SIBO vs IBS: how to tell them apart.

People treat SIBO and IBS like two different diseases. In clinic, they overlap so much that the label often matters less than what you do next.

SIBO stands for small intestinal bacterial overgrowth. IBS stands for irritable bowel syndrome, one of the disorders of gut-brain interaction (DGBI). Both can show up as bloating, gas, loose stools, constipation, or some rotating mix. The symptoms are nearly identical at the kitchen table. The difference lives in mechanism, testing, and what the test result actually changes.

The real story is not which one you have. It is whether the breath test changes the plan.

What each label actually means

IBS is a clinical diagnosis. A gastroenterologist works through symptom pattern, duration, and red flags, and rules out anything structural. Colonoscopy is normal. Bloodwork is normal. The gut looks fine on paper. The wiring between gut and brain is the issue.

SIBO is a mechanistic claim. The argument is that too many bacteria are living in the small intestine, where bacteria are usually sparse. Those bacteria ferment carbohydrates upstream of where fermentation is supposed to happen. The output is hydrogen, methane, or hydrogen sulfide, and the bedside experience is bloat, distension, and altered stool.

One is a syndrome defined by symptoms. The other is a proposed cause for some of those symptoms. That asymmetry is why the two get tangled.

How the breath test works, and where it fails

A lactulose or glucose breath test is what most clinicians use to call SIBO. You drink the sugar, exhale into a tube on a schedule, and a machine measures the gases that come off. An early rise in hydrogen or methane is read as bacteria fermenting in the small bowel before it reaches the colon.

The North American Consensus on hydrogen and methane breath testing (Rezaie 2017) tried to standardize the cutoffs because labs were calling positives inconsistently. The consensus tightened thresholds, set the relevant rise at 20 ppm hydrogen within 90 minutes, and recommended glucose over lactulose when you want fewer false positives. Even with that work, breath testing remains an imperfect proxy. Fast transit, prior antibiotics, recent diet, and even smoking can all shift the curve.

The gold standard for SIBO is actually a culture of small bowel fluid taken during endoscopy. Almost no one does that test in routine practice because it is invasive, contamination is hard to avoid, and not every lab can run it. So the breath test wins by default, not by accuracy.

Ghoshal and colleagues have published several meta-analyses showing that roughly 30 to 40 percent of patients meeting IBS criteria also have a positive SIBO breath test, depending on the population and the cutoff used (Ghoshal 2017, Shah 2020). That overlap is the whole problem. Either SIBO is driving a big chunk of IBS, or the breath test is picking up something that lives downstream of the actual mechanism. The honest read is that both are partly true.

Side by side

What the antibiotic data actually shows

Rifaximin is the drug that put SIBO on the gastroenterology map. It barely absorbs into the bloodstream, so it acts locally in the gut. Pimentel and colleagues ran the TARGET 1 and TARGET 2 trials, which were randomized and placebo controlled, in patients with non-constipation IBS (Pimentel 2011). Two weeks of rifaximin produced modest but real improvement in global IBS symptoms and bloating that outlasted the treatment window.

The follow-up trial, TARGET 3, tested retreatment in people who responded and then relapsed (Lembo 2016). Many responded again. That is useful and also tells you something uncomfortable. Symptoms come back. The label of SIBO does not change the underlying terrain that lets bacteria overgrow in the first place.

So the bedside translation is straightforward. A positive breath test in someone with IBS-style symptoms is a reason to consider a course of rifaximin. It is not a reason to expect a cure.

Where the two labels diverge in practice

If your workup points to SIBO and your clinician offers a targeted antibiotic course, that is a defined intervention with a defined endpoint. You either improve or you do not, and you know within weeks. The decision is bounded.

If your workup points to IBS without a positive breath test, antibiotics are not the move. The longer game matters more. Diet structure, fiber type and dose, sleep regularity, stress load, and how the gut-brain axis is firing all sit upstream of the next flare. A symptom tracker that captures pattern over time is more useful than any one-time test. The data you collect on a normal week is what tells you what a bad week is actually responding to.

Methane-predominant patterns are their own story. High methane on breath testing tracks with constipation and slower transit, and the treatment logic shifts. Rifaximin alone tends to underperform here, and a second agent is often added. That is a conversation with a gastroenterologist, not a self-directed protocol.

Hydrogen sulfide is the third gas in the story and the newest. Labs that measure it are still few, and the clinical playbook is still being written. Treat it as an active research area, not a fixed diagnosis you can build a plan around yet.

What to do

1. Get the diagnosis worked up properly. Red flag symptoms like weight loss, blood in stool, anemia, or new symptoms after age 50 need a colonoscopy and bloodwork before anyone calls it IBS or SIBO.
2. If a breath test is offered, ask which sugar, which gases, and which cutoffs the lab uses. Glucose with the North American Consensus cutoffs is the cleaner test.
3. Treat a positive breath test as one data point, not a diagnosis you carry forever. Re-test only if symptoms return and a decision hinges on it.
4. Track symptoms daily for at least 30 days. Pattern beats any single test for figuring out what actually moves your gut.
5. Choose one variable to change at a time. Diet, sleep, stress load, and medications all interact. Changing four things at once tells you nothing.

The label is a tool, not the answer. The plan is what matters.