SIBO Symptoms: Signs, Causes, Tests and When to See a Doctor

SIBO stands for small intestinal bacterial overgrowth. Your small intestine normally has far fewer bacteria than your large intestine. In SIBO there are too many, and they cause gut symptoms. [1]

The symptoms are common and not specific, so SIBO is easy to mix up with IBS and other gut problems. [1] This guide covers the symptoms, what causes SIBO, how it is tested and treated, and what to write down for your doctor.

What are the symptoms of SIBO?

The most common symptom is bloating. [1] Tummy pain, bloating, gas, a swollen tummy, farting and diarrhoea are the most common symptoms, and are found in more than two-thirds of people with SIBO. [1]

• Bloating: your tummy feels full, tight or uncomfortable. [1]
• A swollen tummy (distension) that you can see. [1]
• Gas: more farting or burping than usual. [1]
• Tummy pain or cramps. [1]
• Diarrhoea or loose poo. [1]
• Constipation, mainly when methane-making microbes overgrow. [1]
• Nausea. [1]
• Feeling full quickly. In 740 people with gastroparesis, a positive hydrogen breath test went with worse bloating, fullness after meals and feeling full early. [2]

Some people also feel tired or find it hard to concentrate. [1] Our guides to feeling tired after eating and brain fog cover the other causes.

No single symptom proves you have SIBO. In one study, people with and without SIBO on a small bowel culture reported the same pain, bloating, fullness, nausea, diarrhoea and gas. [1] That is why testing matters.

What are the less common signs of SIBO?

In severe cases, SIBO can stop your body absorbing food properly (malabsorption). This can cause:

• fatty poo (steatorrhoea), which can be greasy, foul-smelling and hard to flush [1] [3] [10]
• weight loss without trying [1]
• low vitamin B12, vitamin D or iron [1]
• anaemia [1]

These are not common. They are mostly seen when SIBO comes from bowel surgery that left a “blind loop”, or from scleroderma. [1] Blood tests can show raised folate, because the bacteria make folate, and less often low vitamin B12. [1] [3]

Weight gain isn’t listed as a SIBO symptom in the ACG guideline. [1] One study of 792 people found that those with both methane and hydrogen on a breath test had a higher BMI and more body fat, but it didn’t show that the gases cause weight gain. [14]

Low vitamin B12 can cause tiredness, pins and needles, numbness and memory problems. See a GP if you notice these, because some problems from B12 deficiency can become permanent if it isn’t treated. [11]

What are SIBO symptoms in women?

Guidelines don’t list separate SIBO symptoms for women. Bloating, gas, tummy pain and changes in your poo are the main signs for everyone. [1] [3]

Being female is linked with a higher chance of SIBO, possibly because food moves through the gut more slowly. [1]

Several conditions that are common in women cause similar symptoms, which can make SIBO harder to spot:

• IBS: women are up to twice as likely as men to have it. [5] It causes tummy pain, bloating, diarrhoea and constipation. [6]
• Endometriosis: it can affect the bowel and cause pain when you poo, and its symptoms can be similar to IBS. [9] Our endometriosis flare-ups guide explains how to log bowel symptoms with your cycle.
• Periods: some people feel bloated around the time of their period. [8]

Bloating that doesn’t go away can sometimes be a sign of something more serious, such as ovarian cancer. See a GP if you feel bloated regularly. [8]

What causes SIBO and who is at risk?

SIBO almost always comes from another problem, usually one that lets food and bacteria sit too long in the small intestine. [1] Your body normally keeps bacteria down with stomach acid, digestive enzymes, bile, waves of gut muscle movement that sweep the small intestine clean, and a valve between the small and large intestine. When one of these fails, bacteria can build up. [1]

• Slow gut movement. Diabetes and scleroderma can damage the nerves or muscles that move food along. [1] In a small study of 50 people with gastroparesis and tummy pain or bloating, 30 (60%) had a positive breath test for SIBO. [4] Gastroparesis itself causes early fullness, nausea and bloating, and diabetes is its most common known cause, so the two can be hard to tell apart. [16]
• Past stomach or bowel surgery. For example, surgery that leaves a blind loop, a Roux-en-Y gastric bypass, or removing part of the colon, especially the valve between the small and large intestine. [1]
• Low stomach acid, for example from autoimmune gastritis or after part or all of the stomach is removed. [1]
• Proton pump inhibitors (PPIs). A review of 19 studies found up to 3 times the risk of SIBO in people taking these acid-lowering medicines, but the link is complex and it isn’t proven that PPIs cause SIBO. [1]
• Opioid painkillers and some other medicines that slow the gut. [1]
• Some other conditions, including IBS, coeliac disease, Crohn’s disease, chronic pancreatitis, cirrhosis, an underactive thyroid, Parkinson’s disease and fibromyalgia. [1] In one study, 16.8% of people with Crohn’s disease in remission had SIBO. [1]
• Older age. [1]

Don’t stop a prescribed medicine without talking to your doctor. Finding the cause matters, because treating it is the main way to stop SIBO coming back. [1]

If you live with gastroparesis, Crohn’s disease or IBS, our condition pages show what is worth tracking for your doctor.

What is the difference between SIBO and IBS?

They share symptoms, and they can happen together. The ACG guideline says a subset of people with IBS have SIBO, but experts still disagree about how much SIBO explains IBS symptoms. [1] [3]

The ACG guideline suggests breath testing for SIBO in people with IBS. [1] If you have been told you have IBS and treatment hasn’t helped, it is reasonable to ask your doctor whether a SIBO breath test makes sense for you.

How is SIBO tested?

The usual test is a breath test. It is cheap and simple, and doesn’t need an endoscopy. [1]

How it works. Your body can’t make hydrogen or methane gas; only gut microbes can. You drink a sugar solution, either glucose or lactulose, and then give breath samples at set times over the next few hours. If bacteria in the small intestine ferment the sugar early, the gases show up in your breath. [1] [2]

Before the test. Your clinic will give you instructions. The 2017 North American consensus on breath testing recommends: [2]

• no antibiotics for 4 weeks before
• stopping laxatives and medicines that speed up the gut at least 1 week before, if you can manage without them
• avoiding fermentable foods, such as complex carbohydrates, the day before
• fasting for 8 to 12 hours
• no smoking on the day of the test

You don’t need to stop proton pump inhibitors (PPIs) before the test. [2]

What counts as positive. Under the 2017 consensus thresholds: [1] [2]

• Hydrogen: a rise of 20 ppm (parts per million) or more above your starting level within 90 minutes points to SIBO.
• Methane: a level of 10 ppm or more at any point in the test points to intestinal methanogen overgrowth (IMO).

Methane is made by microbes called archaea, not bacteria, and they can overgrow in the large intestine too. That is why the ACG guideline calls this IMO rather than SIBO. Methane on a breath test is linked with constipation. [1]

Limits of the test. Glucose is absorbed early in the small intestine, so a glucose test can miss overgrowth further down. Lactulose that moves quickly into the large intestine can give a false positive. [1] Compared with small bowel samples, studies found glucose tests picked up between 20% and 93% of cases, and lactulose tests between 31% and 68%. [1] [2]

Home breath test kits exist. They are often analysed by certified labs, but the diet preparation and sampling happen at home, where they are harder to control. [1]

Small bowel sample. Less often, a doctor takes a sample of fluid from the small intestine during an endoscopy and counts the bacteria. A count of 1,000 or more colony-forming units per millilitre suggests SIBO. This test is more invasive and costly, and needs sedation. [1]

How is SIBO treated?

Treatment has three parts: finding and fixing the cause where possible, correcting any vitamin or nutrient shortfalls, and antibiotics. [3]

Antibiotics. The ACG guideline suggests antibiotics for people with SIBO who have symptoms, though the evidence is low quality. [1] Rifaximin, an antibiotic that stays in the gut and is not absorbed into the blood, is the most studied. In a review of 32 studies with 1,331 people, it cleared SIBO in about 71%, and about 5% had side effects. [1] Other antibiotics are also used. Your doctor chooses the medicine and the length of the course.

Treating the cause. This might mean looking at medicines that slow the gut, or managing a condition such as diabetes, scleroderma or gastroparesis. Treating the cause is the main way to prevent SIBO coming back. [1]

SIBO can come back. In one study of 80 adults, it came back after antibiotics in 13% within 3 months, 28% within 6 months and 44% within 9 months. [1]

Diet. SIBO diets usually cut down on foods that ferment in the gut. But most of the evidence comes from IBS research, and the evidence for a low-FODMAP diet in IBS is very low quality. [1]

Elemental diet. This replaces all food with a special formula drink for about 2 weeks. In one study of 93 people with IBS and an abnormal breath test, 80% had a normal breath test after 14 days. [12] The study had no comparison group, and it is only something to try with medical support.

Probiotics. There isn’t enough consistent evidence to recommend a probiotic for SIBO. [1]

What are SIBO die-off symptoms?

People use “die-off” to describe feeling worse after starting SIBO treatment, for example more bloating, tiredness or flu-like feelings. There is little evidence for it, and the ACG guideline on SIBO doesn’t mention it. [1]

A well-described reaction of this kind, the Jarisch-Herxheimer reaction, happens with a different group of infections. Within 24 hours of antibiotics for syphilis, Lyme disease and some similar infections, people get shaking chills, a fever and worse skin rashes, which settle a few hours later. [13] It is linked to those infections, not to SIBO.

If you feel worse on treatment, it could be a side effect, a different problem, or SIBO symptoms that haven’t settled yet. In the rifaximin review, about 5% of people had side effects. [1] Contact the doctor who prescribed it, and don’t stop a medicine without advice.

When should you see a doctor about SIBO symptoms?

See a GP or doctor if:

• you feel bloated regularly, or changing your diet hasn’t helped [8]
• you have bloating and are losing weight without trying [8]
• you have gut symptoms and have had bowel or stomach surgery, or live with diabetes, scleroderma or gastroparesis, which raise the chance of SIBO [1] [4]
• you have signs of low vitamin B12, such as tiredness, pins and needles or memory problems [11]

Ask for an urgent GP appointment or call 111 (UK) if you have bloating with vomiting, a high temperature, or a lump or swelling in your tummy, if you are bleeding from your bottom, or if you have diarrhoea that doesn’t go away after a few days. [6] [8] [15]

What should you write down for your doctor?

A doctor will want to know what your symptoms are, how often and when you get them, and how long you have had them. [7] A short note each day is enough. Write down:

• which symptoms you had: bloating, gas, pain, diarrhoea, constipation or nausea
• when they started, and how long after eating
• roughly what you ate
• what your poo was like, and how often you went
• any weight change
• your medicines, especially PPIs, opioids and recent antibiotics
• any past stomach or bowel surgery, and conditions such as diabetes

Our free food and symptom log gives you three days of meals with columns for symptoms and the delay before they start.

If writing it down is too much, you can tell Juno your symptoms and meals by voice or text. Juno logs them and turns them into a dated summary to take to your doctor. See how the Juno symptom tracker works.

When are tummy symptoms an emergency?

Bloating and gas are not an emergency on their own. Get help straight away if you have any of these.

This article is for general information only and does not replace advice from your own clinician. Juno is a health assistant, not a medical provider.

Last updated 21 October 2026