Overview
Bacteria in the small intestine produce hydrogen and methane gas when they ferment certain sugars. These gases are absorbed into the blood and exhaled in your breath. By measuring breath gas levels at intervals after a sugar drink, the test identifies bacterial overgrowth or sugar malabsorption.
Who needs it
Suspected SIBO (bloating, gas, diarrhea, especially after meals)
Suspected lactose intolerance for confirmation
Suspected fructose malabsorption
IBS workup in selected cases
Risks
Temporary GI symptoms (intended — that's how the test works)
Diarrhea after lactulose
No serious risks
Before the procedure
No fermentable foods (high-FODMAP) the day before — your unit will give a list.
No antibiotics for 4 weeks before; no bowel prep for 2 weeks; no probiotics for 2 weeks.
Fast 12 hours before.
No smoking the day of the test.
No vigorous exercise just before or during.
When to See a Doctor
Call your doctor or go to emergency if after the procedure you have severe or worsening abdominal pain, fever, repeated vomiting, heavy bleeding, black stools or fainting.
Why it is done
To diagnose SIBO (cause of bloating, diarrhea, gas), confirm lactose or fructose intolerance, evaluate post-cholecystectomy diarrhea, work up unexplained bloating.
During the procedure
You arrive at the unit and provide a baseline breath sample. You drink a measured sugar solution (lactulose for SIBO, lactose or fructose for those tests). You then breathe into a device every 20 minutes for 2-3 hours. Side effects during the test (cramping, gas, diarrhea) are part of the diagnosis. Bring a book or watch something on your phone.
After the procedure
You may have continued bloating or loose stool for several hours. Resume normal diet — but if SIBO confirmed, your doctor will start treatment. Results usually within 1-3 days.
When you get the results
Within 1-3 days.
Alternatives
Empirical antibiotic trial for SIBO. Trial elimination of lactose or fructose for intolerances. Direct small bowel aspirate culture (invasive, requires endoscopy).
Questions to ask your doctor
• What sugar solution will be used?
• How will my treatment depend on the result?
Source and review
This guide comes from the Saudi Gastroenterology Association (SGA) patient library, prepared by the SGA Patient Education Panel. Last reviewed: 2026-04-24. It is for education and does not replace your doctor's advice.
Frequently Asked Questions
What is Hydrogen Breath Test (SIBO & Lactose)?
Why is Hydrogen Breath Test (SIBO & Lactose) done?
What are the risks of Hydrogen Breath Test (SIBO & Lactose)?
How do I prepare for Hydrogen Breath Test (SIBO & Lactose)?
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