The gas smells like rotten eggs. The urgency hits fast after meals, and “diarrhea-predominant IBS” is the label you’ve collected. Maybe you even did a SIBO breath test — and it came back normal — so you were told your gut is fine while you’re still planning every outing around the nearest bathroom.
If any of that lands, I want you to know something that a normal breath test may have literally been unable to see. There’s a third, newly recognized type of small-intestine overgrowth called hydrogen sulfide SIBO — clinically, intestinal sulfide overgrowth, or ISO — and until very recently, the standard test couldn’t measure it at all. Which means you can be genuinely sick and “normal on paper” at the same time.
A third type, hiding in plain sight
For years, SIBO was understood as two flavors: hydrogen-dominant (tends toward diarrhea) and methane-dominant (tends toward constipation). Hydrogen sulfide is the third. Certain gut bacteria produce hydrogen sulfide gas — the same compound behind that unmistakable sulfur/rotten-egg odor. In small amounts it’s a normal part of gut chemistry. In excess, it irritates the gut lining and is now strongly associated with diarrhea, urgency, and pain.
Some people also notice symptoms that feel bigger than the gut – because hydrogen sulfide is a signaling gas that affects nerves and blood vessels throughout the body. That’s still an emerging area, but it’s part of why this type can feel so distinctly awful and so hard to explain.
Why your “normal” breath test may have missed it
Here’s the clever, maddening part. The bacteria that make hydrogen sulfide consume hydrogen to do it. So on an older two-gas breath test, your hydrogen line could look flat or unusually low — and a flat line was often read as “negative.” In reality, that flat hydrogen was a clue: the hydrogen wasn’t missing, it was being converted into sulfide the test couldn’t measure.
That gap is now being closed by a three-gas breath test (you may see it branded as trio-smart) that measures hydrogen, methane, and
hydrogen sulfide together. It’s the reason people who were told they were fine for years are finally getting an answer that matches how they feel.
How hydrogen sulfide SIBO is tested
- A three-gas breath test that specifically includes hydrogen sulfide — the piece older tests left out. Same simple format: a sugar solution, then breath samples over a couple of hours, done with proper prep.
- GI-MAP or comprehensive stool testing to see the fuller microbial picture and other drivers of diarrhea and inflammation.
- Supportive bloodwork to check for the nutrient gaps that chronic diarrhea and malabsorption can create.
Specific test, specific answer — instead of one more “everything looks normal.”
The root cause under the root cause
As with every form of this condition, the overgrowth is usually
downstream of something else, and that’s why it comes back if you only chase the gas. Worth investigating:
- Gut motility — the between-meal cleaning wave that keeps microbes moving. It can be blunted by stress and a dysregulated nervous system, a prior gut infection or food poisoning, or thyroid issues.
- Low stomach acid, which loosens a key check on microbial numbers. (Something to revisit with your provider — never stop a
prescribed medication on your own.) - Diet and sulfur load, which can influence how much hydrogen sulfide the bacteria produce for some people — though, importantly, this varies a lot person to person.
Trace the overgrowth back to its driver and you stop mowing the lawn and start pulling roots.
What treatment actually looks like (and why it’s its own animal)
Hydrogen sulfide SIBO is treated differently from the hydrogen and methane types — matching the plan to the wrong type is a common reason people stall. This is provider-directed care, not a self-treat protocol, but here’s the shape of it so you’re an informed partner.
Because the problem is an overproduction of a specific gas, some approaches aim to bind or neutralize the sulfide — bismuth-based strategies are one your provider may consider (worth discussing with them rather than reaching for the medicine cabinet on your own). Provider-chosen antimicrobial approaches — prescription or herbal — are used to bring the overgrowth down. Diet is genuinely individual here: a lower-sulfur trial helps some people and is unnecessary or even distracting for others, so it’s best done as a short, guided experiment while tracking your own symptoms rather than as a permanent restriction. And, as always, the upstream driver — motility, stomach acid, nervous system, a past infection — gets addressed so the overgrowth doesn’t simply return.
No one honest will hand you a guaranteed cure. But a type that was invisible a few years ago is now testable and treatable — and that changes everything for people who were told, wrongly, that nothing was wrong.
You were never “making it up”
If you’ve been quietly managing rotten-egg gas, sudden urgency, and a test result that didn’t match your reality, please hear this: you weren’t imagining it, and you weren’t being difficult. The tool to see it simply arrived late. If you’re in Bucks County and done being told you’re fine while you feel anything but, this is the kind of detective work we do at Thrive Nest.
Ready to finally test for the type older panels missed? Book a consult and let’s find what’s actually going on.
This article is for educational purposes only and is not medical advice, diagnosis, or treatment. Hydrogen sulfide SIBO/ISO shares symptoms with several other conditions, and testing and treatment should be guided by a qualified healthcare provider who knows your history. Do not start or stop any medication or supplement without talking to your provider.
