The IBS and SIBO Connection

If you’ve been told you have IBS, you may have been left with more questions than answers. IBS is often handed out as a diagnosis of exclusion — meaning everything else has been ruled out, but nobody has explained why you feel so terrible. Here’s what many doctors don’t tell you: for a significant number of people with IBS, there’s an identifiable, testable, and treatable underlying driver. It’s called SIBO — small intestinal bacterial overgrowth. In this blog we’re going to dive deep in to the IBS and SIBO connection to help you understand the condition hiding inside your IBS diagnosis that most GPs never test for.

What Is IBS — And Why Is It So Frustrating?

Irritable bowel syndrome (IBS) affects an estimated 5–10% of the global population, making it one of the most common gastrointestinal conditions in the world. It’s more prevalent in women, and it places a significant burden on both quality of life and healthcare resources.

The classic symptoms of IBS include:

• Recurrent abdominal pain or cramping
• Bloating and distension
• Diarrhoea, constipation, or an alternating pattern of both
• Urgency and unpredictability with bowel movements
• Excessive gas and flatulence

To receive an IBS diagnosis, these symptoms need to have been present for at least six months, with recurrent abdominal pain at least one day per week in the last three months — and no identifiable structural cause found on investigation.

That last part is the problem. IBS is classified as a disorder of gut-brain interaction — a functional disorder — meaning standard investigations like colonoscopies and blood tests come back normal. You’re told there’s nothing structurally wrong. Yet you feel awful, daily. That experience of being dismissed or left without answers is incredibly common.

But functional doesn’t mean fictional. And IBS doesn’t mean “nothing is wrong.”

Enter SIBO: The Missing Piece for Many IBS Sufferers

Small intestinal bacterial overgrowth — SIBO — occurs when bacteria that should predominantly live in your large intestine migrate upward and colonise your small intestine in excessive numbers. The gold standard for diagnosis has traditionally been a jejunal aspirate culture showing 10³ colony-forming units per millilitre or more, though this invasive test is rarely done in clinical practice. More commonly, SIBO is identified using a hydrogen and methane breath test, which measures the gases produced when small intestinal bacteria ferment carbohydrates.

Normally, your small intestine has very few bacteria. It’s kept that way by a remarkable set of protective mechanisms:

• Gastric acid — kills most bacteria before they reach the small bowel
• Bile and pancreatic enzymes — have antibacterial properties
• The migrating motor complex (MMC) — a wave of muscular contractions that sweeps bacteria out of the small intestine during fasting (think of it as a built-in housekeeping mechanism)
• The ileocecal valve — a physical barrier preventing bacteria from migrating backwards from the colon
• Secretory IgA and defensins — immune defences in the gut lining

When any of these mechanisms fail, bacteria accumulate in the small bowel. And once they’re there, they do what bacteria always do: they ferment whatever food is passing through, producing gases and metabolites that cause symptoms.

The Research Is Clear: SIBO and IBS Are Deeply Linked

This isn’t a fringe theory. The connection between SIBO and IBS is supported by a substantial and growing body of peer-reviewed research.

A 2020 meta-analysis of 25 case-control studies involving over 3,000 IBS patients found that SIBO was present in 31% of those with IBS, with an odds ratio of 3.7 compared to controls — meaning IBS patients were nearly four times more likely to have SIBO than healthy individuals. When only the highest-quality studies were included, that odds ratio rose to 4.1.

Earlier research using lactulose breath testing found even higher rates — some studies reporting SIBO in up to 60–78% of IBS patients, though later work using more rigorous methods settled on prevalence figures more typically in the range of 20–50%, depending on the IBS subtype and diagnostic method used.

Crucially, studies have consistently shown that treating SIBO improves IBS symptoms. One landmark paper found that when SIBO was successfully eradicated, IBS symptoms significantly improved — and in some patients, IBS criteria were no longer met at all. This is perhaps the most clinically meaningful piece of evidence: if treating the bacterial overgrowth resolves the IBS symptoms, that tells us the overgrowth was a meaningful driver, not just an incidental finding.

Why SIBO Causes IBS Symptoms: The Mechanisms

Understanding why SIBO causes IBS symptoms helps to demystify what’s happening in your body.

1. Gas Production and Bloating

When bacteria in your small intestine ferment carbohydrates — sugars, fibres, FODMAPs — they produce gases. Three gases are clinically relevant:

• Hydrogen (H₂) — produced by bacteria fermenting undigested carbohydrates; associated with diarrhoea-predominant symptoms
• Methane (CH₄) — produced by archaea (specifically Methanobrevibacter smithii); directly slows intestinal transit and is strongly linked to constipation; this pattern is now referred to as intestinal methanogen overgrowth (IMO)
• Hydrogen sulphide (H₂S) — emerging research links this to diarrhoea-predominant presentations

So when patients say “I bloat almost immediately after eating” or “I’m either constipated for days or rushing to the toilet” — these aren’t random complaints. They map directly onto specific gas profiles that can be identified on breath testing.

2. Damage to the Gut Lining and Leaky Gut

Bacteria in the wrong place can damage the brush border — the delicate surface of your small intestinal cells — impairing your ability to absorb nutrients. Bacterial toxins increase intestinal permeability (commonly known as leaky gut), allowing bacterial byproducts to enter the bloodstream and trigger immune activation. This low-grade inflammation then further sensitises the gut, making it more reactive and painful.

3. Bile Acid Disruption

SIBO bacteria in the proximal small intestine deconjugate bile acids prematurely. This interferes with fat digestion and can produce toxic bile compounds (lithocholic acid) that act as intestinal irritants, contributing to diarrhoea.

4. Visceral Hypersensitivity

The low-grade inflammation and immune activation triggered by SIBO sensitises the enteric nervous system. Your gut essentially becomes hypervigilant — perceiving normal levels of gas or distension as painful. This is visceral hypersensitivity, and it’s one of the core features of IBS. In this way, SIBO doesn’t just cause symptoms directly — it can reprogram your gut’s sensitivity over time.

5. The Gut-Brain Axis

SIBO-driven dysbiosis affects the gut-brain axis — the bidirectional communication network between your gut and your brain. Microbial imbalance alters tryptophan metabolism (affecting serotonin production), impacts vagal signalling, and can shift mood and pain perception. This explains why so many people with IBS also experience brain fog, anxiety, and fatigue alongside their digestive symptoms.

IBS Subtypes and SIBO: It’s Not One-Size-Fits-All

One of the most clinically useful insights from recent research is that different SIBO gas patterns correlate with different IBS subtypes.

IBS-D (diarrhoea-predominant) is most strongly associated with hydrogen-positive SIBO. A 2026 Swedish study found that hydrogen-positive SIBO was overwhelmingly concentrated in the IBS-D group — 73% of all high-hydrogen patients had IBS-D. The same paper found a sensitivity of 61% for SIBO in IBS-D using the lactulose breath test with an 80-minute readout at a ≥20 ppm hydrogen cut-off, compared to just 10% for IBS-C.

IBS-C (constipation-predominant) is most strongly associated with methane — or more accurately, with intestinal methanogen overgrowth (IMO). Methane gas has a direct physiological effect of slowing intestinal transit and augmenting small intestinal contractile activity. Research has shown that methane levels on breath test directly correlate with constipation severity in IBS patients. The worse the constipation, the higher the methane.

IBS-M (mixed type) tends to show intermediate patterns, with some hydrogen and sometimes methane positivity, reflecting the mixed clinical picture.

This subtype-specific understanding is clinically important. It means that identifying your gas profile isn’t just academic — it guides what treatment approach is most likely to help.

Why Your Doctor May Not Have Tested You for SIBO

Despite the strength of the evidence, SIBO testing is not yet universally integrated into standard IBS care pathways. There are a few reasons for this:

Diagnostic heterogeneity. Different studies have used different breath test substrates (lactulose vs glucose), different timing cut-offs (60, 80, or 90 minutes), and different hydrogen thresholds. This has made it difficult to agree on a single gold standard. Consensus is improving — the North American Consensus recommends ≥20 ppm hydrogen rise within 90 minutes — but practice varies.

Awareness gaps. Many GPs and general gastroenterologists were trained when IBS was still understood primarily as a brain-gut disorder driven by stress and psychological factors. While the gut-brain axis is genuinely important, it’s an incomplete picture for many patients. The research base supporting SIBO’s role in IBS has grown enormously over the past decade, but clinical uptake lags behind.

The “functional” label. Once a condition is labelled functional, there can be a tendency to stop looking for underlying causes. For a meaningful subset of IBS patients, this is a disservice.

Who Is Most Likely to Have SIBO-Driven IBS?

Research has identified several clinical features that make SIBO more likely in an IBS patient:

• Diarrhoea-predominant IBS (IBS-D) — highest SIBO prevalence across studies
• Prominent bloating and flatulence — often the first and most distressing symptom
• Constipation-predominant IBS with methane elevation — a near-diagnostic pattern
• Symptom onset after a gut infection — post-infectious IBS is strongly linked to dysbiosis and SIBO; gastroenteritis can damage the MMC and alter gut immunity
• PPI (proton pump inhibitor) use — acid suppression removes a key anti-bacterial barrier
• History of antibiotic use — disrupts the microbiome and can set the stage for overgrowth
• Older age — reduced motility, more likely to have used PPIs or antibiotics, increased structural risk factors
• Diabetes or other conditions affecting gut motility — impaired MMC function is a direct SIBO risk factor
• Symptoms that worsen shortly after eating — rapid fermentation in the small bowel is characteristic of SIBO

What Does SIBO Testing Involve?

A SIBO breath test is non-invasive, straightforward, and can be done at home with a testing kit. Here’s what to expect:

1. You follow a preparation diet for 24 hours (typically low-fermentable foods) and fast overnight
2. You ingest a measured dose of a fermentable substrate — typically lactulose or glucose
3. You collect breath samples at regular intervals over 2–3 hours
4. The samples are analysed for hydrogen and methane (and increasingly hydrogen sulphide)

Interpreting the results:

• A rise of ≥20 ppm in hydrogen within 90 minutes (or 80 minutes by more recent research) indicates hydrogen-positive SIBO
• A methane level of ≥10 ppm at any point indicates intestinal methanogen overgrowth (IMO)
• Elevated hydrogen sulphide may suggest ISO (intestinal sulphide overproduction)

The pattern of gases, the timing of rises, and the baseline levels all provide clinically meaningful information. It’s not just a positive or negative result — it’s a picture of what’s happening in your gut.

Think you have SIBO?

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What Can Be Done About SIBO?

If your breath test comes back positive, the good news is that SIBO is treatable, and treating it can significantly improve IBS symptoms. A comprehensive approach typically involves several elements.

Targeted Antimicrobial Treatment

The most studied treatment is rifaximin, a minimally absorbed antibiotic that acts locally in the gut. Multiple large randomised controlled trials have shown it significantly improves global IBS symptoms and bloating, with effects outlasting the treatment period by weeks to months. For methane/IMO, combining rifaximin with neomycin appears to be more effective than either alone.

Herbal antimicrobials are a well-evidenced alternative, particularly for patients who prefer to avoid antibiotics or who have had recurrent SIBO. A randomised trial by Chedid et al. found that a botanical protocol (containing oregano, thyme, and berberine) achieved SIBO eradication rates comparable to rifaximin — approximately 46% vs 34% — and some patients who failed rifaximin responded to herbal treatment.

Common recommendations include:

• ADP (Oregano Oil).
• Neem Plus.
• Allimax.
• Berberine.

Dietary Modification

The low-FODMAP diet is one of the best-evidenced dietary interventions for IBS, with meta-analyses reporting symptom response rates of around 70%. For SIBO specifically, reducing fermentable carbohydrates removes the substrate that small intestinal bacteria use to produce gas, providing symptomatic relief while other treatments address the overgrowth itself.

Partially hydrolysed guar gum (PHGG) — a soluble fibre — has shown particular promise in SIBO, with one trial demonstrating that adding it to rifaximin improved eradication rates, possibly by enhancing small bowel motility.

The elemental diet — a pre-digested liquid formula — has been used in more refractory cases, with one small study reporting approximately 80% breath test normalisation after 14 days.

Addressing the Root Cause

Treating an episode of SIBO without addressing why it developed is likely to result in recurrence. Key considerations include:

• Prokinetics to support migrating motor complex function and prevent bacterial stasis
• Reviewing PPI use where clinically appropriate and considering hypochlorhydria (low stomach acid) as an underlying contributory factor.
• Consider biliary health – bile has antibacterial properties and is known as ‘defence mechanism’ preventing SIBO from occurring in the first place. I think bile issues are an incredibly common underlying issue in chronic gut issues.
• Addressing motility disorders that predispose to SIBO
• Testing for and treating H. pylori if present — evidence suggests H. pylori infection increases SIBO prevalence, and eradication improves breath test outcomes. Our Ultimate Gut Health Test includes H. Pylori testing.

Probiotics

Evidence for probiotics in SIBO is growing. A meta-analysis found probiotics were associated with SIBO eradication in some studies and with symptom improvement, and they appear particularly useful for preventing relapse after antibiotic treatment. Lactobacillus-containing preparations have shown reductions in hydrogen levels and symptom improvement in SIBO patients.

For IBS more broadly, probiotics have a solid evidence base — a comprehensive meta-analysis of 53 randomised controlled trials found probiotics as a class significantly improved the probability of IBS symptom relief compared to placebo.

You may like to read our blog Best Probiotics For SIBO.

You Deserve More Than a Diagnosis of Exclusion

IBS as a label is not wrong. The symptoms are real, the gut-brain interactions are real, and the impact on quality of life is real. But for many people with IBS, that label has functioned as a door closing rather than opening. It’s too often handed out without investigation of what is actually driving the dysbiosis.

The research is consistent: SIBO is significantly more prevalent in IBS patients than in healthy controls. The more we investigate, the more we find it. And when we treat it — when we actually address the bacterial overgrowth — patients get better. Not always completely, not always permanently without addressing underlying drivers, but meaningfully.

If you’ve been living with IBS symptoms and never been tested for SIBO, you are missing potentially the most important piece of your clinical picture.

The Next Step: Test, Don’t Guess

A SIBO breath test is a simple, at-home investigation that could fundamentally change your understanding of your gut health and open a pathway to targeted, effective treatment.

If you’re tired of being told “your tests are normal” while feeling anything but, it’s time to look deeper.

Think you have SIBO?

Shop our SIBO test

Frequently Asked Questions

Can you have both IBS and SIBO at the same time? Yes — and this is very common. SIBO is not a separate condition that replaces IBS; it’s often the underlying driver of IBS symptoms. Addressing the SIBO can significantly resolve the IBS symptom picture.

Is SIBO the cause of all IBS? No. IBS is multifactorial, and not every IBS patient will have SIBO. Other factors — including visceral hypersensitivity, gut-brain axis dysregulation, and post-infectious changes — also play a role. But SIBO is one of the most common and clinically actionable drivers, and it’s underdiagnosed.

Will antibiotics cure my IBS? Not necessarily on their own, but treating SIBO — whether with antibiotics or herbal antimicrobials — can produce substantial and durable symptom improvement in SIBO-positive IBS patients. A comprehensive approach also addresses dietary triggers, motility, and microbiome restoration.

How do I know if my IBS is SIBO-related? The only way to know is to test. Clinical clues — particularly bloating shortly after eating, diarrhoea, or constipation with significant gas — are suggestive, but a breath test gives you objective data to act on.

Can SIBO come back after treatment? Yes, SIBO has a significant recurrence rate if underlying predisposing factors aren’t addressed. This is why treatment should always be paired with a strategy to support long-term gut motility and microbiome resilience.