Irritable bowel syndrome (IBS) and small intestinal bacterial overgrowth (SIBO) are two of the most common causes of ongoing digestive symptoms. They share many of the same complaints, including bloating, abdominal pain, diarrhea, and constipation. In many cases, people are diagnosed with IBS while SIBO is the actual cause of their symptoms. 

But not everyone who has SIBO has IBS; and not everyone with IBS has SIBO.

Understanding how these diagnoses differ and when they overlap can help explain why a treatment that works well for one person may not work for another. It also highlights the importance of identifying the root cause of digestive symptoms. 

Defining SIBO and IBS

SIBO is a measurable medical condition in which excessive bacteria grow in the small intestine. Unlike IBS, which is diagnosed by symptoms alone, SIBO is identified through clinical testing, most commonly breath tests. 

IBS, in comparison, is a symptom-based diagnosis. It is defined by recurring abdominal pain linked to changes in bowel habits, such as diarrhea, constipation, or a mix of both. IBS is not a single disease but a broad clinical category that can reflect different underlying drivers.

When IBS and SIBO Overlap and When They Don’t

Although IBS and SIBO symptoms can look very similar, they do not always occur together. Research suggests that their relationship falls into three broad patterns, depending on what is driving a person’s symptoms.

1. IBS Without SIBO

Many people meet criteria for IBS without having bacterial overgrowth in the small intestine. In these cases, symptoms may be driven by factors such as heightened sensitivity in the gut, altered communication between the gut and brain, or changes in the bacteria of the large intestine rather than the small intestine.

2. SIBO Without IBS

SIBO can also occur without IBS. Because SIBO often develops as a secondary condition, it is commonly found in people with other medical issues that affect digestion such as neurological, autoimmune, metabolic, or structural conditions. Symptoms may include bloating, gas, or nutrient malabsorption but may not include the recurring abdominal pain pattern required for an IBS diagnosis.

3. IBS With Underlying SIBO

The strongest overlap between IBS and SIBO appears particularly in patients whose symptoms begin after a clear trigger such as food poisoning or another gastrointestinal infection. In these cases, an initial event disrupts normal gut function. Over time, this disruption can impair motility and allow bacteria to accumulate in the small intestine. The resulting bacterial overgrowth can then drive the symptoms that are diagnosed clinically as IBS.

SIBO and IBS Connection

In many cases where IBS in linked to underlying SIBO, symptoms follow a recognizable pattern:

1. A primary event disrupts normal gut function

2. A secondary condition develops: bacterial overgrowth in the small intestine (SIBO)

3. The clinical outcome is a set of symptoms that may be labeled as IBS

Understanding this sequence helps explain why symptoms can appear weeks or months after a trigger like food poisoning and why addressing both bacterial overgrowth and gut motility can matter for long-term improvement. 

First: A Primary Event Disrupts Normal Gut Function

IBS symptoms often follow a biological disruption, rather than appearing out of nowhere. One of the best-studied examples is food poisoning, but other triggers that activate the immune system can also play a role as well as neurological, autoimmune, or metabolic conditions that affect digestion.

These events don’t always cause immediate symptoms but can quietly alter gut function over time.

Next: A Secondary Condition Develops: SIBO

When normal gut defenses, especially motility, are disrupted, bacteria are no longer cleared efficiently from the small intestine. Over time, this can lead to SIBO.

In this context, SIBO is considered a secondary condition because it develops after the primary event. It reflects a breakdown in the systems that normally keep bacteria where they belong, rather than a random or isolated problem.

Finally: IBS Symptoms Emerge as the Clinical Outcome

Bacterial overgrowth in the small intestine can ferment food, produce gas, and interact with the gut lining and nervous system. These effects can drive symptoms such as bloating, abdominal pain, diarrhea, and/or constipation. 

For this subset of patients, IBS describes the symptoms, while SIBO helps explain the mechanism behind those symptoms.

Different types of bacterial fermentation may lead to different types of IBS-like symptoms:

• Hydrogen-dominant overgrowth is more often associated with diarrhea (IBS-D)

• Methane overgrowth, now called intestinal methanogen overgrowth (IMO), is strongly linked to constipation (IBS-C)

• Hydrogen sulfide is associated with diarrhea, urgency, and abdominal pain

This helps explain why IBS symptoms differ widely between individuals and why the same diagnosis can include very different experiences.

Why Treating SIBO Can Improve IBS Symptoms

One of the strongest pieces of evidence linking SIBO and IBS is treatment response.

Therapies commonly used to treat SIBO, such as targeted antibiotics, dietary strategies that reduce fermentable carbohydrates, and approaches that support healthy gut motility, often lead to meaningful symptom improvement in people diagnosed with IBS.

This does not mean that all IBS is caused by SIBO.

But it does mean that for a substantial subset of people, SIBO could be the missing link between an initial gut injury and ongoing IBS symptoms.

Why This Distinction Matters

Understanding the possible connection between  IBS and SIBO  helps clarify why, for some people, symptoms can be persistent and difficult to treat.

• IBS describes what someone feels

• SIBO helps explain why those symptoms may be happening

When the underlying condition driving symptoms is identified and addressed, treatment becomes more targeted and, in many cases, more effective.

What This Means for Diagnosis and Treatment

Understanding when IBS and SIBO overlap, and when they don’t, helps explain why diagnosing and treating chronic digestive symptoms can be so inconsistent.

For individuals whose symptoms follow a post-infectious or motility-related pattern, testing for SIBO may provide important insight. 

The Takeaway

IBS and SIBO are not opposing diagnoses. They are often connected through a clear progression:

• A primary event disrupts gut function

• A secondary condition, SIBO, develops

• IBS symptoms emerge as the clinical outcome

Understanding this sequence helps explain why IBS symptoms can appear months or years after an initial trigger, why they may change over time, and why addressing both bacterial overgrowth and gut motility can be so important for long-term improvement.