I brush my teeth but my breath still smells. Why?

If your breath still smells after you brush, the smell usually comes from inside the mouth, and a common source is the coating on the back of the tongue, which brushing your teeth does not reach. In a 2009 study of 2,000 people who visited a bad breath clinic in Belgium, 76% had a cause in the mouth, and tongue coating alone accounted for 43%. Food left between the teeth, a dry mouth, strong-smelling food, smoking and problems with the teeth or gums can add to it. The NHS suggests cleaning your tongue once a day, cleaning between your teeth once a day, and seeing a dentist if the smell does not go away after a few weeks of looking after your mouth yourself.

Where does bad breath usually come from?

Most bad breath starts in the mouth. A smaller share comes from the nose and throat or from elsewhere in the body.

What bad breath clinic studies found

The Belgian clinic visitors had been bothered by their breath for an average of 7 years before coming to the clinic. Bollen and Beikler estimate that bad breath affects around 25% of people.

The limits of these figures

These studies looked at people who went to specialist clinics, so they show where the smell comes from in people who were worried enough to seek help, not in everyone. The Italian study found the gums more often involved than the Belgian one did, which shows that results differ from clinic to clinic. What they agree on is that the mouth is by far the most common source.

Why does the smell stay after I brush my teeth?

A toothbrush cleans the teeth and the gum line. Two places it often misses are the top of the tongue and the gaps between the teeth.

The coating on the tongue

The back of the tongue has a rough surface that holds a whitish or yellowish coating. In the Belgian study, this coating alone was the cause in 43% of the people examined. In Thailand, a study of 839 people attending Chulalongkorn University's dental hospital found tongue coating was the factor most strongly linked to feeling you have bad breath: people with it had about 3.5 times the odds. The NHS suggests gently cleaning your tongue once a day using a tongue scraper or cleaner.

Between the teeth

Food caught between the teeth stays there after brushing. The NHS suggests cleaning between your teeth with interdental brushes or floss at least once a day. In the Chulalongkorn study, brushing less often and never having had a professional clean were also more common among people who felt they had bad breath.

Problems with the gums

In the clinic studies, problems with the gums were the source, alone or with tongue coating, in a sizeable share of the people examined. In the Chulalongkorn study, bleeding when brushing was the second strongest factor after tongue coating. Gums that bleed, hurt or look swollen are a reason to see a dentist.

Why is my breath worse in the morning?

Many people notice it most in the morning, and saliva is a large part of why.

Saliva almost stops during sleep

A review by Colin Dawes at the University of Manitoba notes that saliva flow is negligible during sleep. The US National Institute of Dental and Craniofacial Research (NIDCR) explains that saliva helps break down food and clean the teeth and gums. With little saliva for several hours, the mouth has less of that cleaning overnight.

A study of morning breath

In a 2025 study, Popa and colleagues at universities in Romania measured 92 university students aged 18 to 35 early in the morning.

Lower saliva and heavier tongue coating were each linked to higher levels of sulphur gases, the gases that bad breath devices measure. For every 0.1 ml a minute less saliva, the odds of reporting morning breath were 1.9 times higher. The study was small and from one country, but it shows that morning breath is common even in healthy young adults.

It changes during the day

A study of 428 older adults in Phitsanulok by Samnieng and colleagues measured breath at four different times of day and found that the level of one sulphur gas varied across the day. Thicker tongue coating was linked to higher levels.

What else can make breath smell?

Beyond the mouth itself, several everyday things can add to it.

Everyday examples in Thai food (our examples, not from the NHS): dishes heavy in raw garlic, shallots or sator beans.

The NHS also says that some health conditions can cause bad breath. If your dentist finds your mouth healthy but the smell continues, a doctor can look for other causes.

How common is bad breath in Thailand?

Two Thai studies give a sense of how common it is, in two different groups.

What Thai studies found

Feeling it and measuring it are different

The Chulalongkorn study asked people what they felt; the Phitsanulok study measured gases with a device in older adults. Neither describes all Thai adults. The Belgian clinic study is a reminder that the two do not always match: 16% of the clinic visitors were worried about their breath, but no bad breath was found. In the Chulalongkorn study, people whose bad breath had also been noticed by others reported more of it, including when talking and in the morning. A dentist's check is more reliable than judging your own breath.

What everyday mouth care do health agencies suggest?

These are general mouth-care habits from the NHS and NIDCR.

Every day

The NHS also suggests medium or soft bristles as best for most people. Dawes notes that the most important time to brush is just before bed, because saliva flow is negligible during sleep.

If your mouth feels dry

NIDCR suggests drinking more water, using sugarless gum or sugarless sweets, avoiding caffeine and alcohol, not using tobacco and running a humidifier at night.

When should I see a dentist or doctor?

Most of the time, better mouth care is the first step. See a dentist if it is not enough.

See a dentist if

The NHS advises seeing a dentist if you have:

• bad breath that does not go away after a few weeks of looking after your mouth yourself
• painful, bleeding or swollen gums
• toothache or wobbly adult teeth
• problems with your dentures

Other reasons to get checked

• your mouth often feels dry: NIDCR says a dry mouth is not a normal part of ageing and advises seeing a dentist or doctor
• you take regular medicines and your mouth has become drier since starting them; ask a doctor or pharmacist rather than stopping them yourself
• your dentist finds your mouth healthy but the smell continues; a doctor can look for causes in the nose, throat or elsewhere