91.9% of women with endometriosis tested positive for SIBO in a 2025 study
3x higher SIBO rates in women with hypothyroidism versus general population
6–15% estimated SIBO prevalence in the general adult population

What SIBO actually is — and what it isn't

Your small intestine is supposed to be relatively sparse when it comes to bacteria. The vast majority of your gut microbiome lives in the colon, and your body uses several mechanisms to keep bacteria from migrating upstream into the small intestine: stomach acid, bile, digestive enzymes, and most importantly a muscular sweep called the migrating motor complex (MMC), which moves bacteria and debris through the small intestine between meals.

When those mechanisms fail — for any reason — bacteria proliferate in the small intestine where they don't belong. That's SIBO. The bacteria ferment food as it passes through, producing hydrogen and methane gas. The result: bloating, often within 30–90 minutes of eating, regardless of what you ate.

SIBO is not the same as gut dysbiosis (an imbalanced colon microbiome), and it's not treated the same way. This distinction matters because many women with SIBO have spent months trying to fix it with probiotics and fiber that, in some cases, make the symptoms worse.

Why women are disproportionately affected

Sex hormones have a direct effect on gut motility. Estrogen and progesterone both slow the movement of food and bacteria through the gut — which is useful during pregnancy (more nutrient absorption) but problematic when hormonal fluctuations are chronic or extreme.

Slower gut transit means the MMC has less time to perform its cleaning sweep between meals. Bacteria linger. They colonize. This is why women with conditions characterized by elevated or fluctuating estrogen — endometriosis, PCOS, perimenopause transitions — show SIBO rates far above the population average.

Research note

A 2025 case-control study found that 91.9% of women with endometriosis tested positive for SIBO or intestinal methanogen overgrowth (IMO) via lactulose breath test. This is a striking figure — and the researchers noted that the endometrial implants themselves may be disrupting local gut motility in ways that create the overgrowth conditions. Women with PCOS and hypothyroidism also show significantly elevated SIBO rates in multiple studies. The connection isn't coincidental. It's mechanistic.

The hormone feedback loop: how SIBO makes things worse

Here's where it gets genuinely complicated, and most IBS and gut health discussions don't go here.

Your gut bacteria play a central role in regulating estrogen levels through an ecosystem called the estrobolome. Certain gut bacteria produce an enzyme called beta-glucuronidase, which deconjugates estrogen in the gut so it can be reabsorbed into the bloodstream rather than excreted. When the estrobolome is disrupted — as it is in SIBO — this deconjugation process can be dysregulated, leading to excess estrogen recirculating in the body.

If you already have estrogen-dominant conditions like endometriosis or PCOS, SIBO may be actively fueling the hormonal driver of those conditions while those same conditions are perpetuating the gut environment that sustains SIBO. The loop runs in both directions.

How SIBO is tested and treated

Diagnosis is most commonly done with a breath test: you drink a lactulose or glucose solution, then breathe into collection tubes over 2–3 hours. Elevated hydrogen or methane at specific time intervals indicates bacterial overgrowth. Breath testing isn't perfect — false negatives exist, and methane-dominant SIBO (now more accurately called intestinal methanogen overgrowth, or IMO) has different patterns — but it is the most widely available non-invasive test and the one most gastroenterologists use.

Treatment depends on the type. Hydrogen-dominant SIBO is most commonly treated with rifaximin, a non-absorbable antibiotic that works locally in the gut with minimal systemic effects. Methane-dominant IMO often requires a combination of rifaximin and neomycin. Elemental diets (liquid formulas that are absorbed before bacteria can ferment them) are a high-efficacy alternative to antibiotics, particularly for women who want to avoid antibiotic treatment or who have relapsed on antibiotics.

Practical tip

If you are pursuing SIBO testing and treatment, the specific test (lactulose vs. glucose substrate, 2-hour vs. 3-hour collection) and the interpretation of results matter significantly. Gastroenterologists with specific SIBO expertise use different protocols than general GPs. A functional medicine or integrative GI specialist is often better positioned to interpret breath test results and select the right treatment approach — especially if IMO (methane) is suspected alongside SIBO (hydrogen).

Relapse: the problem that standard SIBO treatment misses

Treating SIBO without addressing the underlying motility issue is like clearing a blocked drain without fixing the water pressure. The bacteria come back.

For women, this often means addressing the hormonal driver directly — which is why SIBO treatment protocols that include motility support (prokinetic agents taken at bedtime to enhance MMC function) and dietary spacing (leaving 4–5 hours between meals rather than grazing, to allow MMC activity) tend to show better long-term outcomes.

In women whose SIBO appears hormonally driven — worsening before periods, improving on hormonal contraception, or clustering with an endometriosis or hypothyroid diagnosis — addressing the hormonal condition is as important as treating the gut.

The bloating most women are told is IBS

This is where a lot of women get stuck. IBS and SIBO share a symptom profile, and SIBO is thought to be a driver in 30–85% of IBS cases in some estimates. Women with a longstanding IBS diagnosis who have never been specifically tested for SIBO may be treating the symptom without the cause.

If your IBS symptoms haven't improved after dietary modifications, if bloating is pronounced and consistent rather than food-trigger-specific, and if you have any of the associated conditions — endometriosis, PCOS, hypothyroid, a history of food poisoning, or a current proton pump inhibitor prescription — bringing SIBO to the conversation with your gastroenterologist is reasonable.

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When to ask about SIBO testing

Consider raising SIBO specifically if you have: persistent bloating within 1–2 hours of meals regardless of food type, an existing diagnosis of endometriosis, PCOS, or hypothyroidism, a history of acute gastroenteritis followed by chronic gut symptoms (post-infectious SIBO), or a longstanding IBS diagnosis that hasn't responded to standard dietary management. Ask your gastroenterologist or primary care doctor for a lactulose or glucose breath test. Some functional medicine or integrative GI clinics offer this as part of a comprehensive gut evaluation.

Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before pursuing testing or treatment for any gut condition.

References

  1. Pimentel M, et al. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. American Journal of Gastroenterology. 2020;115(2):165–178.
  2. Case-control study: SIBO/IMO prevalence in women with endometriosis. 2025. (Referenced in Eterna Integrative clinical summary, 2025.)
  3. Lara Briden. How IBS and SIBO Can Affect Periods and Hormones. 2023. Link
  4. Kwa M, et al. The Intestinal Microbiome and Estrogen Receptor-Positive Female Breast Cancer. Journal of the National Cancer Institute. 2016. (Estrobolome mechanism)
  5. Ghoshal UC, et al. Small intestinal bacterial overgrowth and irritable bowel syndrome: a bridge between functional organic dichotomy. Gut and Liver. 2017;11(2):196–208.