Understanding Small Intestinal Bacterial Overgrowth (SIBO)

What Is SIBO?

Small intestinal bacterial overgrowth (SIBO) is a condition in which there are too many bacteria — or the wrong types of bacteria — in the small intestine. Normally, the small intestine has relatively few bacteria compared to the large intestine (colon). When bacteria build up in the small intestine, they can ferment food before your body has a chance to absorb it properly, leading to uncomfortable symptoms and, in some cases, nutritional deficiencies.

Common Symptoms

SIBO can cause a wide range of digestive symptoms, including:

• Bloating and abdominal distension (feeling “swollen”)
• Excessive gas and flatulence
• Abdominal pain or cramping
• Diarrhea (most common) or, less often, constipation
• Nausea
• Feeling full quickly after eating

In more severe or long-standing cases, SIBO can interfere with nutrient absorption and cause:

• Unintentional weight loss
• Fatigue
• Anemia (low iron or vitamin B12)
• Deficiencies in fat-soluble vitamins (A, D, E, K)
• Oily or foul-smelling stools (steatorrhea)

Many of these symptoms overlap with other conditions such as irritable bowel syndrome (IBS), so testing is often needed to confirm the diagnosis.

Why SIBO Happens

Your body has several natural defenses that keep bacteria from building up in the small intestine:

• Stomach acid kills many bacteria before they reach the small intestine.
• The migrating motor complex (MMC) — a wave-like “housekeeping” contraction that sweeps through the stomach and small intestine between meals — clears out leftover food and bacteria.
• Bile and pancreatic juices have natural antimicrobial effects.
• The ileocecal valve (a one-way valve between the small and large intestine) prevents bacteria from backing up from the colon.
• The immune system produces antibodies that help control bacterial growth.

When one or more of these defenses is weakened, bacteria can accumulate. Common risk factors include:

• Prior abdominal or intestinal surgery (e.g., gastric bypass, bowel resection)
• Conditions that slow gut motility (e.g., diabetes, scleroderma, hypothyroidism)
• Structural problems such as strictures, diverticula, or blind loops in the intestine
• Long-term use of acid-suppressing medications (proton pump inhibitors)
• Chronic use of opioid pain medications or other drugs that slow the gut
• Immune deficiency conditions
• Chronic conditions such as Crohn’s disease, celiac disease, chronic pancreatitis, or liver cirrhosis
• Older age

How SIBO Is Diagnosed

There are two main ways to test for SIBO:

1. Breath test (most common method): This is a simple, noninvasive test. You drink a sugar solution (glucose or lactulose) after an overnight fast. Breath samples are then collected every 15 minutes for 2 to 3 hours. If bacteria in the small intestine ferment the sugar, they produce hydrogen or methane gas, which is absorbed into the blood and exhaled through the lungs. A rise in hydrogen of 20 parts per million (ppm) or more above baseline within 90 minutes is considered a positive result. Methane levels of 10 ppm or more at any point may indicate a related condition called intestinal methanogen overgrowth (IMO), which is often associated with constipation.
2. Small bowel aspirate (less common): During an upper endoscopy, a small sample of fluid is collected from the small intestine and sent to a laboratory to count bacteria. This is considered the reference standard but is used less often because it is invasive and more expensive.

Your doctor will decide which test is most appropriate based on your symptoms and medical history. It is important to follow the preparation instructions carefully (such as dietary restrictions and stopping certain medications) to ensure accurate results.

Treatment

Treatment of SIBO focuses on three goals: treating the bacterial overgrowth, correcting any nutritional deficiencies, and addressing the underlying cause when possible.

Antibiotics

Antibiotics are the main treatment for SIBO. A typical course lasts 7 to 14 days. The most commonly used antibiotic is rifaximin, which works primarily in the gut and has few side effects. Other antibiotics your doctor may prescribe include metronidazole, ciprofloxacin, amoxicillin-clavulanate, doxycycline, or others. For intestinal methanogen overgrowth (high methane), a combination of rifaximin and neomycin may be recommended.

A single course of antibiotics improves symptoms in many patients. Your doctor will determine the best antibiotic and duration for your situation.

Correcting nutritional deficiencies

If SIBO has caused vitamin or mineral deficiencies, your doctor may recommend supplements such as vitamin B12, iron, vitamin D, or fat-soluble vitamins.

Treating the underlying cause

Whenever possible, your doctor will try to address the condition that led to SIBO in the first place — for example, adjusting medications that slow the gut, managing diabetes, or correcting a structural problem surgically.

Diet and SIBO

Dietary changes can help manage symptoms, though they are generally used alongside — not instead of — antibiotic treatment.

Low-FODMAP diet

FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols. These are types of carbohydrates that are easily fermented by bacteria. Reducing high-FODMAP foods (such as onions, garlic, wheat, certain fruits, beans, and artificial sweeteners) may help reduce gas, bloating, and discomfort. This diet should ideally be guided by a registered dietitian, as it is meant to be temporary — foods are gradually reintroduced to identify personal triggers.

Elemental diet

In some cases, your doctor may recommend a specialized liquid diet (elemental diet) that is fully absorbed in the upper small intestine, essentially “starving” the overgrown bacteria. This is typically used for a short period (2 to 3 weeks) under medical supervision.

Important: Do not start a highly restrictive diet on your own. Work with your healthcare team to ensure you are getting adequate nutrition.

Recurrence

SIBO commonly comes back, especially if the underlying cause cannot be fully corrected. Studies show recurrence rates of approximately 13% at 3 months, 28% at 6 months, and up to 44% at 9 months after a course of antibiotics.

Factors that increase the risk of recurrence include:

• Older age
• Ongoing use of proton pump inhibitors (acid-suppressing medications)
• Prior appendectomy
• Conditions that permanently affect gut motility (e.g., scleroderma, diabetes)

If SIBO recurs, your doctor may prescribe another course of antibiotics — sometimes rotating between different antibiotics. In some cases, a prokinetic medication (a drug that helps stimulate the gut’s natural sweeping motions) may be recommended to help prevent recurrence.

When to Call Your Doctor

Contact your healthcare provider if you experience:

• Persistent or worsening bloating, diarrhea, or abdominal pain despite treatment
• Unintentional weight loss
• Signs of nutritional deficiency such as fatigue, weakness, numbness or tingling in the hands or feet, or easy bruising
• New or worsening symptoms after completing antibiotic treatment
• Symptoms that return after initial improvement

This site draws on the ACG Clinical Guideline on SIBO, which outlines diagnostic criteria (breath test positivity defined as ≥20 ppm hydrogen rise within 90 minutes, or methane ≥10 ppm), antibiotic treatment options, and recurrence data.[1] The AGA Clinical Practice Update provides additional context on risk factors, the role of the migrating motor complex, and antibiotic regimens including rifaximin’s ~70% eradication rate.[2] Recurrence rates (up to 44% at 9 months) and associated risk factors (older age, PPI use, appendectomy) are drawn from both guidelines.[1][2] Dietary recommendations, including the low-FODMAP approach, are based on ACG guidance noting that evidence is limited but supports reduced fermentation products, and the AGA notes that implementation should be done with dietitian guidance given potential negative impacts on the microbiome.[1][6] The breath test sensitivity and specificity data are from a recent JAMA review.[4] The pathophysiology section reflects the body’s natural defenses against SIBO as described in the ACG guideline and a recent JAMA review.[1][4]

References

1. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. Pimentel M, Saad RJ, Long MD, Rao SSC. The American Journal of Gastroenterology. 2020;115(2):165-178. <doi:10.14309/ajg.0000000000000501>
2. AGA Clinical Practice Update on Small Intestinal Bacterial Overgrowth: Expert Review. Quigley EMM, Murray JA, Pimentel M. Gastroenterology. 2020;159(4):1526-1532. <doi:10.1053/j.gastro.2020.06.090>
3. Epidemiology of Small Intestinal Bacterial Overgrowth. Efremova I, Maslennikov R, Poluektova E, et al. World Journal of Gastroenterology. 2023;29(22):3400-3421. <doi:10.3748/wjg.v29.i22.3400>
4. Chronic, Noninfectious Diarrhea. Singh P, Lee A, Sheth NM, Chey WD. JAMA. 2026;:2845755. <doi:10.1001/jama.2026.0872>
5. Small Intestinal Bacterial Overgrowth: Current Update. Zafar H, Jimenez B, Schneider A. Current Opinion in Gastroenterology. 2023;39(6):522-528. <doi:10.1097/MOG.0000000000000971>
6. AGA Clinical Practice Update on Evaluation and Management of Belching, Abdominal Bloating, and Distention: Expert Review. Moshiree B, Drossman D, Shaukat A. Gastroenterology. 2023;165(3):791-800.e3. <doi:10.1053/j.gastro.2023.04.039>