What Is Small Intestine Bacterial Overgrowth (SIBO)?

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Small intestinal bacterial overgrowth (SIBO) is excessive gut bacteria in the small intestine. SIBO symptoms may include abdominal bloating, constipation, and nutritional deficiencies. A range of conditions can cause SIBO, such as celiac disease, Crohn's disease, and blind loop syndrome.

SIBO can affect adults and children at any age. However, it may be more prevalent and problematic for older adults.

Intestinal bacteria

Experts suggest that SIBO could be fairly common in the population, especially among people who have a digestive disease, with an incidence of about 44% in women who have irritable bowel syndrome (IBS) and 14% in men who have IBS.

Small Intestine Bacterial Overgrowth Symptoms

With SIBO, you can experience vague symptoms and it can be hard to differentiate these effects from those associated with other common gastrointestinal (GI) problems. In fact, SIBO can worsen another GI condition, while other GI conditions can worsen SIBO, resulting in a cycle of illness.

Common effects of SIBO include:

  • Abdominal bloating
  • Flatulence and gas
  • Abdominal pain
  • Abdominal distension
  • Diarrhea, sometimes with urgency or soiling accidents
  • Foul smelling stools
  • Constipation
  • Nausea
  • Weight loss
  • Fatigue

You may experience all or some of these symptoms, and they may come and go.


SIBO can interfere with your absorption of essential nutrients. The health effects of malnutrition cause vague, subtle symptoms, such as fatigue, weight loss, and depression. You can also develop serious health issues, such as osteoporosis (fragile bones) and anemia (low red blood cell function).

Nutritional deficits associated with SIBO include:

  • Carbohydrates: Excess bacteria in the small intestine can interfere with carbohydrate absorption. Sometimes, people who have SIBO avoid carbohydrates to prevent bloating and diarrhea. Overall, carbohydrate deficiency causes weight loss and low energy.
  • Protein: SIBO-induced changes in the small intestine prevent proper protein absorption, resulting in weight loss and decreased immune function.
  • Fat: With SIBO, the bile acids responsible for the breakdown and absorption of fat are deficient. Fat malabsorption produces visible signs, including oily, smelly, and floating stools. Health effects include weight loss and fatigue.

Fat-soluble vitamins, A, D, E, and K, may not be properly absorbed if you have SIBO:

  • Vitamin A deficiency can cause vision problems and immune deficiency.
  • Vitamin D deficiency causes osteoporosis and depression.
  • Vitamin E deficiency interferes with healing.
  • Vitamin K deficiency can cause easy bruising and bleeding.

Vitamin B12 deficiency can occur with SIBO because the excess bacteria in the small intestine utilize the vitamin themselves, decreasing its availability for your body. Vitamin B12 deficiency can result in peripheral neuropathy, resulting in pain of the fingers and toes. This vitamin deficiency also causes megaloblastic anemia (enlarged, dysfunctional red blood cells), resulting in fatigue and irritability.

SIBO can cause iron deficiency, which results in microcytic anemia (small, dysfunctional red blood cells) and a number of other health effects, such as fatigue.


The bacterial overgrowth in SIBO is believed to result from alterations in the acidity (change in pH) of the small intestine and decreased movement of the intestinal muscles.

  • The pH changes make it easier for bacteria from other areas to grow in the small intestine and allow different types of bacteria to thrive.
  • Decreased intestinal motility, such as occurs with blind loop syndrome, keeps bacteria in the small intestine for a longer period of time than usual. This disrupts the normal balance of digestive enzymes.

Bacteria naturally found in the small intestine aid in the breakdown and absorption of nutrients and help prevent infections. The normal pattern of bacterial functions becomes altered as a result of SIBO.

The bacterial overgrowth also results in microscopic damage to the villi lining the walls of the small intestine, which impairs the absorption of nutrients.

A variety of illnesses may predispose you to SIBO. These medical conditions are generally diagnosed years before SIBO and may alter the environment of the small intestine, setting the stage for the condition to develop.

Conditions and circumstances associated with SIBO include:

Keep in mind that you can develop SIBO even if you don't have one of the associated risk factors, and your healthcare provider may consider diagnostic testing if you have symptoms of SIBO without an obvious predisposing cause.


There are three main ways to test for SIBO—a breath test, analysis of samples collected from the small intestine, and use of an antibiotic trial. Each method has benefits and limitations, but the direct sampling method is considered to be the most reliable.

Breath Testing

Hydrogen breath testing is a fast, non-invasive, and safe diagnostic testing method. You drink a solution and then have your breath analyzed a few hours later. The presence of hydrogen or methane is indicative of malabsorption.

However, this test is not considered highly reliable because there are other causes of malabsorption besides SIBO, and your diet and medication use in the weeks prior to the test can alter the results.

Jejunal Aspiration

This is an invasive test in which endoscopy is used to sample fluid from the small intestine. An endoscopy is a test in which a thin, camera-equipped tube is placed down the throat to visualize the upper GI structures; a biopsy or fluid sample can be taken while the endoscope is inserted.

The test can provide helpful information regarding the bacterial content of the small intestine, but there can be other causes of bacterial overgrowth, including an infection.

Antibiotic Trial

Another diagnostic method involves beginning antibiotic treatment of SIBO and assessing the response. This is usually a safe process, but you should let your healthcare provider know immediately if your symptoms worsen while taking antibiotics.


If you have SIBO, you will need treatment for the condition. Treatment options include antibiotics to treat the bacterial overgrowth, management of your underlying medical condition (such as pancreatitis or scleroderma), and nutritional supplementation.

You might not need to use all of these methods, and your own treatment will be tailored to your symptoms and the effects that you are experiencing from the condition.

Keep in mind that SIBO can fluctuate over time, so you may be able to take a break from treatment for months, or even years, at a time. Be sure to discuss recurring symptoms with your healthcare provider so that flare-ups can be treated promptly.


A number of antibiotics can be used to reduce bacteria in the small intestine. Xifaxan (rifaximin) is one of the most commonly prescribed antibiotics for SIBO. Your healthcare provider may select a different antibiotic based on your jejunal sample or your symptoms.

Underlying Condition Management

If you have a medical problem that predisposes you to SIBO, management of that problem can reduce bacterial overgrowth in the small intestine.

Your treatment plan depends on a number of factors. For example, some medical conditions, such as Crohn's disease, may act up periodically, while others, such as those that result from intestinal surgery, are permanent.

Medication Discontinuation

While it is not clear whether medications used for the treatment of heartburn promote SIBO, these medications can alter the intestinal pH. Some healthcare providers recommend stopping these treatments if you are diagnosed with SIBO.

Nutritional Supplementation

When you have a GI condition that causes nutritional deficiencies, your healthcare provider may test your vitamin levels and prescribe supplementation as needed. Because SIBO-associated nutritional deficits stem from malabsorption, you may need injections or intravenous (IV) supplementation rather than oral pills.

Dietary Modifications

Dietary modifications may be helpful as adjunctive therapy for the treatment of SIBO—for example, the use of FODMAP (fermentable oligo-, di-, mono-saccharides and polyols) diet. However, the diet should only be undertaken with the consultation of your GI doctor and nutritionist.

Some people with the condition have an increase in symptoms after consuming certain items, such as lactose or fructose-containing foods.

If you have a specific dietary intolerance, then avoiding the food that exacerbates your symptoms can help prevent SIBO from acting up.

You can identify dietary intolerance by using a food diary and keeping track of your symptoms.

Herbs have not been found effective for managing SIBO, and you should tell your healthcare provider about any supplements that you use, because they may actually promote pH changes or bacterial overgrowth.

A Word From Verywell

SIBO is a condition that is becoming more commonly recognized as a cause of GI disturbance and malnutrition. Getting a diagnosis and formulating a treatment plan for SIBO may take some time, but you should begin to feel more comfortable and energetic once your condition is properly managed.

13 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Almeida JA, Kim R, Stoita A, Mciver CJ, Kurtovic J, Riordan SM. Lactose malabsorption in the elderly: role of small intestinal bacterial overgrowth. Scand J Gastroenterol. 2008;43(2):146-54.doi:10.1080/00365520701676617

  2. Sachdeva S, Rawat AK, Reddy RS, Puri AS. Small intestinal bacterial overgrowth (SIBO) in irritable bowel syndrome: frequency and predictors. J Gastroenterol Hepatol. 2011;26 Suppl 3:135-8.doi:10.1111/j.1440-1746.2011.06654.x

  3. Bures J, Cyrany J, Kohoutova D, et al. Small intestinal bacterial overgrowth syndrome. World J Gastroenterol. 2010;16(24):2978-90. doi:10.3748/wjg.v16.i24.2978

  4. Grace E, Shaw C, Whelan K, Andreyev HJ. Review article: small intestinal bacterial overgrowth--prevalence, clinical features, current and developing diagnostic tests, and treatment. Aliment Pharmacol Ther. 2013;38(7):674-88. doi:10.1111/apt.12456

  5. Gasche C, Lomer MC, Cavill I, Weiss G. Iron, anaemia, and inflammatory bowel diseasesGut. 2004;53(8):1190‐1197. doi:10.1136/gut.2003.035758

  6. Dukowicz AC, Lacy BE, Levine GM. Small intestinal bacterial overgrowth: a comprehensive reviewGastroenterol Hepatol (N Y). 2007;3(2):112‐122.

  7. Rezaie A, Buresi M, Lembo A, et al. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American ConsensusAm J Gastroenterol. 2017;112(5):775‐784. doi:10.1038/ajg.2017.46

  8. Simrén M, Stotzer PO. Use and abuse of hydrogen breath testsGut. 2006;55(3):297‐303. doi:10.1136/gut.2005.075127

  9. Corazza GR, Menozzi MG, Strocchi A, et al. The diagnosis of small bowel bacterial overgrowth. Reliability of jejunal culture and inadequacy of breath hydrogen testingGastroenterology. 1990;98(2):302‐309. doi:10.1016/0016-5085(90)90818-l

  10. Shah SC, Day LW, Somsouk M, Sewell JL. Meta-analysis: antibiotic therapy for small intestinal bacterial overgrowthAliment Pharmacol Ther. 2013;38(8):925‐934. doi:10.1111/apt.12479

  11. Gatta L, Scarpignato C. Systematic review with meta-analysis: rifaximin is effective and safe for the treatment of small intestine bacterial overgrowthAliment Pharmacol Ther. 2017;45(5):604‐616. doi:10.1111/apt.13928

  12. Gaffar SMA, Sarker SA, Mahfuz M, Donowitz JR, Ahmed T. Impact of Small Intestine Bacterial Overgrowth on Response to a Nutritional Intervention in Bangladeshi Children from an Urban CommunityAm J Trop Med Hyg. 2019;100(1):222‐225. doi:10.4269/ajtmh.18-0759

  13. Ghoshal UC, Shukla R, Ghoshal U. Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome: A Bridge between Functional Organic Dichotomy. Gut Liver. 2017;11(2):196-208. doi:10.5009/gnl16126

Additional Reading

By Barbara Bolen, PhD
Barbara Bolen, PhD, is a licensed clinical psychologist and health coach. She has written multiple books focused on living with irritable bowel syndrome.