Can Mould Toxicity Cause SIBO?

“Mould toxicity/mould illness” is an informal term for the symptoms some people report after long exposure to damp buildings and the mycotoxins certain moulds release. SIBO is a specific gut condition where bacteria build up in the small intestine where they shouldn’t be. If you have lived or worked in a damp building and also struggle with bloating, wind and discomfort, you may have wondered whether the two are connected.

Key takeaways

• SIBO means too many bacteria have built up in the small intestine, where numbers should normally stay low. Estimates of how common it is vary widely, from roughly 2.5% to 22% of the general population, all the way up to 100% in clinical populations [4] [13].
• Research does show that mycotoxins can disturb the gut lining, the microbiome, and gut immunity. The same systems that keep the small intestine in balance [6][7][8][9].
• There is no direct proof that mould toxicity causes SIBO in people. The link rests on shared mechanisms and clinical observation, not on trials that show cause and effect.
• Testing for mycotoxin exposure and for SIBO can each add a useful piece to the picture. Every result needs reading in context by an experienced mould literate practitioner.

So, is there a connection between SIBO and mould illness?

It is a fair question, and one I hear often in clinic. The honest answer is that the science is still young. We have reasonable research on how mould and its by-products affect the gut, and solid research on what drives SIBO. What we do not yet have is a clear trial showing that one causes the other in humans. This article walks through what we do know, in plain language, so you can make sense of your own picture.

What is SIBO?

SIBO stands for small intestinal bacterial overgrowth. It describes a state where too many bacteria collect in the small intestine, fermenting food too early and causing symptoms such as bloating, gas, abdominal pain and changes in bowel habit [3].

Your large intestine is meant to be busy with bacteria. Your small intestine is not — it usually holds far smaller numbers. Your body keeps it that way using several tools: stomach acid, bile, digestive enzymes, a working ileocaecal valve and steady movement through the gut [3] [5]. When these protective mechanisms slip, bacteria can build up where they should be sparse.

That overgrowth ferments food too early in the digestive process. Some people also notice fatigue or nutrient shortfalls, because the extra bacteria can interfere with how you absorb certain nutrients. SIBO is usually described in three subtypes, named after the gas the overgrowing microbes produce: hydrogen, methane (now often called intestinal methanogen overgrowth, or IMO) and hydrogen sulphide. The subtype can influence which symptoms dominate — for example, methane is more often linked with constipation.

What is mould toxicity?

“Mould toxicity” or “mould illness” is not a formal medical diagnosis. It is a term commonly used to describe a cluster of symptoms that some people report after long or heavy exposure to mould, usually in a damp or water-damaged building.

The concern centres on mycotoxins, natural compounds that certain moulds produce. Common indoor moulds include Aspergillus, Penicillium, Cladosporium and Stachybotrys (often called “black mould”), and different species can produce different mycotoxins, such as aflatoxins, ochratoxin A and trichothecenes. You can meet mycotoxins in two main ways: through food, and through indoor environments where damp has allowed mould to grow. The World Health Organisation has reviewed the evidence on damp indoor spaces and concluded that their most important effects are increased respiratory symptoms, allergies and asthma, along with disturbance of the immune system [1]. NHS guidance similarly notes that moulds can produce allergens, irritants and, in some cases, toxic substances [2].

Can mould toxicity cause SIBO?

Here is the direct answer: there is currently no study that shows mould toxicity causes SIBO in humans.

That does not mean the two are unrelated. It means the evidence we have is mechanistic rather than causal. We can see biological reasons why mould exposure might create conditions that make SIBO more likely, but we cannot yet point to a trial that proves it does.

This distinction is worth holding onto. Association is not the same as cause. Two things can travel together, share triggers, or feed into each other without one directly producing the other. With that caveat in place, the mechanisms are genuinely interesting — and they are the reason I now spend so much of my time on this question.

But I would like to raise an important point here – absence of evidence is not evidence of absence.

Why do I look at mould in stubborn SIBO cases?

Because a striking share of my treatment-resistant SIBO clients (people who had genuinely tried everything) turned out to have a history of living or working in a water-damaged building. That was not a hypothesis I set out to test; it was a pattern that surfaced once I started asking.

A large part of my work has gradually shifted towards mould-related illness. It happened because so many people came to me with chronic SIBO and long-standing gut issues who had worked through elimination diets, rounds of supplements, fasting, vagus nerve exercises and careful stress management. Some had improved for a while, but many lapsed over time.

As I took detailed histories, one theme kept surfacing. Once I started asking about damp and mould as a matter of routine, I stopped being surprised by how often it came up. For people who had already turned over every other stone, it was often the piece no one had thought to look at.

I want to be clear about what this is and is not. This is a clinical observation, not a controlled study. I cannot tell you that mould caused their SIBO, and I have not run the kind of trial that could. But when a pattern appears this consistently in people who have exhausted the usual routes, it earns a closer look. The mechanisms below are what make that pattern make sense to me.

How might mould exposure affect the gut?

SIBO tends to develop when the systems that keep the small intestine tidy stop working as well as they should. Several of those systems are ones that mycotoxins have been shown to disturb, at least in laboratory and animal research. If mould does play a part in SIBO, this is the most likely route: not one dramatic effect, but several smaller disruptions that stack up in the same person.

Gut movement and the migrating motor complex

Mycotoxins can cause inflammation and damage in the gut, and inflammation can affect how well the gut moves food along. That matters because poor gut movement is one of the most well-established drivers of SIBO [11]. If heavy mycotoxin exposure interferes with gut movement, that is a plausible route to overgrowth, though the direct evidence for an effect on this specific system is still thin.

The system in question is the migrating motor complex, or MMC: a slow, sweeping wave your small intestine performs between meals. Think of it as the gut’s housekeeper, clearing leftover food and bacteria down towards the large intestine so nothing lingers where it should not. When this housekeeping wave is weak or disrupted, bacteria are not cleared properly, and overgrowth becomes more likely [11]. Where mould may fit in is by adding inflammation that slows this wave — a plausible mechanism, not a proven one.

The gut lining and the microbiome

This is where the research is strongest. Studies show mycotoxins can damage your gut lining. They can disturb the protective mucus layer and the bacteria living there, and loosen the tight seals between gut cells that normally control what gets through [8].

Ochratoxin A is one of the most-studied mycotoxins. Research shows it can throw off the balance of gut bacteria, reducing the helpful kinds and letting harmful ones take over more space [7]. A broader review found the same pattern with several other mycotoxins too. [6][8].

An out-of-balance microbiome and a weakened gut lining are the kind of background conditions in which overgrowth can take hold. It is worth stressing that most of this work comes from animal and cell studies, often at exposure levels above everyday human intake. The findings are a signal, not a settled human story.

Gut immunity

Your gut has its own immune defences, which helps keep bacteria numbers under control. Research suggests mycotoxins can weaken this gut immunity, Ochratoxin A, for example, has been shown to damage the gut’s immune tissue [9].

If those defences are dampened, the small intestine may be less able to keep its bacterial population under control. This is a mechanism that makes sense on paper. It has not been shown to produce SIBO in people.

Putting the mechanisms together

Lay the three threads side by side — motility, the microbiome and barrier, and gut immunity. The housekeeping wave that clears the small intestine may run less smoothly. The microbiome shifts, and the barrier that holds it in place becomes leakier. The immune defences that normally keep numbers down grow quieter. Not one of these alone would reliably cause SIBO. Together, in a susceptible person with ongoing exposure, they tilt the odds in the wrong direction.

That is the mechanistic case, and I think it is a reasonable one. It is also why the clinical pattern I described earlier does not feel like coincidence to me. The biology gives the observation somewhere sensible to sit. It still stops short of proof, and I would rather say that plainly than overstate it.

Why do mould illness and SIBO symptoms overlap?

Part of the confusion between these two comes down to symptoms: they look alike. Bloating, wind, loose stools or constipation, fatigue, brain fog and new food reactions can all appear with SIBO, and many of the same complaints are reported by people who feel unwell after mould exposure. When two conditions share a symptom list, it is easy to assume they share a cause. Sometimes they may. Sometimes a person simply has two separate things happening at once.

The Histamine Connection

One thread that comes up in clinic is histamine. Some people with suspected mould exposure describe classic histamine-type reactions, such as flushing, itching and food sensitivity. There are proposed mechanisms linking mould, immune activation and histamine [14], and SIBO itself may affect histamine balance in the gut — some overgrowing bacteria produce histamine, and inflammation can reduce the DAO enzyme that breaks it down.

How are mould exposure and SIBO tested?

Three tests do most of the work here, and each answers a different question: a SIBO breath test looks for overgrowth in the small intestine, a urine mycotoxin test looks for recent mycotoxin exposure, and a comprehensive stool test maps the balance of your gut bacteria. The key is knowing what each one can and cannot tell you.

A word on urine mycotoxin testing, since Healthpath is introducing one. Used in the right way, it can be a genuinely valuable part of a wider investigation — and in the right way is doing real work in that sentence, because it is the whole point of how we offer it.

Start with what the test is not. A urine mycotoxin test does not diagnose mould illness, and it is not designed to. Healthpath’s test is not a diagnostic test, and we do not diagnose mould toxicity or mould illness from a result. What it gives you is a snapshot of the mycotoxins your body is currently excreting — a single data point that only becomes meaningful when it is read alongside your history, your environment and your symptoms. The same number can point in very different directions depending on the person it belongs to, which is exactly why interpretation matters more than the figure on the page.

This is also where the recognised limits of these tests come in — and, handled properly, they make the case for how the test should be used rather than a reason to avoid it. Results can be influenced by recent food intake, because some everyday foods carry small amounts of mycotoxins, so a single value cannot by itself tell you whether an exposure came from a building or from your diet. There are also no universally validated reference ranges that neatly separate “healthy” from “unwell”. It is on these grounds that public health bodies, including the US Centers for Disease Control and Prevention, have cautioned against treating a urine mycotoxin result as a stand-alone diagnosis [10]. We think that caution is right — and our response to it is not to withhold the test, but to make sure it never travels alone.

That is why practitioner review is built into every Healthpath test as standard. It does not sit right with us to hand someone a set of numbers and leave them to interpret their own health — that is precisely the situation those warnings describe, and it helps no one. Instead, your result is read in context by a practitioner, weighed against your history and symptoms, and turned into a plan you can actually act on. It is also why we do not offer a cheaper results-only version of this test: the interpretation is not an optional extra, it is the part that makes the test worth doing.

There is a simple step that makes the result cleaner, too. Because some of the signal reflects recent diet, we ask you to follow a low-mould diet for a few days before testing. It is worth being realistic about how far this goes: it lowers the recent dietary contribution from fast-clearing mycotoxins, but it does not reset everything — ochratoxin A, one of the most common dietary mycotoxins in human blood, is cleared very slowly, with a half-life of around 35 days [12]. So the preparation sharpens the picture without turning the test into a diagnosis on its own. Read in context, with support, it becomes a genuinely useful piece of the puzzle — which, for us, is the only way it should ever be used.

If you want a broad look at your gut first, the Ultimate Gut Health Test uses comprehensive stool testing with a practitioner review, so you receive a personalised plan and a recorded video rather than a page of numbers to interpret alone. If SIBO is the specific question, the SIBO Test uses a hydrogen breath test with the same practitioner-led support. And if your history includes a damp or water-damaged building, adding mycotoxin testing to that picture is often the piece that has been missing — with interpretation included, so you are never left holding a result you cannot make sense of. These can work well together when your picture is complicated, which mould-related cases often are.

What can you do if you suspect both?

If you think mould exposure and gut symptoms are tangled together for you, three steps tend to help, in this order.

1. Start with the environment. If there is visible damp or mould where you live or work, addressing the source matters more than any supplement. Good ventilation, fixing leaks and reducing moisture are the foundation — the NHS and WHO both point to preventing persistent damp as the priority [1][2].
2. Support your gut with the basics that are always worth doing. A varied diet rich in plants feeds a diverse microbiome; steady meals give the MMC time to do its housekeeping between them; and managing stress, protecting sleep and moving your body all support healthy gut function. One caveat if your gut is currently reactive: some fermentable fibres, such as the inulin found in onions, garlic, chicory and Jerusalem artichoke, can worsen bloating and wind in people with IBS or SIBO. Build plant variety up gradually, and with guidance if symptoms flare.
3. Then test with purpose. Rather than testing everything at once, consider what came first. Did your gut symptoms start while living/working in a mouldy building? If so, perhaps mycotoxin testing is most worthwhile.

Where treatment is needed, SIBO usually calls for its own targeted approach — which may combine dietary change, herbal antimicrobials, and work to correct the underlying driver (for example, motility). Mould-focused strategies, including the use of binders, are sometimes layered in, but the evidence for them is more limited than it is for SIBO and they belong under practitioner supervision rather than self-directed protocols. Evidence is in italics because, again, absence of evidence is not evidence of absence. We know what works in mould illness as experienced mould literate practitioners have spent decades figuring out what helps, and what doesn’t.

Above all, be patient with yourself. Long-standing gut symptoms are draining, and layered problems take time to untangle. A methodical, root-cause approach tends to serve people far better than a scramble for the newest protocol.

When to see a doctor

Most bloating and changes in bowel habit are not dangerous, but some symptoms need prompt medical assessment rather than testing or self-management. See a GP — and do not simply put these down to SIBO or mould — if you have any of the following:

• Unintentional weight loss
• Blood in your stool, or black, tarry stools
• Difficulty swallowing, or persistent vomiting
• A change in bowel habit lasting three weeks or more, especially if you are over 50
• Iron-deficiency anaemia, or a family history of bowel or ovarian cancer
• A lump or swelling in your tummy, or persistent, severe or worsening abdominal pain

These are general red flags, not a diagnosis. If in doubt, get checked — the same symptoms can have many causes, and it is always reasonable to ask a clinician.

Frequently asked questions

Does mould exposure always cause gut problems?

No. Many people exposed to damp and mould have no gut symptoms at all. The best-established effects of mould are on the airways. Gut effects are less consistent and appear to depend on the level of exposure and on individual susceptibility.

Can addressing mould exposure improve SIBO?

There is no trial that answers this directly. In principle, reducing a source of gut inflammation and immune stress could support recovery. In practice, SIBO usually needs its own targeted approach as well, alongside dealing with the environment.

Should I get a mycotoxin test if I have SIBO?

Not automatically. A mycotoxin test is most useful when there is a real reason to suspect mould exposure, such as a history of living or working in a damp building. It is one input into a wider assessment and is worth considering in certain cases, but isn’t a necessity in all. The results of a mycotoxin test should always be read in context by a practitioner. If you’re unsure, book a call with one of our friendly team and we will advise you on the best set of tests for you.

Is black mould the main concern?

Black mould gets the most attention, but the colour of mould is not a reliable guide to risk. What matters more is the amount of mould, how long it has been present, and how sensitive the person exposed is. If you have significant damp, the sensible move is to deal with it rather than to focus on identifying the species first.

The bottom line

Can mould toxicity cause SIBO? On current evidence, we cannot say that it does. What we can say is that mycotoxins are capable of disturbing the gut lining, the microbiome and gut immunity, and that these are the same systems whose failure allows SIBO to develop. That makes a connection plausible and worth investigating, without overstating what the science has shown.

If this resonates with your own experience, the most useful next step is usually to understand your gut properly, in context, with support. That is where testing and a personalised plan earn their place.

This article is for information only. It is not intended to diagnose, treat, cure or prevent any disease. Please speak to a qualified healthcare professional about your individual circumstances. If you are in the UK and need urgent help, call NHS 111, or 999 in an emergency.

Scientific references

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   2. NHS inform. Damp and mould indoors. https://www.nhsinform.scot/healthy-living/indoor-health/damp-and-mould-indoors/
   3. Sorathia SJ, Chippa V, Rivas JM. Small Intestinal Bacterial Overgrowth. StatPearls, NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK546634/
   4. Efremova et al., (2023) Epidemiology of small intestinal bacterial overgrowth, World J Gastroenterol. 14;29(22):3400–3421 https://pmc.ncbi.nlm.nih.gov/articles/PMC10303511/
   5. Skrzydło-Radomańska B, Cukrowska B. The importance of food quality, gut motility, and microbiome in SIBO development and treatment. 2024. https://www.sciencedirect.com/science/article/abs/pii/S089990072400114X
   6. Robert H, et al. Impact of mycotoxins on the intestine: are mucus and microbiota new targets? J Toxicol Environ Health B. https://pubmed.ncbi.nlm.nih.gov/28636450/
   7. Gut as a Target of Ochratoxin A: Toxicological Insights and the Role of Microbiota. Int J Mol Sci, 2025. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12525421/
   8. Gao Y, et al. The Compromised Intestinal Barrier Induced by Mycotoxins. Toxins. https://www.mdpi.com/2072-6651/12/10/619
   9. Islam MdR, et al. Gut Microorganism-Mediated Neutralization of Mycotoxins. Adv Gut Microbiome Res, 2024. https://onlinelibrary.wiley.com/doi/10.1155/2024/8448547
   10. Kawamoto M, Page E. Use of Unvalidated Urine Mycotoxin Tests for the Clinical Diagnosis of Illness — United States, 2014. MMWR, 2015. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4584707/
   11. Husebye E. Gastrointestinal motility disorders and bacterial overgrowth. J Intern Med, 1995. https://pubmed.ncbi.nlm.nih.gov/7714466/
   12. EFSA Panel on Contaminants in the Food Chain. Risk assessment of ochratoxin A in food. EFSA Journal, 2020. https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2020.6113
   13. Pimentel et al., (2004) A link between irritable bowel syndrome and fibromyalgia may be related to findings on lactulose breath testing, Ann Rheum Dis;63(4):450-2 https://pubmed.ncbi.nlm.nih.gov/15020342/ 
   14. Kritas et al. (2018) Impact of mold on mast cell-cytokine immune response, J Biol Regul Homeost Agents;32(4):763-768