The halitosis meter catches only half.
The molecules that create bad breath fall into two broad groups. The VSC meter used in most clinics sees only the sulfur group.
VSC · sulfur group
Hydrogen sulfide and methyl mercaptan. The familiar "rotten egg, onion, garlic" breath. Shows up as a VSC meter reading.
Non-VSC · indole group
Odor made by non-sulfur compounds. The VSC meter reads "normal," yet the smell is strong — this is the real identity of "the test is normal, but it's there."
If the halitosis meter reads normal but the subjective odor is strong, a non-sulfur compound like indole or skatole is likely the culprit. A normal reading does not mean "no odor."
Biofilm and the gut — two sources.
Biofilm — the tongue coating
Bacteria settle as a sticky film (biofilm) on the tongue coating at the back of the mouth and on oral surfaces. Some of these bacteria give off non-sulfur odor compounds without producing VSC at all.
- Clean only the surface and it improves briefly, but while the film remains the smell rises again.
- That is why brushing and scaling alone rarely fix it.
The gut — the real identity of "garlic breath"
The bad breath that lingers up to two days after eating garlic is not the garlic itself — it is a metabolite made by gut bacteria (methyl mercaptan).
- Standard SIBO breath tests (hydrogen, methane, hydrogen sulfide) leave out methyl mercaptan, so it is easily missed.
- This gas carries mitochondrial toxicity in the same class as cyanide.
A "normal" misdiagnosis, and a settled film.
Bad breath is often not "incurable" — it has simply not been seen properly. Usually for three reasons.
The standard test misses half
Non-VSC (indole group) and methyl mercaptan are left out of the standard panel, producing a "normal" misdiagnosis.
A biofilm has settled
Without also managing the film on the tongue and mouth, cleaning the surface means relapse the moment you stop.
The upstream is in the gut
If the odor is downstream of gut bacterial overgrowth, managing only the mouth leaves the root — which is why it comes with systemic symptoms (bloating, anxiety, fatigue).
The best option is to actually measure the causative gases — hydrogen sulfide and methyl mercaptan. Standard tests tend to omit methyl mercaptan and read "normal," and when this gas is not measured there is currently little other suitable test.
But this is not a dead end. The key point is this: most bad breath and body odor involves bacteria. If a smell genuinely exists (someone can perceive it), it is hard to deny that the cause is bacterial. Recognizing that, one can try empiric antibiotic treatment even when the gas is not measured. Many cases improve; a very few do not respond — either way we tell you honestly. Any specific prescription is decided through medical care.
Fish odor syndrome — a disease where the smell comes from bacteria and is managed with antibiotics.
If "the smell is from bacteria" sounds unfamiliar, medicine already has a recognized example — fish odor syndrome (trimethylaminuria, TMAU).
Gut bacteria make the "fish smell"
When gut bacteria break down the choline and carnitine in foods like eggs, fish, beans, and red meat, they produce trimethylamine (TMA) — the rotten-fish smell. Normally the liver (the FMO3 enzyme) converts it to an odorless form, but when gut bacteria overproduce it or the liver can't keep up, the odor leaks out through breath, sweat, and urine.
One established management is low-dose antibiotics to reduce the gut bacteria that make TMA (together with dietary change). In other words, "odor → gut bacteria → antibiotics" is an approach medicine has already accepted — the basis for viewing much of halitosis and body odor the same way. (Antibiotics are usually short-course, with care about resistance.)
Follow the full essay series.
The English-language writing on breath gases, sulfur-reducing bacteria, and the gut origins of odor lives on Substack.
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