Symptoms of SIBO

In short: the classic symptoms of SIBO are bloating, diarrhea and abdominal pain or discomfort. In more severe cases, fat is poorly absorbed and stools become pale, greasy and floating². Of all the symptoms, diarrhea has the strongest link to SIBO³. Methane overgrowth is linked to constipation instead⁷. Because the symptoms overlap with IBS and many other conditions, they cannot diagnose SIBO on their own; a test is needed³.

Small intestinal bacterial overgrowth (SIBO) produces symptoms that range from mild bloating to chronic diarrhea, weight loss and malabsorption⁴. This guide covers the digestive symptoms, how they differ between hydrogen, methane and hydrogen sulfide overgrowth, the nutrient problems SIBO can cause, and the symptoms outside the gut that have been linked to it.

If you are preparing for an appointment, our symptom checker turns the symptoms you tick into a printable summary with questions to ask your doctor.

Digestive symptoms

A 2025 position paper from a national gastroenterology society lists the main symptoms of SIBO as bloating, distension, gas, abdominal pain, diarrhea and constipation. Among these, diarrhea, rather than bloating, has the strongest association with SIBO³.

• Bloating and distension. Bloating is the feeling of pressure or trapped gas; distension is a visible increase in waist size. Gas made by bacteria fermenting food in the narrow small intestine stretches the gut wall. In some people, distension is made worse by an abnormal reflex after meals: the diaphragm contracts and the front abdominal wall relaxes, pushing the belly outward even when the amount of gas is only slightly raised⁵.
• Diarrhea. Bacteria ferment carbohydrates before they can be absorbed, and they break down bile acids needed to absorb fat. Both can cause loose stools³.
• Constipation. This is typical of methane overgrowth (IMOIntestinal methanogen overgrowth (IMO) Overgrowth of methane-producing archaea, mainly Methanobrevibacter smithii. It is diagnosed by a breath methane level of 10 ppm or more and is linked to constipation.), because methane slows the gut. In a 2006 animal study, methane slowed small-intestinal transit by an average of 59%⁶.
• Abdominal pain and discomfort, often worse after meals².
• Gas and belching, from fermentation in the small intestine³.
• Fatty stools (steatorrhea) in more severe cases: pale, greasy, foul-smelling stools that float or are hard to flush².

Why pain can feel worse than the gas

Many people with SIBO or IBS feel pain or bloating from normal or only slightly raised amounts of gas. This is called visceral hypersensitivityVisceral hypersensitivity Heightened sensitivity of the gut's nerves, so that normal amounts of gas or stretching are felt as pain.: the nerves of the gut have become more sensitive, so normal stretching is felt as pain⁵²⁰. Stress and the gut-brain connection can make it worse²⁰. Our guide to stress and SIBO covers approaches that may help.

Symptoms by gas type

The patterns come from a 2022 study that combined breath tests with stool sequencing in people with IBS. Higher breath methane went with constipation and more Methanobrevibacter, while higher hydrogen and hydrogen sulfide went with diarrhea and with more hydrogen and sulfide-producing bacteria⁷. A 2020 meta-analysis found the same split: SIBO was more common in diarrhea-predominant IBS, and methane-positive tests were more common in constipation-predominant IBS⁸.

Nutrient problems

When SIBO is more severe, it can interfere with nutrition²³:

• Fat and fat-soluble vitamins. Bacteria break down bile acids, which are needed to absorb fat, so fat and vitamins A, D, E and K can be poorly absorbed³.
• Vitamin B12. Bacteria use up B12 before it can be absorbed, so deficiency can develop, sometimes with anemia or nerve symptoms such as tingling. Folate and vitamin K, by contrast, are often high, because bacteria make them²³.
• Weight loss or, in children, poor weight gain³.

SIBO can also change the lining of the small intestine. In a 2010 study of duodenal biopsies, flattened villi were more common in people with SIBO than in controls (24% vs. 7%), although more than half of the SIBO biopsies looked normal¹⁰. Our blood tests guide covers how these deficiencies are checked.

Symptoms outside the gut

Several conditions outside the gut have been linked to SIBO. These studies show associations; they do not prove that SIBO causes the condition.

• Brain fog. In a 2018 study of 38 people with gas and bloating, those with brain fog were more likely to have SIBO (68% vs. 28%) and D-lactic acidosis, an excess of acid made by gut bacteria. All of those with brain fog were taking probiotics, and the brain fog cleared in 77% after they stopped the probiotics and took antibiotics⁹.
• Rosacea. In a 2008 study, 52 of 113 people with rosacea had SIBO, compared with 3 of 60 controls, and the skin cleared or greatly improved in most of those whose SIBO was treated successfully¹¹.
• Restless legs syndrome. In a 2011 study, 69% of 32 people with restless legs syndrome had a positive breath test, compared with 28% of controls from the general population¹².
• Histamine-type symptoms. People with histamine intolerance (headaches, flushing, itching or a racing heart after high-histamine foods) have been found to have more histamine-producing bacteria in their stool¹⁶. Whether SIBO itself drives histamine intolerance has not been studied directly; see our guide to sauerkraut and histamine.
• Leaky gut. Bacteria in contact with the small intestine can loosen the seals between gut cells. Our leaky gut guide covers what is known.

Symptoms in children and older adults

Children. SIBO is a common cause of non-specific digestive symptoms in children, such as chronic abdominal pain, distension, diarrhea and gas, and it has been implicated in stunted growth in children worldwide¹³. In a 2010 study of 50 children with stool soiling caused by severe constipation (encopresis), 56% produced methane on a breath test, compared with 23% of children with other gut problems, and methane went with more severe stool retention¹⁴.

Older adults. Older age is a risk factor for SIBO. In studies of people with IBS, those with SIBO were on average older. Long-term use of acid-reducing medicines, which is common in older adults, is also linked to SIBO: in one study of older people, 77% of long-term PPIProton pump inhibitor (PPI) A class of acid-reducing medication used for reflux, such as omeprazole. Long-term use lowers stomach acid and is associated with a higher risk of SIBO. users had SIBO, compared with 58% of non-users¹⁵.

SIBO, IBS and other look-alike conditions

The symptoms of SIBO overlap closely with irritable bowel syndrome (IBS). A 2020 meta-analysis of 25 studies found that people with IBS had 3.7 times the odds of SIBO compared with controls, but rates varied widely with the test used, and the evidence quality was low⁸. SIBO is one possible explanation for IBS symptoms in some people, not the cause of most IBS.

Other conditions with similar symptoms include celiac disease, inflammatory bowel disease, lactose or fructose intolerance, bile acid diarrhea, pancreatic insufficiency and fungal overgrowth (SIFO). This is why the national guidelines stress that SIBO cannot be diagnosed from symptoms alone and should be confirmed with a test, especially in people with risk factors³¹. Our diagnosis guide explains which test to use.

Warning signs

See a doctor promptly, whatever the cause, if you have blood in your stool, unintended weight loss, anemia, difficulty swallowing, persistent vomiting, symptoms that wake you at night, or new symptoms after age 50.

Easing symptoms while you investigate

These steps can ease symptoms but do not clear SIBO:

• Low-FODMAPFODMAPs Fermentable oligo-, di- and monosaccharides and polyols: short-chain carbohydrates that are poorly absorbed and quickly fermented by gut microbes. diet. Cutting down on rapidly fermented carbohydrates reduces gas. In a 2026 online survey of 98 people with IBS, SIBO or IMO who had followed the diet, most reported fewer symptoms, especially less bloating, but fewer than half completed both the elimination and reintroduction phases¹⁹. See our low-FODMAP guide and SIBO diet guide.
• Meal spacing. The migrating motor complexMigrating motor complex (MMC) A wave of muscle contractions that sweeps the stomach and small intestine between meals, roughly every 90 to 120 minutes, pushing leftover food and bacteria into the colon. Eating interrupts it., which sweeps bacteria out of the small intestine between meals, only runs while fasting, and eating interrupts it¹⁷. SIBO practitioners recommend spacing meals 4 to 5 hours apart without snacks and fasting about 12 hours overnight¹⁸. See our motility guide.

For treatment itself, see our guides to antibiotics, herbal antimicrobials and the elemental diet, or the side-by-side comparison of SIBO treatments.

Common questions

What does SIBO bloating feel like?

Usually a feeling of pressure or fullness that builds after meals, often with a visibly larger waist by the evening. The amount of gas is not always large; a more sensitive gut and an abnormal belly-wall reflex can make normal amounts feel uncomfortable⁵.

Can SIBO cause constipation?

Yes, especially methane overgrowth (IMO). Methane slows gut transit⁶, and methane-positive breath tests are more common in constipation-predominant IBS⁸.

Can SIBO cause fatigue or brain fog?

It can contribute. B12 deficiency can cause fatigue², and one study linked brain fog to SIBO, probiotic use and D-lactic acidosis⁹. A national position paper notes that it is unclear whether brain fog, anxiety and depression are directly caused by SIBO³.