Most cases of bad breath (halitosis) come from germs (bacteria) or debris that build up within the mouth.
What are the causes of bad breath?
Morning bad breath
Most people have some degree of bad breath after a night's sleep. This is normal and occurs because the mouth tends to get dry and stagnate overnight. This usually clears when the flow of saliva increases soon after starting to eat breakfast.
Dry mouth (xerostomia)
Bad breath associated with a dry mouth is caused by a reduction in the cleansing mechanism of the mouth as a result of reduced flow of saliva. There are many causes of a dry mouth. The most common cause is after a night's sleep (discussed above). Dry mouth may also occur:
- Due to a lack of fluid in the body (dehydration).
- As a side-effect of some medicines (for example, tricyclic antidepressants).
- As a symptom of some diseases (such as Sjögren's syndrome).
- Following radiotherapy to the head and neck region.
Foods, drinks and medicines
Chemicals in foods can get into the bloodstream and then be breathed out from the lungs. Most people are familiar with the smell of garlic, spicy foods and alcoholic drinks on the breath of people who have recently eaten or drunk these. Various other foods and medicines can cause a smell on the breath. This type of bad breath is temporary and easily cured by not eating the food. (However, some people eat spiced food every day. As a result, they will constantly have a typical smell on their breath.)
If a medicine is causing the problem then discuss possible alternatives with your doctor. Medicines that have been associated with bad breath include:
- Chloral hydrate
- Nitrites and nitrates
- Dimethyl sulfoxide
- Some chemotherapy medicines
- 'Water' tablets (diuretics)
Check the list of side-effects on the leaflet that comes with your medicine if you think it might be causing bad breath.
Most non-smokers can tell if a person is a smoker by their breath which "smells like an ashtray". Stopping smoking is the only cure for this type of bad breath. Smoking also increases the risk of developing gum disease - another cause of bad breath.
Crash dieting or fasting can cause a sickly sweet smell on the breath. This is due to chemicals called ketones being made by the breakdown of fat. Some ketones are then breathed out with each breath. Low-carbohydrate diets also can cause this sort of bad breath.
Medical causes, other than problems in the mouth, are relatively uncommon. Possible causes include:
- Nasal problems. For example, a lump (polyp) in the nose or a small object stuck in a nostril (occurs most commonly in children) can cause a bad smell. In this situation, the smell tends to occur only, or more severely, when you breathe out through your nose. It is not so noticeable when you breathe out through your mouth.
- Sinus infections.
- Throat infections such as tonsillitis.
- Mouth infections, such as dental abscesses.
- Infections or tumours of the lung.
- Reflux of acid from the stomach ora stomach infection called Helicobacter pylori.
- Severe kidney or liver problems.
- Fish odour syndrome (trimethylaminuria). This is a rare medical cause but worth being aware of. It typically causes breath and body odour that is often like a fishy smell. It occurs because the body loses the ability to properly break down trimethylamine which is found in certain foods. There is then a build-up of trimethylamine in the body which is released in sweat, urine and breath. Urine and blood tests can help to confirm this diagnosis if it is suspected.
In these medical cases, there are usually other symptoms that would indicate the cause. For example, a blocked nose, sinus pain, chest symptoms, a high temperature (fever), etc. If you are otherwise well and have no other symptoms apart from bad breath, the smell is likely to be coming from a build-up of bacteria in the mouth and other medical causes are unlikely.
Bad breath caused by germs
In most people who have bad breath (halitosis), the bad smell is thought to come from germs (bacteria) and debris within the mouth.
As the bacteria break down proteins and other debris in the mouth, they release foul-smelling gases. One or more of the following may contribute to the build-up of bacteria, debris and bad breath:
- Food stuck between teeth. Normal teeth brushing may not clear bits of food which can get stuck between teeth. The food then rots and becomes riddled with bacteria. Regular cleaning between the teeth can clear and prevent this problem.
- Plaque, tartar (calculus) and gum disease. Dental plaque is a soft whitish deposit that forms on the surface of teeth. It forms when bacteria combine with food and saliva. Plaque contains many types of bacteria. Calculus is hardened calcified plaque. It sticks firmly to teeth. Gum disease means infection or inflammation of the tissues that surround the teeth. If your gums look inflamed, or regularly bleed when you clean your teeth, you are likely to have gum disease. The severity can range from mild to severe.
- Coating on the back of the tongue. In some people, a coating develops on the back part of the tongue. It is not clear why this occurs. It may be from mucus that drips down from the back of the nose (postnasal drip). The coating can contain many bacteria. This explains why bad breath can sometimes occur in people with otherwise good oral hygiene.
- Tonsil stones (tonsilloliths). These are clusters of calcified material that form in the tonsillar crypts, or crevices of the tonsils. They are made up mostly of calcium but can contain other ingredients such as magnesium and phosphorus, and can feel like a small lump in the tonsils. Rarely harmful, they can be a nuisance and hard to remove and can often cause bad breath.
Further reading and references
Porter SR, Scully C; Oral malodour (halitosis). BMJ. 2006 Sep 23333(7569):632-5.
Fedorowicz Z, Aljufairi H, Nasser M, et al; Mouthrinses for the treatment of halitosis. Cochrane Database Syst Rev. 2008 Oct 8(4):CD006701.
Phillips IR, Shephard EA; Trimethylaminuria, University of Washington, 2007 (updated 2011)
Yaacob M, Worthington HV, Deacon SA, et al; Powered versus manual toothbrushing for oral health. Cochrane Database Syst Rev. 2014 Jun 17(6):CD002281. doi: 10.1002/14651858.CD002281.pub3.
Poklepovic T, Worthington HV, Johnson TM, et al; Interdental brushing for the prevention and control of periodontal diseases and dental caries in adults. Cochrane Database Syst Rev. 2013 Dec 18(12):CD009857. doi: 10.1002/14651858.CD009857.pub2.
Kapoor U, Sharma G, Juneja M, et al; Halitosis: Current concepts on etiology, diagnosis and management. Eur J Dent. 2016 Apr-Jun10(2):292-300. doi: 10.4103/1305-7456.178294.
Halitosis; NICE CKS, November 2014 (UK access only)
Kuo YW, Yen M, Fetzer S, et al; Toothbrushing versus toothbrushing plus tongue cleaning in reducing halitosis and tongue coating: a systematic review and meta-analysis. Nurs Res. 2013 Nov-Dec62(6):422-9. doi: 10.1097/NNR.0b013e3182a53b3a.
Reidy JT, McHugh EE, Stassen LF; A review of the role of alcohol in the pathogenesis of oral cancer and the link between alcohol-containing mouthrinses and oral cancer. J Ir Dent Assoc. 2011 Aug-Sep57(4):200-2.
Bollen CM, Beikler T; Halitosis: the multidisciplinary approach. Int J Oral Sci. 2012 Jun4(2):55-63.