is off-label, usually self-funded, and typically dosed at 550 mg three times daily for 14 days. The full picture - access routes, realistic costs, and a GP script - is in the rifaximin UK access guide.
Methane changes the plan. Methane-positive overgrowth (IMO) responds poorly to rifaximin alone. Adding neomycin substantially improves methane clearance in studies - combination therapy normalised breath tests in far more patients than a single agent.11 If your test shows methane, expect a different regimen.
Herbal antimicrobials
Botanical antimicrobials - berberine, oregano oil, allicin (from garlic), and neem among them - are a credible alternative. In a single-centre study, a herbal protocol cleared SIBO on repeat breath testing at least as often as rifaximin (about 46% versus 34%, a difference that was not statistically significant), with fewer side effects.9 The evidence is weaker than for rifaximin - one non-randomised study - but it is a reasonable option, particularly where cost or antibiotic avoidance matters.
Elemental diet
An elemental diet replaces food with a pre-digested liquid formula of amino acids, simple sugars and fats for two to three weeks, effectively starving the overgrowth while still feeding you. A 2004 study reported breath-test normalisation in around 80-85% of patients.10 It works, but it is demanding: the formula is unpalatable, expensive, and hard to sustain. Most people reserve it for stubborn or relapsing cases.
| Option | Indicative success | Evidence | Main drawback |
|---|---|---|---|
| Rifaximin (14 days) | ~60-70% breath normalisation | Strong (RCTs in IBS-D; meta-analysis in SIBO) | Off-label & self-funded in the UK; cost |
| Rifaximin + neomycin | Best for methane/IMO | Moderate (small trials) | Second antibiotic; supervision needed |
| Herbal antimicrobials | ~46% in one study | Weak (single non-randomised study) | Less standardised; quality varies |
| Elemental diet | ~80-85% | Weak (small, uncontrolled) | Unpalatable, costly, hard to sustain |
Relapse prevention
Clearing the bacteria is the easy part; keeping them cleared is the real test. Because the usual root cause is poor motility, the standard approach after eradication is a prokinetic - a medicine that strengthens the migrating motor complex - together with leaving four to five hours between meals so the housekeeping wave can run. Agents used (mostly off-label for this purpose) include prucalopride, low-dose erythromycin, and low-dose naltrexone. Be aware that the evidence here is thin: the rationale is sound and widely followed, but it rests on small, observational data rather than large trials. The sequencing logic is laid out in the protocols guide.
Diet plays a supporting, not starring, role. A temporary lower-fermentation approach (such as low-FODMAP) can reduce symptoms while you treat, but restricting food does not eradicate overgrowth and long-term restriction can harm the wider microbiome. Use it as a comfort measure, not a cure. If you are rebuilding gut health more broadly afterwards, the 90-day gut healing protocol is a useful companion, and why you bloat after meals covers the symptom most people want gone first.
One condition often travels with SIBO: fungal overgrowth. If antibacterial treatment helps only partially, it is worth reading the candida and SIFO guide, since the two can coexist.
What to ask your GP
- Given my symptoms, could SIBO be contributing - and can I be referred to gastroenterology for a hydrogen/methane breath test?
- Is SIBO testing or treatment funded on my local NHS formulary, or would I need to self-fund?
- If rifaximin isn’t available on the NHS for this, would you consider a private prescription so I can self-fund a course?
- My symptoms started after food poisoning / a course of antibiotics / long-term acid suppression - could that be relevant?
- Are there licensed alternatives or dietary steps you’d suggest first?
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.
- Pimentel M, et al. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. Am J Gastroenterol. journals.lww.com, 2020.
- Pimentel M, et al. Rifaximin therapy for patients with IBS without constipation (TARGET 1 & 2). N Engl J Med. NEJMoa1004409, 2011.
- Lembo A, et al. Repeat Treatment With Rifaximin Is Safe and Effective in IBS-D (TARGET 3). Gastroenterology. PMID 27528177, 2016.
- Gatta L, Scarpellini E, et al. Meta-analysis: rifaximin is effective and safe for SIBO. Aliment Pharmacol Ther. PMID 28078798, 2017.
- Chedid V, et al. Herbal Therapy Is Equivalent to Rifaximin for the Treatment of SIBO. Glob Adv Health Med. SAGE, 2014.
- Pimentel M, et al. A 14-Day Elemental Diet Normalises the Lactulose Breath Test. Dig Dis Sci. PMID 14992438, 2004.
- Low K, Pimentel M, et al. A combination of rifaximin and neomycin is most effective in treating IBS patients with methane. Am J Gastroenterol. journals.lww.com, 2008.
- Lo WK, Chan WW. Proton Pump Inhibitor Use and the Risk of SIBO: meta-analysis. Clin Gastroenterol Hepatol. cghjournal.org, 2013.
- Pimentel M, et al. Autoimmunity links vinculin to the pathophysiology of chronic functional bowel changes following Campylobacter jejuni infection. PMID 25424202, 2015.
- Rifaximin 550 mg film-coated tablets (Targaxan), Summary of Product Characteristics. medicines.org.uk (emc), 2025.
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.