Small Intestinal Bacterial Overgrowth—better known as SIBO—is a digestive condition that affects millions of people. Many are misdiagnosed or never diagnosed at all. If you experience chronic bloating, gas, abdominal discomfort, or bowel changes, SIBO may be an underlying cause. This guide explains what SIBO is and what it isn’t, the three types of microbial overgrowth, and how they are diagnosed and treated. 

What Is SIBO?

SIBO occurs when too many microbes grow in the small intestine, where they don’t belong.

Unlike the colon, the small intestine is meant to have very few bacteria. When excess microbes grow there, they ferment food too early and produce gases such as hydrogen, methane, or hydrogen sulfide. These gases can trigger:

• Bloating

• Gas

• Discomfort or cramping

• Diarrhea or constipation

• Nausea

• Fatigue or brain fog

What SIBO IS vs. What SIBO Is NOT

SIBO is not about “bad bacteria.” It’s about too many microbes in the wrong location.

Despite the high prevalence of SIBO in the population, 75% of people in the U.S. have never heard of it. 

What Could Cause SIBO?

Anything that slows or disrupts gut motility can create the right environment for microbes to build up in the small intestine.

• Post-infectious nerve damage

• Migrating motor complex dysfunction

• Adhesions or scar tissue

• Crohn’s or celiac disease

• Diabetes

• Hypothyroidism

• Surgeries

• Medications that slow digestion

The Three Types of SIBO

SIBO is classified by the main gas produced by the offending microbes. Both the gas and the microbes can cause symptoms. Hydrogen, methane, and hydrogen sulfide gases each come from different organisms, cause different symptoms, and require different treatments. Knowing your subtype helps you get the right treatment.

1. Hydrogen-Dominant SIBO (H₂-SIBO)

What it is: Hydrogen-producing bacteria ferment carbohydrates too early, creating excess hydrogen.

Common symptoms:

• Bloating

• Gas

• Loose stools or diarrhea

Diagnosis: Hydrogen rise ≥20 ppm within 90 minutes on a breath test.

Treatment: Often responds to rifaximin. Motility support is crucial for preventing recurrence.

2. Intestinal Methanogen Overgrowth (IMO)

What it is: Archaea—not bacteria—use hydrogen to produce methane, which slows gut movement.

Common symptoms:

• Constipation

• Hard stools

• Bloating

Diagnosis: Any methane ≥10 ppm at any time confirms IMO.

Treatment: Requires combination antibiotics (e.g., rifaximin + neomycin) and motility support.

3. Hydrogen Sulfide–Dominant SIBO (H₂S-SIBO or ISO)

What it is: Sulfate-reducing bacteria produce hydrogen sulfide, a gas with a strong sulfur smell that can irritate the gut lining. Research into this type of SIBO is still emerging as professionals debate whether it is its own category.

Common symptoms:

• Rotten-egg smelling gas

• Urgent or “burning” diarrhea

• Nausea

Diagnosis: Newer three-gas breath tests can detect hydrogen sulfide directly.

Treatment: May involve antimicrobials targeting sulfur-reducing bacteria, bismuth therapy, diet modification, and motility support.

How Is SIBO Diagnosed?

There are various mechanisms for diagnosing SIBO:

• Breath Testing: Measures hydrogen, methane, and sometimes hydrogen sulfide.

• Small Bowel Aspirate: The gold standard for bacterial overgrowth and the only way to diagnose SIFO.

• Differential Diagnosis: Your provider may review your clinical presentation (symptoms) and past medical history to determine if SIBO could be the cause of your issues.

Visit our SIBO Diagnosis page to learn more.