Yoon Hang Kim, MD, MPH

Board-Certified in Preventive Medicine | Integrative & Functional Medicine Physician

Direct Integrative Care  |  Telemedicine Practice (TX, IA, IL, MO, GA, FL)

Introduction

Hydrogen sulfide small intestinal bacterial overgrowth — increasingly reclassified by leading gastroenterologists as Intestinal Sulfide Overproduction (ISO) — represents one of the most clinically challenging and underdiagnosed subtypes of gut dysbiosis. Unlike hydrogen-dominant SIBO (associated primarily with diarrhea) or methane-dominant SIBO/IMO (associated with constipation), ISO results from the unchecked proliferation of sulfate-reducing bacteria (SRBs) in the small intestine, producing hydrogen sulfide (H₂S) gas in pathological quantities.

For decades, standard three-gas breath testing only measured hydrogen and methane, leaving ISO systematically invisible. The advent of the TrioSmart™ breath test (Gemelli Labs) — the first commercially available test to measure all three SIBO gases simultaneously — has finally given clinicians a diagnostic window into this condition. Even so, many patients with textbook ISO symptoms go years without a diagnosis, and formal treatment guidelines remain absent.

This article synthesizes the current evidence base — including data presented at Digestive Disease Week (DDW) 2025 — to provide clinicians and informed patients with a practical, evidence-stratified approach to treating ISO/H₂S SIBO.

Nomenclature: SIBO, IMO, and ISO

The terminology in this field is rapidly evolving. At DDW 2025, Dr. Mark Pimentel of Cedars-Sinai Medical Center formally articulated the modern tripartite classification of small intestinal microbial overgrowth:

• SIBO (Small Intestinal Bacterial Overgrowth): hydrogen-producing bacteria; associated with diarrhea-predominant symptoms
• IMO (Intestinal Methanogen Overgrowth): methane-producing archaea; associated with constipation
• ISO (Intestinal Sulfide Overproduction): hydrogen sulfide-producing bacteria; mixed bowel pattern with distinct systemic features

ISO is distinguished pathophysiologically by the fact that H₂S, at excessive concentrations, acts as a mitochondrial toxin — inhibiting cytochrome C oxidase and impairing ATP production. When confined to the small intestine (where detoxification capacity is limited), H₂S can produce downstream effects well beyond the gut, potentially contributing to fatigue, brain fog, and systemic inflammation that patients with this condition so commonly report.

Diagnosis

Breath Testing

Standard two-gas lactulose or glucose breath tests will not identify ISO. The TrioSmart three-gas breath test is currently the only validated commercial test for direct H₂S measurement. Clinicians should suspect ISO when a patient demonstrates a flat-line pattern on standard hydrogen and methane testing despite compelling gastrointestinal symptoms — a pattern that has been shown in the literature to represent occult hydrogen sulfide activity.

A 2023 medRxiv case registry — the largest collection of H₂S SIBO cases to date — found that diagnosis was made via flatline lactulose breath test in 42.5% of cases, empirically based on symptom presentation in 31.9%, and via direct H₂S measurement in only 25.7%. Notably, empiric diagnosis was associated with significantly lower treatment response rates compared to flatline-confirmed diagnosis (relative risk 0.60; p=0.04), underscoring the clinical value of objective testing when available.

Key Organisms

The primary sulfate-reducing bacteria associated with ISO include Desulfovibrio, Fusobacterium, and Bilophila wadsworthia. These organisms thrive on sulfur-containing substrates — including dietary sulfur amino acids (methionine, cysteine, taurine) and sulfate — to produce H₂S as a metabolic end-product. GI-MAP stool testing may provide indirect evidence of their overgrowth but is not diagnostic for ISO.

Symptom Profile

The largest case registry reports the following as the most common presenting features:

• Bloating: 77% of cases
• Constipation: 50.8% of cases
• Abdominal pain: 50.8% of cases
• Distinctive sulfurous / "rotten egg" odor to flatulence or belching
• Systemic features: fatigue, brain fog, and neurological symptoms disproportionate to gut complaints

Treatment: A Tiered Approach

ISO currently has no formal clinical guidelines. The evidence base is derived from case registries, small open-label trials, mechanistic studies, and clinical experience. The following framework organizes interventions by evidence tier, prioritizing H₂S-specific mechanisms.

Tier 1: H₂S-Specific Interventions (Strongest Evidence)

Bismuth Subsalicylate

Bismuth subsalicylate (BSS) — most familiar as Pepto-Bismol® — is arguably the most evidence-supported treatment specifically for ISO. BSS produces greater than 95% reduction in H₂S gas through direct binding (Suarez et al., 1998, PMID: 9576524), while simultaneously exerting antimicrobial activity against sulfate-reducing bacteria via its bismuth moiety.

The 2023 case registry — the largest ISO dataset to date — demonstrated a 76% treatment responder rate with bismuth (p=0.01), making it the most statistically robust intervention identified. Standard dosing is 524 mg (two standard tablets) taken four times daily for 3 to 7 days, often paired with rifaximin for synergistic effect. An anti-biofilm protocol combining bismuth with biofilm disruptors (e.g., N-acetylcysteine, EDTA-based agents) may further enhance penetration.

Rifaximin + N-Acetylcysteine (NAC) — DDW 2025 Update

One of the most clinically significant developments in SIBO treatment in recent years emerged from Cedars-Sinai investigators at DDW 2025. Low-dose rifaximin combined with the antioxidant N-acetylcysteine (NAC) demonstrated improved outcomes for bloating, diarrhea, and pain compared to rifaximin alone — with the combination also reducing H₂S levels on breath testing. The proposed mechanism is elegant: NAC disrupts the intestinal mucus biofilm, allowing rifaximin to penetrate and more effectively target the primary ISO culprits, including Desulfovibrio and Fusobacterium.

This NAC + rifaximin combination represents an important update to standard practice and is not yet reflected in existing guidelines. This aligns with the broader shift in expert thinking from simple microbial eradication toward strategic microbial manipulation — a framing that accounts for the risk of further microbiome depletion with aggressive antibiotic use. Typical NAC dosing used in this context is 600 mg twice daily, initiated several days prior to or concurrent with rifaximin.

Tier 2: Herbal/Botanical Antimicrobials (Promising Trial Data)

A 2024 open-label clinical study published in Nutrients (Min et al., 2024, PMID: 39339748) evaluated a broad-spectrum botanical antimicrobial regimen in patients with confirmed SIBO. The regimen was most effective for hydrogen sulfide-dominant SIBO, yielding negative breath test results in 66.7% of H₂S participants at week 10. The protocol also favorably shifted the microbiome — increasing SCFA-producing organisms (Akkermansia muciniphila, Faecalibacterium prausnitzii) and decreasing the Firmicutes/Bacteroidetes ratio.

A separate clinical study using Biocidin® (a broad-spectrum herbal antimicrobial) combined with a GI binder demonstrated resolution or significant improvement in approximately 67% of participants. The rationale for pairing a binder with antimicrobials is mechanistically sound: as SRBs are killed, they release H₂S; a binder traps this liberated gas, reducing the symptomatic flare that often accompanies treatment initiation.

Other herbal antimicrobials with evidence in general SIBO and theoretical applicability to ISO include high-dose oregano oil, berberine, uva ursi, and pomegranate extract.

Tier 3: Dietary Modifications

Low-Sulfur Diet

The 2023 case registry identified a 73% responder rate for a low-sulfur diet (p=0.01), making it one of only two interventions to achieve statistical significance. However, a low-sulfur diet reduces available substrate for SRBs without eliminating the overgrowth itself. It should therefore be used as a short-term adjunct to active antimicrobial treatment, not as a standalone strategy.

High-sulfur foods to limit during active treatment include: eggs, garlic, onion, cruciferous vegetables (broccoli, cauliflower, cabbage), asparagus, meat (particularly processed meats with sulfite preservatives), and alcohol. The goal is temporary substrate restriction to reduce bacterial fuel load during the treatment window.

Low-FODMAP and Low-Fermentation Diets

General fermentable carbohydrate restriction remains the dietary backbone for all SIBO subtypes. One systematic review of 10 clinical trials found that the low-FODMAP diet produced clinical response in 50-80% of IBS/SIBO patients. For ISO specifically, a combined approach — reducing both fermentable carbohydrates and sulfur substrates during treatment — is the most logical dietary strategy. Meals should be spaced at least four hours apart to allow adequate migrating motor complex (MMC) cycling between feeds.

Tier 4: Supportive and Adjunctive Therapies

Probiotics

Probiotic therapy for ISO is supported more by general SIBO data than ISO-specific trials. Bacillus clausii has demonstrated 47% breath test normalization as monotherapy — comparable to antibiotic response rates in some studies — with excellent tolerability (Soifer et al., 2010, PMID: 21082900). Lactobacillus plantarum 299v has shown significant reductions in abdominal pain and bloating in IBS/SIBO populations (Ducrotté et al., 2012, PMID: 22529959). A triple probiotic approach combining a Lactobacillus/Bifidobacterium blend, Saccharomyces boulardii, and a soil-based probiotic (Bacillus species) represents a reasonable clinical strategy, though direct ISO-specific evidence remains limited.

Molybdenum

Molybdenum is an essential cofactor for the enzyme sulfite oxidase, which converts sulfite (an intermediate in sulfur metabolism) to sulfate for excretion. At doses of 500–1000 mcg daily, molybdenum supplementation can support sulfur detoxification pathways and reduce symptomatic flares — particularly in patients with impaired sulfur metabolism or genetic variants affecting sulfur processing (e.g., CBS or SUOX pathway variants).

Hydroxocobalamin (B12) and Butyrate

Hydroxocobalamin — a form of B12 that acts as a direct H₂S scavenger — provides additional mechanistic support for symptom management. H₂S can deplete intestinal butyrate production by suppressing butyrate-producing organisms; butyrate supplementation (sodium or calcium butyrate, 300–600 mg two to three times daily) supports intestinal epithelial repair and barrier integrity during and after treatment.

Prokinetics and MMC Support

Addressing impaired small bowel motility is essential for relapse prevention. The migrating motor complex (MMC) — the interdigestive "housekeeper wave" that clears bacteria from the small intestine — is frequently dysregulated in SIBO and ISO. Natural prokinetics with clinical evidence include ginger extract (180–360 mg between meals), Iberogast® (a botanical formula), and low-dose naltrexone (LDN), which in addition to its immune-modulating effects has demonstrated prokinetic activity. Pharmaceutical prokinetics (low-dose erythromycin, low-dose naltrexone, prucalopride) may be warranted in cases with documented MMC dysfunction.

A Note on the Elemental Diet

The elemental diet — a liquid formula providing pre-digested nutrients absorbed proximally, thereby starving small bowel bacteria — has a well-established track record in hydrogen and methane SIBO (Pimentel et al., 2004, PMID: 15058720). Its role in ISO is more nuanced. Most elemental formulas contain sulfur-containing amino acids (methionine, cysteine, taurine), which may theoretically sustain or exacerbate sulfate-reducing bacterial activity. Clinical experience reports more mixed results for ISO compared to other SIBO subtypes. If used, a 2-to-3-week protocol is standard, and patients should be monitored for symptom exacerbation. Sulfur-free elemental formulations, when available, may be preferable.

Evidence Summary

The table below summarizes the current evidence base for H₂S SIBO/ISO treatment, including verification against the most current published and conference data.

A Paradigm Shift: From Eradication to Manipulation

A critical clinical reframe emerging from DDW 2025 deserves emphasis. Current antibiotic therapies are effective in only approximately 44% of SIBO cases — a sobering figure that calls into question the traditional eradication model. Leading researchers now advocate for a shift in clinical thinking: rather than attempting to eliminate the small bowel microbiome, the goal should be to strategically manipulate it — reducing pathological overgrowth while preserving and supporting beneficial microbial populations.

For ISO specifically, this means: combining bismuth (H₂S binding) with targeted antimicrobials (rifaximin ± NAC), supporting the microbiome with probiotics, restoring MMC function to prevent recurrence, and addressing the underlying physiological vulnerability — whether that is motility dysfunction, immune dysregulation, prior gastrointestinal infection, or anatomical factors — that allowed the overgrowth to establish in the first place.

Conclusion

Hydrogen sulfide SIBO — now more precisely termed Intestinal Sulfide Overproduction — is a real and underdiagnosed clinical entity with significant symptomatic burden and a growing evidence base. The absence of formal clinical guidelines does not preclude a systematic, evidence-stratified approach to treatment. Bismuth subsalicylate remains the most H₂S-specific and robustly evidenced intervention. The emerging rifaximin-plus-NAC protocol from Cedars-Sinai, botanical antimicrobial regimens with 66-67% resolution rates, and strategic probiotic support each represent meaningful clinical tools.

Clinicians practicing integrative and functional medicine are in a particularly strong position to manage ISO, given the emphasis on root-cause analysis, gut-immune-motility interplay, and individualized therapeutic sequencing. As research in this area accelerates — driven in part by the widespread adoption of the TrioSmart breath test and the growing recognition of ISO as a distinct clinical entity — treatment protocols will continue to evolve.

Patients interested in a comprehensive evaluation and individualized treatment approach for ISO/H₂S SIBO may schedule a consultation through Direct Integrative Care.

References

1. 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. PMID: 9576524

2. Min M, Nadora D, Chakkalakal M, et al. An oral botanical supplement improves small intestinal bacterial overgrowth (SIBO) and facial redness. Nutrients. 2024;16(18):3149. PMID: 39339748

3. Pimentel M, Saad RJ, Long MD, Rao SSC. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. Am J Gastroenterol. 2020;115(2):165-178. PMID: 32022728

4. Soifer LO, Peralta D, Dima G, Besasso H. Comparative clinical efficacy of a probiotic vs. an antibiotic in the treatment of patients with intestinal bacterial overgrowth and chronic abdominal functional distension. Acta Gastroenterol Latinoam. 2010;40(4):323-7. PMID: 21082900

5. Pimentel M, Constantino T, Kong Y, Bajwa M, Rezaei A, Park S. A 14-day elemental diet is highly effective in normalizing the lactulose breath test. Dig Dis Sci. 2004;49(1):73-77. PMID: 15058720

6. Ducrotté P, Sawant P, Jayanthi V. Clinical trial: Lactobacillus plantarum 299v (DSM 9843) improves symptoms of irritable bowel syndrome. World J Gastroenterol. 2012;18(30):4012-4018. PMID: 22529959

7. Kashyap P, Moayyedi P, Quigley EMM, et al. Critical appraisal of the SIBO hypothesis and breath testing: A clinical practice update endorsed by ESNM and ANMS. Neurogastroenterol Motil. 2024;36(6):e14817. PMID: 38627907

8. Villanueva-Millan MJ et al. Hydrogen sulfide producers in the duodenum by shotgun sequencing correlate with hydrogen sulfide levels on breath testing. DDW 2025 — Cedars-Sinai Presentation.

9. H2S SIBO Case Registry (medRxiv, 2023). DOI: 10.1101/2023.03.07.23286900