Bad breath causes: where it comes from and what actually fixes it

Most bad breath starts in the mouth, and most of that starts on the back of the tongue. Bacteria there break down food debris, shed cells and proteins in saliva, and release volatile sulfur compounds, chiefly hydrogen sulfide and methyl mercaptan.^[1] That’s the smell.

The split between bad breath causes is lopsided. In a series of 2,000 consecutive patients at a multidisciplinary bad breath clinic in Belgium, an oral cause turned up in 76%: tongue coating alone in 43%, gum disease alone in 11%, and both together in 18%. Ear, nose and throat problems and causes elsewhere in the body accounted for 4%. Another 16% were diagnosed with pseudo-halitosis or halitophobia, meaning a conviction of bad breath that the examiners couldn’t detect.^[2]

So what follows runs roughly in order of share. The stomach theory, which gets a lot of airtime, gets a fair hearing: there is real association data for both reflux and H. pylori. It’s just a much smaller slice than it’s often made out to be.

One disclosure before we start: Evexia’s owner also owns Greco Gum, a mastic gum brand, which comes up in one section near the end.

What causes bad breath? The sulfur compounds

The chemistry, briefly. Mouth bacteria break down food debris, shed cells, saliva proteins and dental plaque, producing volatile sulfur compounds along with other foul-smelling molecules such as indole, skatole, putrescine and cadaverine. The build-up of bacteria and debris at the back of the tongue and in its furrows is considered the major cause, and the pockets that form around teeth in gum disease make a good home for the same process.^[1]

How many people does this affect? Estimates vary wildly, and their reliability has been questioned. A 2018 meta-regression put the prevalence at about 32%, and severe halitosis is thought to affect fewer than 5% of people.^[1]

Where bad breath comes from, at a glance

Source	How often	Clues	Where to start
Tongue coating	43% alone, 61% counting those who also had gum disease (Belgian clinic series)	White or yellowish coating at the back of the tongue	Clean the tongue daily
Gum disease	11% alone, 29% alongside tongue coating	Bleeding gums, deep pockets around teeth	Clean between teeth; see a dentist
Dry mouth	An independent predictor in a 119-patient study	Worse on waking, drying medicines, mouth breathing	Water, chewing, review medicines
Ear, nose and throat	3% to 10% of cases	Tonsil stones, sinus infection, post-nasal drip	GP or ear, nose and throat specialist
Stomach (reflux, H. pylori)	Associated in studies; share unclear	Heartburn, regurgitation, indigestion	Treat reflux; ask about H. pylori testing
Systemic disease	Uncommon	Fruity, ammonia-like or faecal smell, feeling unwell	Doctor, sometimes urgently
Pseudo-halitosis	16% of the Belgian clinic's patients	Nobody else notices it	An objective check, then address the worry

Mouth causes: the big share

Tongue coating

The single biggest source. In the Belgian series, tongue coating was involved in 61% of patients once you count the people who also had gum disease.^[2] In a separate study of 119 patients who came in complaining of bad breath, a higher tongue coating score was one of the strongest independent predictors of strong odour.^[3] If the back of your tongue looks white or yellowish, that’s the likely reservoir, and our white tongue guide covers what that coating is and how to tell it from other things.

Gum disease

Involved in 29% of the Belgian patients, alone or alongside tongue coating.^[2] In the 119-patient study, gum pockets 5 mm or deeper that bled when probed were another strong independent predictor.^[3] How much the pockets themselves contribute is still unclear,^[1] but gum disease is worth treating in its own right, and treating it deals with a source rather than masking one.

Dry mouth and morning breath

In the same 119 patients, resting saliva flow was significantly lower in those with strong bad breath, and low resting flow stood as an independent predictor alongside tongue coating and gum pockets. Stimulated saliva (the flow you produce while chewing) showed no link. Resting flow was lower in women and fell with age.^[3]

Morning breath is the everyday version of this, and it’s so normal that halitosis researchers class it as physiological and exclude it from treatment trials.^[1] If your breath is only bad on waking, you’re probably describing morning breath rather than a problem. If you sleep with your mouth open, that’s worth looking at too; we cover the nasal breathing question in mouth taping.

Food, drink, tobacco and medicines

The standard list includes garlic, raw onions, cabbage, coffee, alcohol use disorder, tobacco smoking, large doses of some vitamin supplements, and a few medicines. Dental causes beyond gum disease belong here too: cavities, an abscessed or impacted tooth, and dentures.^[4] Food-driven odour is temporary and has an obvious cause, which is what separates it from the persistent kind.

Throat and nose: tonsil stones and post-nasal drip

Between 3% and 10% of bad breath cases are put down to ear, nose and throat problems such as tonsillitis, sinusitis and post-nasal drip.^[1] The clinical checklist also includes tonsils with deep crypts, sinus infections, throat infections and, in children, an object stuck up the nose, which often causes discharge from one nostril.^[4]

Tonsil stones get a lot of attention, and there’s some data. In a study of 49 people with chronic tonsil problems and bad breath, all referred for laser treatment of their tonsils, only eight had abnormal sulfur readings. Of those eight, 75% had a visible tonsil stone at the examination, compared with 6% of the people whose readings were normal.^[5] That’s a small, selected group, but it points two ways at once: a tonsil stone can produce measurable bad breath, and most people with troublesome tonsils in that study didn’t have abnormal readings at all.

Can bad breath come from your stomach?

This is the theory most worth taking seriously on its own terms, because the mechanism is plausible and there is data behind it. H. pylori, the stomach bacterium, produces hydrogen sulfide and methyl mercaptan in lab cultures, and one proposed route is that sulfur compounds made in the gut are carried in the blood to the lungs and breathed out.^[6]

Reflux

The main population evidence comes from a German study of 3,005 adults. Among 2,588 people with their own teeth, those with reflux symptoms were more likely to report bad breath (odds ratio 2.24), with a clear dose-response: the worse the heartburn or regurgitation, the stronger the link. Among 417 denture wearers, severe reflux symptoms carried an odds ratio of 12.94 compared with none, though the confidence interval was very wide, from 2.66 to 63.09. The analysis adjusted for age, smoking, dental status and a history of gastritis or ulcers.^[7]

Two limits. Both the reflux and the bad breath were self-reported, and nobody measured anyone’s breath. And the authors themselves said the obvious next step, trials of acid-suppressing drugs for halitosis, still needed to be done.^[7] Set against that, the Belgian clinic, which examined people directly, found ear, nose, throat and all other non-oral causes combined in 4% of patients.^[2]

My read: reflux is a plausible contributor for some people, especially those with frequent heartburn or regurgitation. It isn’t the default explanation, and treating reflux won’t fix a coated tongue. If reflux is part of your picture, our GERD diet guide covers what food changes can and can’t do.

H. pylori

The strongest-sounding evidence here is a 2016 meta-analysis of 21 studies and 5,062 participants. People with bad breath had about four times the odds of carrying H. pylori (odds ratio 4.03), and people with H. pylori had nearly three times the odds of bad breath (odds ratio 2.85). In three studies covering 128 infected people with bad breath, those whose infection was successfully cleared were far less likely to still have bad breath afterwards than those whose treatment failed (relative risk 0.17).^[6]

Now the quality. None of the 21 studies was a randomised trial: 13 were case-control studies, two were case reports, and the rest were cross-sectional or non-randomised treatment studies. The two main pooled results showed very high heterogeneity (I² of 87% and 89%), meaning the studies disagreed with each other a lot. Two studies in children found no significant association. And one study the authors discuss found higher sulfur levels in the mouth air of infected people but no difference in air breathed out from the lungs, which points back at the mouth rather than the stomach. H. pylori has also been found in dental plaque.^[6]

So: a real and repeated association, built on weak designs, with a mechanism that might run through the mouth as much as the stomach. The authors call the correlation clear and suggest eradication may help people whose bad breath has resisted other treatment.^[6] Given the study designs, I’d put more weight on the second half than the first. If you have stomach symptoms as well, or bad breath that has survived genuinely good tongue and gum care, testing for H. pylori is reasonable. Our guide to H. pylori treatment covers what clears it.

Systemic smells worth noticing

A few smells point somewhere other than the mouth entirely, and they aren’t subtle:^[4]

• Fruity: a sign of ketoacidosis, which can happen in diabetes and is potentially life-threatening.
• Ammonia or urine-like, sometimes described as fishy: chronic kidney failure.
• Like faeces: prolonged vomiting, especially with a bowel obstruction.

Liver failure (hepatic encephalopathy), lung infections and abscesses, and some cancers of the oesophagus and stomach also appear on the list of diseases that can change breath odour.^[4]

How to tell if you have bad breath

You are a poor judge of your own breath, and that cuts both ways. In the Belgian series, most people who came to the clinic, many of them after years of worry, scored under 3 on the 0 to 5 odour scale, and 16% had pseudo-halitosis or halitophobia.^[2]

The reference standard is a trained judge smelling your breath, known as an organoleptic test.^[1] Portable sulfide meters are more reproducible but have a blind spot: they’re highly sensitive to hydrogen sulfide and much less sensitive to methyl mercaptan.^[1]

At home, the practical proxies are these:

• Ask someone you trust, and ask them to be blunt.
• Scrape the back of your tongue with a spoon or scraper, wait a few seconds and smell it. That’s where most of the odour is made.
• Smell used floss, which tells you about the spaces between your teeth.
• Don’t judge first thing in the morning, when ordinary morning breath skews the result.

If you’ve been examined and nobody can detect anything, pseudo-halitosis is a recognised diagnosis, not a brush-off. Halitophobia can persist even after genuine bad breath has been treated.^[1] The useful response is to address the worry directly rather than adding another rinse.

How to get rid of bad breath: fixes that work

Clean the back of your tongue, daily. Cochrane’s review of halitosis treatments rated the trial evidence for tongue cleaning as very low certainty: two trials, 46 people, and a small drop of 0.2 points on the 0 to 5 odour scale.^[1] That’s thin. But tongue coating is the leading source, so removing it is the most direct fix available, and it costs nothing. I’d start here.

Clean between your teeth and get your gums checked. Gum disease is the second big source,^[2] and plaque between the teeth contributes too.^[1] Flossing is the one home-care measure the clinical checklist singles out,^[4] and if your gums bleed, that’s a conversation for a dentist.

Use mouthwash for what it is. Most over-the-counter fixes, from gum and mints to scented drops and rinses, mask odour with a competing, temporary smell. Some rinses contain ingredients that act on the bacteria or the sulfur compounds, such as zinc and chlorhexidine. The trial results are modest: brushing plus a cetylpyridinium rinse beat brushing alone by about half a point on the odour scale in one trial of 70 people (low certainty), and a chlorhexidine and zinc rinse against placebo produced a result too uncertain to call. Continuous rinse use can stain teeth and change taste.^[1] And no rinse treats the underlying source.^[4] We explain why a daily antiseptic habit is the wrong default in the oral microbiome.

Keep saliva flowing. Drink enough water, and chew. In a study of 30 young adults, chewing a tasteless wax film raised saliva flow from 0.37 to 1.49 mL per minute.^[8] Chewing gum as a treatment in its own right has barely been tested: one trial of a eucalyptus gum against placebo gum gave a very uncertain result.^[1]

What “permanently” means. There’s no one-time cure for the common kind of bad breath, because the tongue coating grows back, and most treatment trials only ran for one to four weeks.^[1] In practice, “permanently” means a daily tongue-cleaning habit, clean spaces between the teeth, treated gums, and looking beyond the mouth only if that combination fails.

Where mastic fits

If you’re going to chew something for saliva anyway, mastic is one option with some oral-health research of its own, and it’s worth stating exactly how much there is.

Saliva. In the same 30-person study, chewing mastic resin raised saliva flow from 0.41 to 2.10 mL per minute, and the increase was significantly larger than with the tasteless chewing control (a median rise of 1.57 against 1.11 mL per minute).^[8] Those were healthy young dental students, not people with dry mouth.

Oral bacteria. In two small double-blind randomised trials in 20 dental students, chewing mastic gum reduced bacteria in saliva over four hours compared with a placebo gum. In the second, run over seven days without toothbrushing in a smaller group, the mastic group had lower plaque and gum inflammation scores (plaque index 2.69 against 3.15, gingival index 0.44 against 0.66).^[9]

Breath itself. The only human trial I could find that measured bad breath used mastic toothpaste, not gum. In 32 teenagers with fixed braces, two weeks of mastic toothpaste cut median hydrogen sulfide from 158 to 26 parts per billion, significantly more than placebo toothpaste, and improved plaque and gum scores. The other two sulfur compounds it measured didn’t fall more than with placebo.^[10]

The bounds matter. I found no trial testing mastic gum against bad breath. A toothpaste isn’t a gum. The studies are small, short and in young, healthy people. So mastic isn’t a halitosis treatment, and it won’t clear a coated tongue or treat gum disease. What it is: something to chew that gets saliva moving, with some evidence of acting on oral bacteria, and without sugar. For the wider picture, see our review of mastic gum benefits.

If you want to try it, Greco Gum is single-ingredient: 100% Chios mastic resin, with no sweeteners and no synthetic gum base. Greco Gum shares our owner (Hubert Ventures LLC).