Bad Breath (Halitosis) - Causes

What are the causes of bad breath?

Coming from within the mouth

Most cases of bad breath (halitosis) come from germs (bacteria) or debris that build up within the mouth. This is discussed in more detail in its own section.

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:

  • Betel
  • Chloral hydrate
  • Nitrites and nitrates
  • Dimethyl sulfoxide
  • Disulfiram
  • Some chemotherapy medicines
  • Phenothiazines
  • Amfetamines
  • Lithium
  • 'Water' tablets (diuretics)
  • Bisphosphonates

Check the list of side-effects on the leaflet that comes with your medicine if you think it might be causing bad breath.

Smoking

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.

Diets

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

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.
  • Bronchiectasis.
  • Reflux of acid from the stomach ora stomach infection called Helicobacter pylori.
  • Diabetes
  • 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.

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Author:
Dr Mary Harding
Peer Reviewer:
Dr Helen Huins
Document ID:
4892 (v43)
Last Checked:
06 July 2017
Next Review:
05 July 2020

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.