SIBO
Small intestinal bacterial overgrowth, including methane overgrowth (IMO)
Chronic conditionToo many bacteria, or methane-making microbes, in the small intestine. It can cause bloating, gas, and bowel changes, but breath tests often over- or under-diagnose it.
Medical Disclaimer
For education only — this guide can't diagnose you. If these signs sound familiar, talk with a healthcare provider. Book a consult
What happens
The small intestine normally has far fewer bacteria than the colon. Waves of gut movement, stomach acid, and the valve between the small and large intestine keep it that way. When these defenses fail, bacteria can build up and ferment food too early, causing gas and bloating, and sometimes poor absorption of B12, iron, and fat. In intestinal methanogen overgrowth (IMO), microbes that make methane are linked to constipation. Glucose or lactulose breath tests are the usual test, but they're imperfect. Fast gut transit can cause a false positive, and real overgrowth can be missed. Testing fluid from the small bowel is more accurate but rarely done. Symptoms overlap with IBS, so a positive breath test alone doesn't prove SIBO is the cause.
Who gets it
SIBO is more likely when the gut moves slowly or its defenses are weakened. Risk factors include past abdominal surgery, long-term acid-reducing medicine (PPIs) or low stomach acid, hypothyroidism, diabetes, scleroderma, celiac disease, Crohn's disease, and opioid use. It becomes more common with age. IBS, which is about twice as common in women, overlaps heavily with SIBO.
Signs worth getting checked
- Bloating that builds through the day or after meals
- Gas and belching
- Abdominal discomfort or cramping
- Diarrhea, or constipation with methane overgrowth
- Nausea or feeling full quickly
- Pale, greasy, or foul-smelling stools
- Fatigue, weight loss, or low B12 or iron in more severe cases
Labs to ask about
- Glucose or lactulose breath test measuring hydrogen and methane, done after a prep diet
- B12, folate, ferritin, and vitamin D
- Celiac screening (tTG-IgA with total IgA)
- TSH, A1c, and CBC to look for underlying causes
- Endoscopy with a small-bowel fluid sample in selected cases
Treatments your doctor may discuss
- Rifaximin, the most studied antibiotic, or other antibiotics
- For methane overgrowth, rifaximin sometimes combined with a second antibiotic
- Treating the underlying cause, such as reviewing PPIs or opioids, treating hypothyroidism, or controlling diabetes
- Prokinetics, medicines that speed gut movement, to help prevent relapse
- Herbal antimicrobial blends have limited evidence from small studies. Ask whether they fit your situation.
Supportive steps to discuss with your provider
- Ask whether you still need acid-reducing medicine, and don't stop it without a plan.
- Try a short-term low-FODMAP diet with a dietitian to ease symptoms while the cause is addressed.
- Space meals a few hours apart if it helps you. It's often advised to support the gut's cleaning waves, though it hasn't been well studied.
- Ask about a plan to prevent relapse. SIBO often comes back when the cause isn't treated.
- Be cautious about repeated antibiotic courses or long restrictive diets without a clear diagnosis.
See a doctor promptly if you have
- Vomiting blood or black stools: go to the emergency room
- Severe belly pain and swelling with vomiting, or not passing gas or stool: go to the emergency room (possible bowel blockage)
- Blood in the stool, unintended weight loss, or diarrhea that wakes you at night
- Numbness, tingling, or balance problems, which can mean low B12
- Symptoms that come back soon after treatment
Related conditions
Ready to find the root cause?
Dr. Lesher is a Naturopathic Doctor who focuses on autoimmune and chronic conditions in women. Since 2020, she's helped hundreds of patients stop managing symptoms and start healing.