SIBO: Symptoms, Diagnosis and Treatment Explained

01Definition

What small intestinal bacterial overgrowth means

Mayo Clinic explains that, unlike the large intestine, the small intestine normally has relatively few bacteria, because its contents move quickly and bile is present. In SIBO there is an abnormal increase in bacteria in the small intestine, particularly of types not usually found there.

MedlinePlus explains that the excess bacteria may use up nutrients the body needs and that their breakdown of nutrients can damage the lining of the small intestine, making absorption even harder. Mayo Clinic notes that SIBO is sometimes called blind loop syndrome, a name that reflects one of its causes after some operations, although not every case is related to surgery.

02Symptoms

Symptoms and why they are nonspecific

Symptoms described by Mayo Clinic and MedlinePlus include:

• bloating, gas and abdominal fullness, including an uncomfortable feeling of fullness after eating
• abdominal pain and cramps
• diarrhea, often watery, and sometimes fatty stools
• nausea and loss of appetite
• unintentional weight loss and, in more severe cases, malnutrition

Mayo Clinic points out that bloating, nausea and diarrhea are symptoms of many intestinal problems. Having bloating does not mean you have SIBO, and the appearance of stools cannot confirm it. Our guide on why you may feel bloated covers other common causes.

03Contributing conditions

Motility, anatomy and other contributing conditions

SIBO commonly develops when surgery or disease slows the passage of food and waste through the gut, allowing bacteria to multiply. The two sources list:

• complications of abdominal surgery, such as gastric bypass for obesity or some types of stomach removal, especially operations that create a loop of intestine where contents stagnate
• structural problems such as scar tissue (adhesions), pouches in the wall of the small intestine (diverticulosis), blockages, an abnormal connection between parts of the bowel (fistula) or an injury
• conditions that slow intestinal movement, such as diabetes and scleroderma
• Crohn’s disease, celiac disease, radiation damage to the intestine and intestinal lymphoma
• short bowel syndrome after surgical removal of part of the small intestine
• immune deficiencies
• some cases of irritable bowel syndrome (IBS)

Knowing these conditions helps explain why the doctor asks about previous operations and other illnesses: treating SIBO often means addressing what made it possible.

04Warning signs

When symptoms need prompt assessment

Mayo Clinic advises a full evaluation, especially if you have had abdominal surgery, for persistent diarrhea, rapid unintentional weight loss or abdominal pain lasting more than a few days, and immediate care for severe abdominal pain.

Signs of nutritional deficiency also deserve attention. Mayo Clinic explains that SIBO can lead to vitamin B12 deficiency, which may cause weakness, fatigue, tingling and numbness in the hands and feet and, in advanced cases, confusion; nervous system damage from B12 deficiency may be irreversible.

05Evaluation

Clinical evaluation and alternative explanations

MedlinePlus explains that the doctor will perform a physical examination and ask about your medical history. Because the symptoms overlap with many other intestinal problems, the evaluation looks at the whole picture rather than a single symptom.

It helps to bring information about previous abdominal surgery, diabetes or other conditions, medicines you take, how long symptoms have lasted, weight changes and how your stools have changed. Our guide on IBD vs IBS describes two conditions that are often considered.

06Tests

Selected testing and interpretation limits

MedlinePlus lists tests that may be used, depending on the case:

• hydrogen and methane breath tests
• blood tests, including a complete blood count, albumin and vitamin levels
• a fecal fat test
• small intestine endoscopy, sometimes with a biopsy or a culture of intestinal fluid

No single test answers every question. A breath test result needs to be interpreted by a clinician together with symptoms, history and other findings, and home tests should not be self-interpreted to start treatment. Blood and vitamin tests also show whether SIBO is affecting nutrition.

07Treatment

Treating overgrowth and the underlying cause

MedlinePlus states that the goal is to treat the cause of the bacterial overgrowth. Mayo Clinic notes that antibiotics are the most common treatment and that surgery is sometimes needed to correct a structural problem. MedlinePlus also lists medicines that speed intestinal movement and, in people who are severely ill or malnourished, intravenous fluids or nutrition given through a vein.

Antibiotics and other medicines are chosen and prescribed by a doctor; the type, length and any repeat courses depend on the person. Herbal or “natural” antimicrobial products are not described by these sources as treatments, and stopping prescribed medicines on your own is not advised.

08Nutrition

Nutritional effects, support and returning symptoms

Malnutrition is a feature of severe cases. Mayo Clinic explains that excess bacteria break down the bile salts needed to digest fat, causing poor absorption of fats and of the fat-soluble vitamins A, D, E and K, and can damage the intestinal lining, reducing absorption of carbohydrates and proteins. Over time, poor calcium absorption may lead to weakened bones and kidney stones. MedlinePlus adds dehydration, bleeding problems from vitamin deficiency and liver disease among possible complications.

Nutritional support aims to correct deficiencies and maintain weight. MedlinePlus notes that a lactose-free diet can be helpful; that does not mean every dairy food must be avoided by everyone. Very restrictive diets do not cure SIBO and can worsen nutrition. For adults with a medical diagnosis, our nutrition practice in Bilbao offers dietary support coordinated with your doctor; we do not diagnose or treat SIBO itself (consultations are in Spanish).

If symptoms return after treatment, it is worth reviewing with the doctor whether the underlying cause is still present and whether another diagnosis should be considered. A simple record helps: the agreed treatment goal, the dates of treatment, symptoms, stools and weight before and after, any diet changes and the planned review.

09Sources

Where the sources differ

Mayo Clinic emphasizes abdominal surgery, structural problems and slow intestinal movement, and names antibiotics as the most common treatment. MedlinePlus, reviewed in April 2026, lists a wider range of causes, including immune deficiencies and some cases of irritable bowel syndrome, and focuses on treating the cause, with antibiotics as one option alongside motility medicines and nutritional support.

MedlinePlus mentions breath tests among several possible investigations, while the Mayo Clinic symptoms page does not discuss testing. Neither source supports diagnosing SIBO from symptoms alone or treating it with self-chosen diets or supplements. The Mayo Clinic page is dated November 2024.

10Summary

In summary

SIBO is an excess of bacteria in the small intestine, usually linked to surgery, structural problems or conditions that slow the movement of food through the gut. Its symptoms, such as bloating, fullness, pain, gas and diarrhea, are shared with many other conditions, so it needs a medical evaluation with selected tests.

Treatment aims at both the overgrowth and its cause, most often with prescribed antibiotics, sometimes with motility medicines or surgery, and with nutritional support to correct deficiencies and maintain weight. Seek prompt care for severe pain, persistent diarrhea or rapid weight loss, and review with your doctor if symptoms come back.