Common factors that can lead to bad breath

Poor oral hygiene

It is commonly believed that the primary cause of bad breath is due to oral hygiene problems such as decaying teeth or tartar build-up. While these causes can certainly lead to bad breath, they are rather less common than one might imagine. Indeed, even individuals with a spotless dental record may still suffer from bad breath.

Impacted Food or Debris

The food and drink we consume can result in bacterial growth. When food debris become trapped between teeth, they can decay and help odour-causing bacteria to flourish.

Just as importantly, studies suggest that bacteria will also feed on sloughed cells from within the mouth. This can manifest itself in the form of “morning breath“. When our normal daytime activities cease during sleep they fail to wash away epithelial cells as they are exfoliated.

Tongue Coating

Approximately half of bad breath cases are thought to be caused by a residue of bacteria on the tongue – leading to what dentists refer to as a “tongue coating”(1).

Changes to Salivary Flow or Composition

Human saliva has highly effective properties for fighting tooth decay and the bacteria which can lead to halitosis. This is one reason why chewing sugar-free gum after eating can be beneficial for maintaining standards of oral care.

However, situations which lead to changes in the composition of the saliva, such as during pregnancy or a decline in the flow of saliva e.g. while sleeping, can have a deleterious effect on your breath. The latter is a major cause of so-called “morning breath“.

Something as simple as sleeping with your mouth open or breathing through your mouth while exercising can also dry out the palette, increasing the chances of bad breath shortly afterwards.

Odorous Food Consumption

For those of us who enjoy garlic, it should come as no surprise that some strongly-flavoured foods can result in bad breath. Typically this cause of halitosis has less to do with bacterial activity, and more to do with the compounds contained in the food.

Examples of foods which have been shown to cause bad breath include garlic (see Garlic Breath), onions, spices, cabbages, cauliflowers and radishes(2).

Alcohol Consumption

Alcohol is a diuretic so can lead to dehydration. Studies have found that alcohol consumption varies the blood composition leading to a dry mouth and excessive thirst(3). This can impact the breath, by allowing oral bacteria to proliferate.

Additionally, it has been noted that the day after alcohol consumption, levels of ketones in the blood may rise. This can result in “ketosis breath” as also experienced by some low-calorie dieters.


The smell of smoke on the breath isn’t the only cause of bad breath found among smokers. Smoking also dries out the mouth, preventing saliva from continuing its protective role against unpleasant-smelling breath.

Medical Conditions

A surprisingly diverse range of medical conditions have been shown to have an impact on halitosis(4).

Cirrhosis of the liver, for example, can lead to bad breath. However the problem in this case isn’t actually caused from within the mouth. The odour is actually produced by the liver which then escapes through exhalation.

There are plenty more examples. Pulmonary disease, kidney disorders and diabetes can all be considered common causes of bad breath, even without any adverse oral conditions present. Acid reflux can also lead to a sour taste in the mouth and unpleasant breath.

The important point here therefore, is to appreciate that while such conditions are far less common that many of the previously-discussed causes, it is still a possibility that someone with bad breath may have an undiagnosed, underlying medical condition.

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  • (1) Krespi, Y et al. 2006. The relationship between oral malodor and volatile sulfur compound-producing bacteria. Otolaryngology 135(5), pp 671-676.
  • (2) Scully, C and Greenmam, J. 2008. Halitosis (breath odor). Periodontology 2000 48(1), pp 66-75. Available at:
  • (3) Verster, J. 2008. The alcohol hangover – a puzzling phenomenon. Alcohol and Alcoholism 43(2), pp 124-126. Available at:
  • (4)Attia, E and Marshall, K. 1982. Halitosis. Canadian Medical Association Journal 126(11), pp 1281-1285. Available at:


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