SIBO Explained: Symptoms, Breath Tests and Treatment

Small intestinal bacterial overgrowth, usually shortened to SIBO, describes an excessive number or altered mix of microorganisms in the small intestine. These organisms can ferment food before it has been absorbed properly, potentially causing bloating, abdominal discomfort, excessive wind, diarrhoea and, in some cases, nutritional deficiencies.

SIBO is real, but it is also difficult to diagnose reliably. Its symptoms overlap with irritable bowel syndrome, coeliac disease, lactose intolerance, constipation and several other digestive conditions. Breath tests are convenient and widely used, but they can produce false-positive and false-negative results.

A positive breath test should therefore be interpreted alongside symptoms and risk factors—not treated as proof that every digestive problem is caused by bacterial overgrowth.

Treatment may involve a short course of antibiotics, correction of an underlying condition, nutritional support and management of impaired bowel movement. Recurrence is common if the factor allowing overgrowth remains.

This guide provides general information. Persistent or severe digestive symptoms need an appropriate medical assessment rather than repeated home testing or self-treatment with antibiotics and supplements.

What is SIBO?

The digestive tract naturally contains microorganisms, but they are not distributed evenly. The large bowel normally contains a much denser microbial population than the small intestine.

Several mechanisms help prevent excessive microbial growth in the small bowel:

• stomach acid;
• normal muscular movement through the digestive tract;
• bile and pancreatic secretions;
• an intact valve between the small and large bowel;
• immune defences within the intestinal lining;
• regular clearance of food and bacteria between meals.

If these defences are disrupted, microorganisms can accumulate or remain in the small intestine for longer than usual. They may consume nutrients, alter bile acids and produce gases as they ferment carbohydrates.

The American College of Gastroenterology guideline defines SIBO as excessive bacteria in the small bowel associated with gastrointestinal symptoms. However, experts continue to debate the best diagnostic definition and test.

SIBO is not the same as general “gut dysbiosis”, candida overgrowth or an abnormal result from a commercial stool-microbiome test. Those terms and tests assess different—and sometimes poorly defined—concepts.

What are the symptoms of SIBO?

The most commonly reported symptoms are:

• bloating or visible abdominal distension;
• excessive wind;
• abdominal discomfort or cramping;
• diarrhoea or loose stools;
• constipation, particularly with methane production;
• alternating diarrhoea and constipation;
• feeling unusually full after eating;
• nausea;
• belching;
• fatigue.

More significant overgrowth can interfere with digestion and absorption. Possible consequences include:

• unintentional weight loss;
• greasy, pale or difficult-to-flush stools;
• vitamin B12 deficiency;
• iron or folate abnormalities;
• low levels of fat-soluble vitamins;
• anaemia;
• poor bone health in longstanding malabsorption;
• malnutrition in severe cases.

None of these symptoms is specific to SIBO. Bloating after meals, for example, can arise from constipation, IBS, coeliac disease, food intolerance or ordinary fermentation in the colon.

See our guide to bloating, common causes and warning signs for a wider explanation.

What causes SIBO and who is at greater risk?

SIBO is more plausible when something changes the structure, movement or protective mechanisms of the small intestine.

Reduced intestinal movement

Food and bacteria are normally moved through the bowel by coordinated muscular contractions. Conditions that slow this movement can allow bacteria to accumulate.

Possible causes include:

• systemic sclerosis or scleroderma;
• diabetes affecting digestive nerves;
• intestinal pseudo-obstruction;
• Parkinson’s disease and some other neurological conditions;
• severe longstanding constipation;
• medicines that slow the gut, including opioids;
• certain connective-tissue disorders.

Previous abdominal surgery

Surgery can alter the normal anatomy or create areas where intestinal contents stagnate. Risk may be increased by:

• blind loops;
• small-bowel diverticula;
• strictures or narrowed sections;
• removal or dysfunction of the ileocaecal valve;
• some gastric or bariatric operations;
• small-bowel resection;
• surgical connections that bypass part of the intestine.

Digestive and systemic conditions

SIBO may occur alongside:

• Crohn’s disease, particularly with strictures or previous surgery;
• coeliac disease that is not responding as expected;
• chronic pancreatitis or pancreatic insufficiency;
• cirrhosis;
• radiation damage to the bowel;
• immune deficiency;
• short-bowel syndrome.

Proton-pump inhibitors reduce stomach acid and have been associated with SIBO in some studies, but the relationship is not straightforward. Taking a PPI without symptoms is not, by itself, a reason to undergo breath testing. Do not stop prescribed acid-suppressing medication without discussing it with the prescriber.

Is SIBO the same as IBS?

No. Irritable bowel syndrome, or IBS, is a disorder of gut–brain interaction characterised by abdominal pain and altered bowel habits without a structural explanation found on routine testing.

SIBO describes suspected microbial overgrowth in the small intestine. The two conditions can produce almost identical symptoms, and some people may have both.

Feature	SIBO	IBS
Main concept	Excessive or abnormal microbial growth in the small bowel	Altered gut function and gut–brain interaction
Common symptoms	Bloating, wind, discomfort, diarrhoea or constipation	Abdominal pain, bloating and altered bowel habit
Diagnosis	Clinical context, sometimes breath testing or small-bowel sampling	Symptom pattern and exclusion of concerning alternatives
Treatment	May include antibiotics and treatment of the underlying cause	Dietary, lifestyle and symptom-directed treatment

A positive lactulose breath test in someone with IBS does not necessarily establish SIBO. Rapid movement of lactulose into the colon can produce an early hydrogen rise that resembles small-bowel fermentation.

This uncertainty is one reason experts disagree about how extensively breath tests should be used in people who have typical IBS without additional SIBO risk factors.

Our guide to IBS symptoms, diagnosis and treatment explains the usual UK assessment pathway.

How is SIBO diagnosed?

There is no simple test that identifies every clinically important case while excluding all people without SIBO.

Assessment normally begins with:

• a detailed history of symptoms;
• previous operations and digestive conditions;
• medicines, including antibiotics, opioids and acid suppression;
• weight change and signs of malabsorption;
• examination of the abdomen;
• blood or stool tests for alternative explanations;
• breath testing when the result may change management.

Hydrogen and methane breath testing

This is the most commonly used non-invasive test. After drinking a glucose or lactulose solution, the patient provides repeated breath samples over approximately two to three hours.

Microorganisms ferment the test sugar and produce gases. Hydrogen and methane pass into the bloodstream, travel to the lungs and can then be measured in exhaled breath.

Small-bowel aspirate and culture

Fluid can be collected from the small intestine during an upper endoscopy and cultured in a laboratory. This has traditionally been treated as a reference test, but it also has substantial limitations:

• it is invasive;
• the sample can be contaminated by the mouth or stomach;
• only one section of the small bowel is sampled;
• not every relevant microorganism grows in routine culture;
• laboratories use varying methods and thresholds.

Small-bowel sampling is therefore not routinely used for every person with bloating or suspected SIBO.

Other investigations

Depending on symptoms, a clinician may check:

• a full blood count;
• ferritin, vitamin B12 and folate;
• coeliac antibodies;
• thyroid, liver and kidney function;
• inflammatory markers;
• faecal calprotectin;
• pancreatic elastase;
• stool tests for infection;
• imaging or endoscopy.

If inflammatory bowel disease is a concern, our guide to faecal calprotectin results explains how this stool marker is used.

How does a SIBO breath test work?

The exact protocol varies by clinic, but a typical test involves:

1. following a restricted preparation diet the day before;
2. fasting overnight;
3. providing a baseline breath sample;
4. drinking a measured glucose or lactulose solution;
5. providing further samples at regular intervals;
6. recording symptoms that occur during the test;
7. sending samples to a laboratory or analysing them in the clinic.

The Royal Free London describes a lactulose test involving breath samples at 15-minute intervals over approximately two and a half hours.

Glucose breath test

Glucose is normally absorbed in the upper small intestine. An early hydrogen rise may indicate fermentation before absorption.

Advantages include a lower likelihood of the sugar reaching the colon and creating a false-positive result. Its limitation is that rapid glucose absorption may prevent it from detecting overgrowth further down the small bowel.

Lactulose breath test

Lactulose is not absorbed and eventually reaches the colon, where normal bacteria ferment it. It can potentially detect overgrowth over a longer section of small intestine.

However, rapid intestinal transit can cause lactulose to reach the colon early. The resulting gas rise may be misinterpreted as SIBO, particularly in people with diarrhoea or IBS.

Hydrogen and methane measurement

A good test should measure both gases where possible. Some people produce little hydrogen because other organisms consume it and generate methane.

Methane is particularly associated with constipation and slower intestinal transit. Because methane is produced by archaea rather than bacteria and may originate in either small or large bowel, the preferred term is intestinal methanogen overgrowth, or IMO, rather than “methane SIBO”.

The British Society of Gastroenterology notes that methods and interpretation vary between centres, which contributes to continuing controversy.

How should you prepare for a SIBO breath test?

Preparation matters because recent antibiotics, fermentable food, smoking, exercise and several medicines can alter breath-gas readings.

Instructions vary between providers. A clinic may ask you to:

• avoid antibiotics for approximately four weeks;
• stop probiotics for a specified period;
• avoid bowel-cleansing preparations for several weeks;
• temporarily stop non-essential laxatives or medicines affecting motility;
• follow a low-residue preparation diet for the day before testing;
• fast for approximately 8 to 12 hours;
• drink only plain water during the fasting period;
• avoid smoking, vaping, chewing gum and vigorous exercise;
• brush your teeth as instructed without swallowing products;
• remain seated and avoid sleeping during the test.

Do not apply generic online instructions without checking the provider’s protocol. Stopping essential medication can be unsafe, and laboratories do not all use the same preparation requirements.

University College London Hospitals advises patients undergoing its hydrogen and methane breath test to avoid antibiotics and probiotics for four weeks, with specific guidance for laxatives and motility medicines.

Tell the provider if you:

• have taken antibiotics recently;
• have had a colonoscopy or bowel preparation;
• take daily laxatives or antibiotics;
• have diabetes;
• are pregnant or breastfeeding;
• have experienced low blood sugar after meals;
• cannot tolerate the test solution;
• have recently had bowel surgery.

What does a positive SIBO breath test mean?

One widely used interpretation considers a rise in hydrogen of at least 20 parts per million above baseline within 90 minutes supportive of SIBO. Methane of at least 10 parts per million at any point is commonly used to support IMO.

These thresholds are not perfectly standardised. Different centres may use different:

• test sugars and doses;
• sample intervals;
• test durations;
• gas analysers;
• cut-off values;
• rules for high baseline readings.

The European breath-test guideline supports breath testing in appropriate clinical circumstances while acknowledging the absence of a universally accepted reference standard.

A positive result is more convincing when:

• the preparation protocol was followed correctly;
• the gas pattern meets an accepted threshold;
• typical symptoms occurred during the test;
• the patient has a recognised risk factor;
• alternative causes have been assessed.

A result may be less reliable when the baseline gas reading is already high, preparation was incomplete or intestinal transit is unusually fast.

A negative result also does not exclude every form of microbial overgrowth. Some people produce low measurable hydrogen and methane, while glucose can miss more distal overgrowth.

How is SIBO treated?

Treatment has three broad aims:

1. reduce problematic microbial overgrowth;
2. correct nutritional deficiencies and manage symptoms;
3. identify and treat the factor allowing SIBO to recur.

Antibiotics

A clinician may prescribe a short course of antibiotics when symptoms, risk factors and investigations support SIBO.

Rifaximin is commonly discussed because it remains largely within the gut rather than being extensively absorbed into the bloodstream. Other antibiotics may be selected according to clinical circumstances, previous treatment and local guidance.

In the UK, rifaximin use for SIBO may be off-label, and NHS availability varies by local formulary. It should be prescribed and monitored by an appropriate clinician.

The evidence is imperfect. A systematic review found that rifaximin appeared effective and generally well tolerated, but also concluded that the quality of the available studies was poor. The ACG guideline supports antibiotics for symptomatic SIBO with a conditional recommendation based on limited-quality evidence.

Potential antibiotic problems include:

• nausea, diarrhoea or abdominal discomfort;
• allergic reactions;
• Clostridioides difficile infection;
• antimicrobial resistance;
• interactions with other medicines;
• temporary improvement followed by recurrence.

Repeated antibiotics should not become the automatic response to every episode of bloating. Persistent or recurrent symptoms need reassessment.

Treating constipation and impaired motility

Slow transit can contribute to ongoing overgrowth. Treatment may involve an individual bowel-management plan, review of constipating medicines and, in selected cases, a medicine that promotes intestinal motility.

Read our guide to constipation, remedies and laxatives for general information.

Correcting the underlying cause

Management may also require treatment of coeliac disease, pancreatic insufficiency, diabetes, a bowel stricture or another associated condition. Structural problems occasionally require surgical or specialist intervention.

Nutritional treatment

Blood tests may identify vitamin or mineral deficiencies requiring replacement. Significant weight loss, restricted eating or malabsorption may justify referral to a gastroenterology dietitian.

Do diet, probiotics or herbal treatments cure SIBO?

Low-FODMAP and other restrictive diets

A low-FODMAP diet can reduce fermentable carbohydrates and may improve bloating, wind and pain. It is an established dietary approach for IBS symptoms, but symptom improvement does not prove that SIBO has been eradicated.

The diet is intended as a structured elimination and reintroduction process rather than a permanently restrictive eating plan. Long-term unnecessary restriction can reduce dietary variety and affect nutritional intake.

Other heavily promoted “SIBO diets” may exclude large numbers of foods without strong evidence. Diet can help manage symptoms, but it may not correct a motility disorder, anatomical problem or other underlying cause.

Probiotics

Evidence for probiotics in SIBO remains uncertain. Products contain different species, strains and doses, making broad claims difficult.

Some people report improvement, while others notice more bloating. Probiotics can also interfere with breath-test preparation. They should not be assumed to replace proper assessment or antibiotic treatment when this is clinically required.

Herbal antimicrobials

Products containing oregano oil, berberine, neem and other substances are promoted online as natural alternatives to antibiotics. Evidence for their effectiveness and safety in SIBO is limited.

“Natural” does not mean harmless. Herbal products can cause liver injury, gastrointestinal side effects, allergic reactions and interactions with prescribed medicines. Product strength and purity may also vary.

Elemental diets

An elemental diet provides nutrients in a pre-digested liquid form. It is sometimes promoted for difficult SIBO, but it is highly restrictive, expensive and challenging to follow.

It should not be attempted without specialist medical and dietetic supervision, particularly by people with diabetes, eating disorders, low weight or significant health conditions.

Why does SIBO come back?

Symptoms and positive breath tests can recur after apparently successful treatment. This is more likely when the original risk factor remains.

Possible reasons include:

• ongoing slow intestinal movement;
• untreated constipation;
• structural changes following surgery;
• a bowel stricture or diverticulum;
• an unmanaged digestive disease;
• continued use of a necessary medicine that slows the gut;
• an incorrect original diagnosis;
• temporary symptom improvement unrelated to bacterial eradication.

When symptoms return, the best next step is not always another identical antibiotic course. The clinician may reconsider:

• whether SIBO was adequately established;
• whether the symptom pattern has changed;
• whether nutritional deficiencies are developing;
• whether another condition has been overlooked;
• whether repeat testing would change management;
• how bowel motility can be improved.

Some people need specialist gastroenterology review. Our guide to private gastroenterologist costs in the UK explains typical consultation, testing and follow-up fees.

What else can cause similar symptoms?

SIBO should not become a catch-all explanation for every chronic digestive symptom.

Alternative or coexisting causes include:

• IBS;
• functional bloating;
• constipation with incomplete emptying;
• coeliac disease;
• lactose or fructose malabsorption;
• inflammatory bowel disease;
• pancreatic insufficiency;
• bile-acid diarrhoea;
• gastroparesis;
• thyroid disease;
• medication side effects;
• gastrointestinal infection;
• ovarian or other pelvic conditions;
• bowel cancer.

Coeliac testing should normally occur while gluten is still being eaten. Starting a gluten-free diet first can make blood tests and biopsies less reliable. See our guides to coeliac disease and coeliac blood-test results.

Depending on symptoms, a clinician may recommend stool tests, blood tests, imaging, colonoscopy or gastroscopy rather than—or before—a SIBO test.

When should you seek medical advice urgently?

Arrange a medical assessment rather than self-treating suspected SIBO if symptoms persist, recur frequently or significantly restrict your diet.

Seek urgent advice if you develop:

• blood in the stool or black, tar-like stools;
• unexplained weight loss;
• persistent vomiting;
• difficulty swallowing;
• severe or worsening abdominal pain;
• a swollen abdomen with inability to pass stool or wind;
• fever or signs of serious infection;
• yellowing of the skin or eyes;
• significant dehydration;
• new bowel symptoms with anaemia;
• symptoms that regularly wake you from sleep.

Call 999 for collapse, severe breathing difficulty, vomiting blood or sudden extreme abdominal pain with marked illness.

A persistent change in bowel habit should not automatically be attributed to SIBO. Our guides to changes in bowel habits and bowel-cancer symptoms and diagnosis explain when further investigation is important.

Frequently asked questions

What does SIBO stand for?

SIBO stands for small intestinal bacterial overgrowth. It describes excessive numbers or an altered pattern of bacteria in the small intestine associated with digestive symptoms.

What are the main signs of SIBO?

Common symptoms include bloating, wind, abdominal discomfort, diarrhoea, constipation and early fullness after meals. More severe cases may cause weight loss or nutritional deficiencies.

Can symptoms alone diagnose SIBO?

No. SIBO symptoms overlap with IBS, constipation, coeliac disease, food intolerance and several other conditions. Risk factors and appropriate investigations are needed.

What is a SIBO breath test?

It measures hydrogen and methane in repeated breath samples after drinking glucose or lactulose. An early rise in gas may suggest fermentation within the small intestine.

Is glucose or lactulose better for SIBO testing?

Neither is perfect. Glucose may produce fewer false positives but can miss overgrowth further down the small bowel. Lactulose reaches the whole colon but can give a false-positive pattern when intestinal transit is rapid.

How long does a SIBO breath test take?

Testing commonly takes two to three hours, depending on the clinic protocol. Samples are usually collected every 15 to 30 minutes.

Can I perform a SIBO test at home?

Some laboratories provide home kits. Their usefulness depends on correct preparation, accurate timing, proper sample collection and qualified interpretation. A commercial result should not replace a medical assessment.

What is hydrogen-dominant SIBO?

An early increase in breath hydrogen may support SIBO and is often associated with diarrhoea, bloating and wind, although symptoms cannot reliably identify the gas pattern.

What is methane SIBO?

The more accurate term is intestinal methanogen overgrowth, or IMO. Methane-producing organisms are archaea rather than bacteria and may be present in either small or large bowel. Methane is commonly associated with constipation.

Can SIBO cause weight gain?

SIBO is not an established general explanation for weight gain. Severe overgrowth is more traditionally associated with malabsorption and weight loss. Bloating can temporarily increase abdominal size or scale weight without representing body-fat gain.

Can SIBO cause vitamin B12 deficiency?

Yes, significant longstanding overgrowth can contribute to vitamin B12 deficiency and other nutritional abnormalities. However, B12 deficiency has many other possible causes.

Can SIBO be cured permanently?

Some people improve after one treatment course. Others relapse, particularly when impaired motility, altered anatomy or another underlying condition remains. Long-term management focuses on the cause as well as the overgrowth.

Does rifaximin cure SIBO?

Rifaximin can improve symptoms and breath-test findings in some people, but it does not work for everyone and recurrence is possible. Evidence quality is limited, and UK prescribing for SIBO may be off-label.

Should I follow a low-FODMAP diet for SIBO?

It may reduce symptoms, particularly when IBS overlaps, but it is not proven to eradicate SIBO. Restriction should be followed by systematic reintroduction, ideally with dietetic support.

Are probiotics good or bad for SIBO?

The evidence is uncertain. Some people report benefit and others experience worse bloating. The effect depends on the product and individual, and probiotics should be stopped before testing if the provider instructs you to do so.

Is SIBO testing available on the NHS?

Hydrogen and methane breath testing is available in some NHS gastroenterology and gastrointestinal-physiology services, services, but access varies by area. A GP or specialist can advise whether testing is available and appropriate locally.