Hydrogen vs. Methane vs. Hydrogen Sulfide SIBO

Hydrogen vs Methane vs Hydrogen Sulfide — scientific infographic poster
SIBO H2/CH4 breath test result patterns
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Table of Contents

  1. What the Three Gas Subtypes Mean for You
  2. Hydrogen-Dominant SIBO (the diarrhea one)
  3. Methane-Dominant SIBO / IMO (the constipation one)
  4. Hydrogen Sulfide SIBO (the rotten-egg one)
  5. How Breath Tests Tell the Subtypes Apart
  6. Why Treatment Differs by Gas
  7. Can You Have More Than One Subtype?
  8. Key Research Papers
  9. Connections
  10. Featured Videos

1. What the Three Gas Subtypes Mean for You

A SIBO breath test can measure up to three gases, and which gas is raised points to different microbes, different typical symptoms and different treatment data. A test that measures hydrogen alone cannot show a raised methane or hydrogen sulfide level.

SIBO is not a single disease. The 2020 American College of Gastroenterology (ACG) guideline defines it as excessive numbers of bacteria in the small bowel, causing gastrointestinal symptoms. Gut microbes ferment food and release gases, and the gas depends on which microbes are present. Three are measured:

Relapse after a course of antibiotics is common (section 7), and the published treatment data differ by gas (section 6). The rest of this article walks through each subtype and what the research reports about it.

2. Hydrogen-Dominant SIBO (the diarrhea one)

Hydrogen-dominant SIBO is the "classic" picture: hydrogen was the gas the original SIBO breath tests measured, and methane was added later. Organisms reported to be increased in SIBO include E. coli, Klebsiella and Enterococcus. When they reach carbohydrates before the body has absorbed them, they ferment them and release hydrogen gas.

What it feels like

The IBS-D overlap

SIBO and IBS-D (diarrhea-predominant irritable bowel syndrome) overlap, but estimates of how much vary widely. A 2020 meta-analysis of 25 case-control studies found SIBO by breath testing in 35.5% of people with IBS versus 29.7% of controls (odds ratio 3.7 overall), with higher odds in IBS-D than in IBS-C (odds ratio 1.86); the authors rated the overall quality of the evidence as low. In 2015 the FDA approved rifaximin for IBS-D on the strength of the TARGET trials, an approval the ACG guideline describes as resting on the understanding that one possible cause of IBS is a disturbed gut microbiome.

What the breath test looks like

The 2017 North American Consensus defines a positive glucose or lactulose breath test for SIBO as a rise in hydrogen of at least 20 ppm above baseline by 90 minutes. The 90-minute limit is meant to separate fermentation in the small intestine from the normal fermentation that starts once the test sugar reaches the colon; the ACG guideline adds that a second, later peak is not required. Rapid transit can carry the sugar to the colon early and produce a false positive.

3. Methane-Dominant SIBO / IMO (the constipation one)

The microbe behind methane-dominant SIBO is not a bacterium. The ACG guideline names Methanobrevibacter smithii, the predominant methanogen in the human gut, as the apparent cause of excess methane. It belongs to the archaea, the third domain of life alongside bacteria and eukaryotes (organisms with nucleated cells, from fungi to people). Archaea look similar to bacteria under a microscope but differ in their ribosomal RNA and cell walls.

This matters because most antibiotics are designed against bacteria. In laboratory testing, M. smithii strains were resistant to ampicillin, gentamicin, tetracycline and rifampicin (a rifamycin, the same drug family as rifaximin), but susceptible to metronidazole. In a retrospective review of methane-positive patients, rifaximin alone cleared methane in only 28% (section 6).

Because the organism is an archaeon, and because people with positive methane tests also carry more methanogens in stool (suggesting overgrowth beyond the small intestine), the authors of the 2020 ACG guideline proposed the term IMO (intestinal methanogen overgrowth) in place of "methane SIBO."

Methane and slower transit

In a 2006 study, Pimentel and colleagues infused methane into the small intestine of dogs and found that it slowed transit in every animal, by an average of 59%. In guinea-pig intestine in an organ bath, methane increased contractile activity, and among people with IBS who had a small-bowel motility study, methane producers had a higher motility index than hydrogen producers. The authors concluded that methane slows small-intestinal transit, apparently by augmenting contractile activity.

The amount matters as well as the presence: in a 2007 study of 87 people with IBS, the quantity of methane on the breath test was directly proportional to the reported degree of constipation (r = 0.60), and greater methane production went with fewer bowel movements and harder stools.

What it feels like

The IBS-C link

Methane is more common in IBS-C than in IBS-D: the 2020 meta-analysis found methane-positive breath tests more prevalent in IBS with constipation (odds ratio 2.3), and a 2022 study combining breath testing with stool sequencing found higher breath methane and more M. smithii in stool in IBS-C. The ACG guideline suggests methane testing to diagnose IMO in people with constipation (a conditional recommendation based on very low-quality evidence).

4. Hydrogen Sulfide SIBO (the rotten-egg one)

Some people with SIBO-type symptoms have breath tests with flat lines on both hydrogen and methane. The research group that developed the three-gas test proposes that some of these "flat-line" results reflect hydrogen sulfide production: hydrogen is consumed to make hydrogen sulfide, so little reaches the breath. Until recently there was no commercial test for the gas — the 2020 ACG guideline noted that none was yet available and that a hydrogen sulfide cutoff still needed to be validated.

The organisms involved are hydrogen sulfide-producing bacteria. A 2022 study linked higher breath H2S in IBS-D with more Fusobacterium and Desulfovibrio in stool, and a 2025 study of duodenal fluid linked it with small-bowel H2S producers such as Proteus mirabilis and the sulfate-reducing Desulfobulbus and Desulfosarcina. Sulfate-reducing bacteria use hydrogen made by other microbes to produce hydrogen sulfide (H2S) — the gas that gives rotten eggs their smell.

What it feels like

Why two-gas tests miss it

A two-gas breath test measures only hydrogen and methane, so it cannot detect hydrogen sulfide. According to the three-gas test's developers, the hydrogen curve can be flat because sulfate-reducing bacteria consume the hydrogen to make H2S, and methane is flat when methanogens are absent. Such a "flat-line" test reads as negative on hydrogen and methane criteria.

A three-gas breath test that adds hydrogen sulfide is now available as an at-home kit in the United States. No guideline has set an H2S cutoff: the 2026 nationwide study defined a positive result as ≥2 ppm (with 1.5 ppm in a sensitivity analysis), and a 2025 duodenal-aspirate study used 1.5 ppm. Most published three-gas studies come from the research group that developed the test.

The bismuth angle

Bismuth subsalicylate, an over-the-counter antidiarrheal, binds hydrogen sulfide. In a 1998 study of 10 healthy volunteers, bismuth subsalicylate (524 mg four times a day for 3–7 days) cut fecal H2S release by more than 95%. No published trial has tested it in people with raised breath H2S; its use alongside antibiotics for hydrogen sulfide is a practitioner approach rather than a trial result.

5. How Breath Tests Tell the Subtypes Apart

The ACG guideline suggests glucose or lactulose breath testing to diagnose SIBO, and methane testing to diagnose IMO in people with constipation (conditional recommendations based on very low-quality evidence). The patterns, using the 2017 North American Consensus cutoffs for hydrogen and methane:

A comparison matrix of the three SIBO gas subtypes set side by side on four axes: which organism makes the gas, the breath-test threshold, the dominant symptom, and the response to rifaximin alone: about 71 per cent SIBO eradication in a meta-analysis, against 28 per cent methane clearance in a retrospective review of methane-positive patients, the methane organism being an archaeon rather than a bacterium. THREE GASES, THREE ORGANISMS one diagnosis · three organisms · about 18% of positive tests show two or more gases H2 hydrogen CH4 methane · IMO H2S hydrogen sulfide ORGANISM BACTERIA ordinary gut flora E. coli, Klebsiella ARCHAEON Methanobrevibacter smithii BACTERIA sulfate-reducing Desulfovibrio BREATH TEST rise ≥20 ppm above baseline within 90 minutes ≥10 ppm absolute level at any point ≥2 ppm absolute level no guideline value DOMINANT SYMPTOM diarrhea, urgency loose stool, bloating, gas constipation hard pellet stools Bristol type 1 or 2 diarrhea, urgency abdominal pain sulfur-smelling gas RIFAXIMIN ALONE 70.8% SIBO eradicated 32-study meta-analysis 28% methane cleared + neomycin → 87% no trial data practitioners use rifaximin + bismuth Hydrogen is the only baseline-relative threshold: a rise of 20 ppm above your own starting value. Methane and H2S are absolute levels at any point in the test. ONE DRUG, THREE ANSWERS rifaximin is a bacterial drug a meta-analysis found 71% SIBO eradication, but methane cleared in only 28% the methane organism is an archaeon a separate domain of life, distinct from both bacteria and eukaryotes the thresholds are not alike hydrogen is a rise of 20 ppm above your own baseline; methane and H2S are absolute levels at any point in the test three organisms, three thresholds, three drug responses — and only a three-gas breath test measures all three.

Test preparation, reading the curves and the causes of false results are covered in the Breath Test Guide.

6. Why Treatment Differs by Gas

The published response to the same antibiotic differs by gas. Most of the evidence is weak: the ACG guideline describes the antibiotic data for SIBO as "small clinical trials of poor to modest quality," and its suggestion of antibiotics for symptomatic SIBO is a conditional recommendation based on low-quality evidence.

Hydrogen-dominant: rifaximin studies

Rifaximin is a minimally absorbed antibiotic, so it acts mainly inside the gut. The TARGET 1 and 2 trials, in people with IBS without constipation (not selected for SIBO), used 550 mg three times daily for 14 days: adequate relief of global IBS symptoms was reported by 40.7% on rifaximin versus 31.7% on placebo, with similar adverse-event rates. Those trials did not measure SIBO eradication. For SIBO itself, a 2017 meta-analysis of 32 studies (1,331 patients; doses of 600–1,600 mg a day for 5–28 days) found an overall eradication rate of 70.8% (95% CI 61.4–78.2%) and adverse events in 4.6%, while rating the quality of the studies as generally poor.

Methane-dominant (IMO): rifaximin with neomycin

The first data came from a 2010 retrospective chart review of 74 methane-positive IBS patients (methane ≥3 ppm, a lower cutoff than today's 10 ppm). Methane was cleared in 28% of those given rifaximin alone (400 mg three times daily for 10 days), 33% with neomycin alone (500 mg twice daily for 10 days) and 87% with the two combined for 10 days; clinical response was 56%, 63% and 85%. A 2014 double-blind randomized trial in 31 methane-positive people with IBS-C then compared 14 days of neomycin plus rifaximin with neomycin plus placebo: the combination improved constipation, straining and bloating more (but not abdominal pain), and the benefit tracked with methane falling below 3 ppm. Other approaches under study:

The doses used in rifaximin trials are covered on the Rifaximin Protocols page.

Hydrogen sulfide: no tested regimen yet

No guideline names a treatment for raised hydrogen sulfide, and the authors of the 2026 nationwide study wrote that human trials had not yet been published; the supporting work they cite is in animals (in a rat model of post-infection IBS, rifaximin combined with N-acetylcysteine improved diarrhea-like features in step with a fall in H2S producers). Approaches described in practitioner accounts, none tested for this purpose in a published trial, include:

Herbal antimicrobials

A 2014 retrospective chart review from Johns Hopkins (Chedid and colleagues) followed 104 patients with a positive lactulose breath test who were treated with either rifaximin 1,200 mg a day or commercial herbal preparations for 4 weeks. The follow-up test was negative in 46% (17/37) of the herbal group and 34% (23/67) of the rifaximin group, a difference that was not statistically significant (P = 0.24). The authors concluded the herbs were at least as effective and called for prospective studies; the comparison was not randomized. The herbs are covered on the Herbal Antimicrobials page.

7. Can You Have More Than One Subtype?

Yes, though in the largest published series it was the minority pattern. In the 2026 nationwide study of 3,004 at-home three-gas tests, 267 of the 1,497 positive tests (about 18%) showed two or more gases above threshold (H2S counted as positive from 2 ppm; counts from the study's Table 2), and high hydrogen and high methane rarely occurred together. What the study reported for each combination:

How treatment sequencing is described

No guideline or trial sets an order for treating mixed results. Practitioner accounts describe two approaches: treating methane first, on the reasoning that methane slows transit, then hydrogen and hydrogen sulfide in a second course; or combining drugs (for example rifaximin, neomycin and bismuth) in a single course.

Relapse is common. In an Italian cohort of 80 people whose glucose breath test had normalized after rifaximin, 12.6%, 27.5% and 43.7% tested positive again at 3, 6 and 9 months; older age, previous appendectomy and long-term proton-pump-inhibitor use were associated with recurrence. Prokinetic drugs, which stimulate gut motility, have been studied for relapse prevention mainly in a retrospective review: among 64 IBS patients whose SIBO had resolved, low-dose bedtime tegaserod was followed by a longer symptom-free interval than erythromycin or no prevention. More is covered on the Prokinetics & Relapse Prevention and Root Causes pages.

Key Research Papers

  1. Pimentel M, et al. Methane, a gas produced by enteric bacteria, slows intestinal transit and augments small intestinal contractile activity. Am J Physiol Gastrointest Liver Physiol. 2006;290(6):G1089-G1095. — doi:10.1152/ajpgi.00574.2004
  2. Pimentel M, Leite G, Joo L, et al. Real-world Study of Three-gas Breath Testing Nationwide and the Association With Symptoms. J Clin Gastroenterol. 2026;60(5):406-417. — PubMed PMID: 41671534
  3. Takakura W, Pimentel M. Small intestinal bacterial overgrowth and irritable bowel syndrome: an update. Frontiers in Psychiatry. 2020;11:664. — doi:10.3389/fpsyt.2020.00664
  4. Pimentel M, Saad RJ, Long MD, Rao SSC. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. Am J Gastroenterol. 2020;115(2):165-178. — doi:10.14309/ajg.0000000000000501
  5. Rezaie A, Buresi M, Lembo A, et al. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. Am J Gastroenterol. 2017;112(5):775-784. — PubMed PMID: 28323273
  6. Shah A, Talley NJ, Jones M, et al. Small Intestinal Bacterial Overgrowth in Irritable Bowel Syndrome: A Systematic Review and Meta-Analysis of Case-Control Studies. Am J Gastroenterol. 2020;115(2):190-201. — PubMed PMID: 31913194
  7. Schiller LR. Evaluation of small bowel bacterial overgrowth. Curr Gastroenterol Rep. 2007;9(5):373-377. — PubMed PMID: 17991337
  8. Kunkel D, Basseri RJ, Makhani MD, et al. Methane on breath testing is associated with constipation: a systematic review and meta-analysis. Dig Dis Sci. 2011;56(6):1612-1618. — PubMed PMID: 21286935
  9. Chatterjee S, Park S, Low K, et al. The degree of breath methane production in IBS correlates with the severity of constipation. Am J Gastroenterol. 2007;102(4):837-841. — PubMed PMID: 17397408
  10. Mathur R, Amichai M, Chua KS, et al. Methane and hydrogen positivity on breath test is associated with greater body mass index and body fat. J Clin Endocrinol Metab. 2013;98(4):E698-E702. — PubMed PMID: 23533244
  11. Samuel BS, Gordon JI. A humanized gnotobiotic mouse model of host-archaeal-bacterial mutualism. Proc Natl Acad Sci U S A. 2006;103(26):10011-10016. — PubMed PMID: 16782812
  12. Dridi B, Fardeau ML, Ollivier B, et al. The antimicrobial resistance pattern of cultured human methanogens reflects the unique phylogenetic position of archaea. J Antimicrob Chemother. 2011;66(9):2038-2044. — PubMed PMID: 21680581
  13. Villanueva-Millan MJ, Leite G, Wang J, et al. Methanogens and Hydrogen Sulfide Producing Bacteria Guide Distinct Gut Microbe Profiles and Irritable Bowel Syndrome Subtypes. Am J Gastroenterol. 2022;117(12):2055-2066. — PubMed PMID: 36114762
  14. Villanueva-Millan MJ, Leite G, Mathur R, et al. Hydrogen Sulfide and Methane on Breath Test Correlate with Human Small Intestinal Hydrogen Sulfide Producers and Methanogens. Dig Dis Sci. 2025;70(11):3846-3856. — PubMed PMID: 40569514
  15. Suarez FL, Furne JK, Springfield J, Levitt MD. Bismuth subsalicylate markedly decreases hydrogen sulfide release in the human colon. Gastroenterology. 1998;114(5):923-929. — PubMed PMID: 9558280
  16. Pimentel M, Lembo A, Chey WD, et al. Rifaximin therapy for patients with irritable bowel syndrome without constipation. N Engl J Med. 2011;364(1):22-32. — PubMed PMID: 21208106
  17. Gatta L, Scarpignato C. Systematic review with meta-analysis: rifaximin is effective and safe for the treatment of small intestine bacterial overgrowth. Aliment Pharmacol Ther. 2017;45(5):604-616. — PubMed PMID: 28078798
  18. Low K, Hwang L, Hua J, et al. A combination of rifaximin and neomycin is most effective in treating irritable bowel syndrome patients with methane on lactulose breath test. J Clin Gastroenterol. 2010;44(8):547-550. — PubMed PMID: 19996983
  19. Pimentel M, Chang C, Chua KS, et al. Antibiotic treatment of constipation-predominant irritable bowel syndrome. Dig Dis Sci. 2014;59(6):1278-1285. — PubMed PMID: 24788320
  20. von Muhlenbrock C, Landskron G, Madrid AM. Treatment of small intestinal bacterial overgrowth in Chilean patients with irritable bowel syndrome: A prospective and comparative study. Rev Gastroenterol Mex (Engl Ed). 2025;90(1):54-62. — PubMed PMID: 40240267
  21. Gottlieb K, Wacher V, Sliman J, Pimentel M. Review article: inhibition of methanogenic archaea by statins as a targeted management strategy for constipation and related disorders. Aliment Pharmacol Ther. 2016;43(2):197-212. — PubMed PMID: 26559904
  22. Chedid V, Dhalla S, Clarke JO, et al. Herbal therapy is equivalent to rifaximin for the treatment of small intestinal bacterial overgrowth. Glob Adv Health Med. 2014;3(3):16-24. — PubMed PMID: 24891990
  23. Lauritano EC, Gabrielli M, Scarpellini E, et al. Small intestinal bacterial overgrowth recurrence after antibiotic therapy. Am J Gastroenterol. 2008;103(8):2031-2035. — PubMed PMID: 18802998
  24. Pimentel M, Morales W, Lezcano S, et al. Low-dose nocturnal tegaserod or erythromycin delays symptom recurrence after treatment of irritable bowel syndrome based on presumed bacterial overgrowth. Gastroenterol Hepatol (N Y). 2009;5(6):435-442. — PubMed PMID: 20574504

PubMed Topic Searches

Live PubMed searches on the specific microbes and mechanisms discussed in this article. Each link opens a fresh query, so the results stay current.

  1. PubMed: Intestinal methanogen overgrowth (IMO)
  2. PubMed: Hydrogen sulfide SIBO
  3. PubMed: Methanobrevibacter smithii and constipation
  4. PubMed: Desulfovibrio and IBS
  5. PubMed: Rifaximin + neomycin for methane
  6. PubMed: Allicin and methanogens
  7. PubMed: Bismuth subsalicylate and hydrogen sulfide
  8. PubMed: Hydrogen sulfide (three-gas) breath testing

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Connections

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