Two Conditions, Overlapping Symptoms, Different Solutions

Irritable bowel syndrome (IBS) and small intestinal bacterial overgrowth (SIBO) are among the most commonly confused gastrointestinal conditions. They share a remarkably similar symptom profile — bloating, abdominal pain, altered bowel habits, and gas — which is why millions of people receive an IBS diagnosis when SIBO is actually the underlying cause.

This distinction is not merely academic. IBS and SIBO require fundamentally different treatment approaches. Managing SIBO as IBS means you are treating symptoms while the underlying bacterial overgrowth continues unchecked. Understanding the difference can be the key to finally finding relief.

What Is IBS?

IBS is a functional gastrointestinal disorder — meaning it is defined by symptoms rather than by a specific structural or biochemical abnormality. The Rome IV diagnostic criteria define IBS as recurrent abdominal pain occurring at least one day per week in the last three months, associated with two or more of the following: related to defecation, associated with a change in stool frequency, or associated with a change in stool form.

IBS is classified into four subtypes:

  • IBS-D: diarrhoea predominant
  • IBS-C: constipation predominant
  • IBS-M: mixed (alternating diarrhoea and constipation)
  • IBS-U: unsubtyped

IBS affects approximately 10 to 15% of the global population and is the most common diagnosis made by gastroenterologists. However, IBS is essentially a label for a symptom pattern — it does not explain why the symptoms are occurring.

What Is SIBO?

SIBO, by contrast, is a specific, measurable condition: an excessive number of bacteria in the small intestine. It has identifiable causes (impaired motility, low stomach acid, structural abnormalities), can be diagnosed with objective tests (breath test or small bowel aspirate), and has targeted treatments (antibiotics or antimicrobial herbs).

SIBO produces symptoms that are virtually identical to IBS — bloating, pain, diarrhoea or constipation, gas — because bacterial fermentation in the small intestine creates the same end result as the visceral hypersensitivity and motility disorders attributed to IBS.

The Overlap: How Many IBS Patients Actually Have SIBO?

This is where the picture gets compelling. Multiple studies have tested IBS patients for SIBO using breath tests, and the results are striking:

  • A meta-analysis in World Journal of Gastroenterology found SIBO in 38% of IBS patients on average, with some studies reporting prevalence as high as 78%
  • Dr. Mark Pimentel's research at Cedars-Sinai demonstrated that treating SIBO with the antibiotic rifaximin resolved IBS symptoms in a significant proportion of patients
  • A 2020 study found that IBS patients who tested positive for SIBO and received targeted treatment had significantly better outcomes than those who received standard IBS management alone
The implication is profound: for a substantial percentage of people carrying an IBS diagnosis, SIBO is the actual underlying cause, and treating it directly produces results that dietary management alone cannot achieve.

Key Differences in Symptoms

While the overlap is significant, there are subtle differences that can help distinguish SIBO from functional IBS:

  • Timing of bloating: SIBO bloating typically occurs within 30-90 minutes of eating (because bacteria are in the small intestine). IBS bloating may be more variable in timing
  • Response to fibre: In SIBO, increasing fibre often worsens symptoms because it feeds the overgrown bacteria. In IBS without SIBO, soluble fibre often helps
  • Response to probiotics: Some SIBO patients worsen with probiotics (adding bacteria to an already overgrown small intestine). IBS patients without SIBO generally tolerate or benefit from probiotics
  • Nutritional deficiencies: SIBO commonly causes iron, B12, and fat-soluble vitamin deficiencies due to bacterial competition for nutrients. Functional IBS does not typically cause measurable deficiencies
  • History: A history of food poisoning, PPI use, or abdominal surgery preceding symptom onset strongly suggests SIBO

Different Treatments

IBS Treatment

Standard IBS management focuses on symptom control: low-FODMAP diet, antispasmodics, stress management, peppermint oil, and sometimes low-dose antidepressants for visceral pain. These approaches do not address bacterial overgrowth.

SIBO Treatment

SIBO treatment targets the overgrowth directly: rifaximin (for hydrogen-dominant) or rifaximin plus neomycin or metronidazole (for methane-dominant), or herbal antimicrobials (oregano oil, berberine, allicin) for those preferring non-pharmaceutical approaches. After eradication, preventing recurrence requires addressing the underlying cause — often through prokinetic agents that restore migrating motor complex function.

Getting the Right Diagnosis

If you have been diagnosed with IBS but have not been tested for SIBO, requesting a lactulose breath test from your gastroenterologist is a reasonable and important next step. If your provider is unfamiliar with SIBO testing, a functional medicine practitioner may be more receptive.

GutIQ can help you assess whether your symptom profile is more consistent with functional IBS or SIBO, guiding you toward the most appropriate next diagnostic and therapeutic steps. The distinction between these conditions is one of the most important differentiations in digestive health.