Around 90% of persistent bad breath originates in the mouth, most often from bacteria on the back of the tongue, gum disease, or food trapped around decayed teeth and failing fillings. These bacteria produce volatile sulphur compounds, which are what actually smells. Mouthwash masks the odour temporarily without removing the source; tongue cleaning, treating gum disease and repairing decay address the cause.
Where the smell comes from
Anaerobic bacteria — those that thrive without oxygen — break down proteins from food debris, dead cells and blood, releasing volatile sulphur compounds. Those compounds smell of rotten eggs and decay because that is chemically what they are.
These bacteria need somewhere oxygen-poor to live. The coating on the back of the tongue, deep gum pockets, and cavities in teeth are all ideal.
The usual sources, in order of likelihood
- The back of the tongueThe single most common source, and the most commonly missed. The rough surface at the back of the tongue holds a bacterial coating that brushing your teeth does not touch.
- Gum diseaseDeep pockets around teeth are exactly the oxygen-poor environment these bacteria need. Bad breath plus bleeding gums is gum disease until proven otherwise.
- Decay and failing restorationsCavities and gaps around old fillings trap food that cannot be cleaned out.
- Dry mouthSaliva is the mouth's natural cleaning system. Medications, mouth-breathing and dehydration reduce it — which is also why morning breath is universal.
- Non-dental causesSinus infection, tonsil stones, acid reflux, and in rarer cases diabetes or liver disease. Worth investigating once dental causes are excluded.
How to check your own breath
Cupping your hands over your mouth does not work — you have long since stopped perceiving your own smell. Instead, lick the inside of your wrist, let it dry for ten seconds and smell it. Or scrape the back of your tongue with a spoon and smell the residue. Both are more honest than you might wish.
What actually resolves it
- Clean your tongue daily with a tongue scraper, reaching as far back as you can tolerate. This alone resolves a large proportion of cases.
- Clean between your teeth every day — a brush reaches only about 60% of tooth surfaces.
- Have gum disease diagnosed and treated properly rather than managed with mouthwash.
- Get decay and failing fillings repaired.
- Stay hydrated, and if a medication is drying your mouth, discuss alternatives or use a saliva substitute.
- Stop tobacco, which causes odour directly and drives gum disease.
When to see a dentist about it
If the problem persists despite daily tongue cleaning and good interdental cleaning for a month, book an examination. Persistent bad breath with a genuinely good routine almost always means there is a source you cannot reach — a deep pocket, a cavity between teeth, or a leaking restoration.
Questions we get asked
Why does my breath smell even though I brush twice a day?
Because brushing cleans teeth, and the most common source is the back of the tongue, followed by the surfaces between teeth. Neither is reached by a toothbrush on the teeth alone.
Do tonsil stones cause bad breath?
Yes, and they are frequently missed. They are small calcified deposits in the tonsil crypts with a characteristic strong odour. If your dental examination is clear, this is worth raising with an ENT surgeon.
Is chewing gum a reasonable solution?
Sugar-free gum stimulates saliva, which genuinely helps in the short term, particularly for dry mouth. It manages the symptom rather than the cause, but it is a harmless adjunct.