Bad breath, medically called halitosis, is most commonly caused by conditions within the mouth. Oral bacteria metabolise proteins and other organic material and produce volatile compounds, including volatile sulphur compounds, that can produce an unpleasant odour. Bacteria accumulate in dental plaque, on the tongue and within periodontal pockets, so inadequate oral hygiene can allow odour-producing microorganisms and their substrates to accumulate. Gum disease and tooth decay are important dental causes, and infections can also contribute. NHS guidance identifies problems with teeth or gums, including gum disease, dental cavities and infection, among the common causes of bad breath. Persistent halitosis therefore warrants attention to oral hygiene and dental health rather than being attributed automatically to the stomach or digestive system.
The tongue is an important source of oral malodour because its surface contains numerous papillae and microscopic irregularities where bacteria, shed epithelial cells, mucus and food residues can accumulate. A coating on the back of the tongue can provide a substantial substrate for odour-producing bacteria. Cleaning the tongue gently once daily can therefore reduce the microbial load contributing to breath odour. Tongue cleaning should be gentle rather than aggressive because scraping until the tissue bleeds can cause irritation without improving oral health. NHS guidance recommends gentle tongue cleaning with a tongue scraper or cleaner as part of self-care for bad breath.
Gum disease can produce persistent bad breath because inflamed gums and periodontal pockets provide a favourable environment for anaerobic bacteria and retained debris. As periodontal disease progresses, pockets can extend deeper below the gum margin, making them inaccessible to ordinary brushing and providing a continuing source of microbial metabolites. Bad breath accompanied by bleeding gums, swollen gums, gum recession or loose adult teeth is therefore particularly suggestive of an underlying dental problem. Treating the odour alone with mints or mouthwash may temporarily mask the smell without addressing the cause. Effective periodontal treatment and daily plaque control are more important when gum disease is responsible.
Tooth decay and dental infection can also produce bad breath. Cavities allow bacteria and food debris to accumulate within damaged tooth structure, while an abscess can contain pus and inflammatory material with a strong odour. Broken fillings, cracked teeth, poorly cleaned dentures and other dental problems can similarly create plaque-retentive areas. A person who develops persistent halitosis alongside toothache, a bad taste, a wobbly tooth, swollen gums or denture problems should arrange dental assessment. NHS guidance specifically advises seeing a dentist when bad breath persists after self-care or occurs with toothache, bleeding or swollen gums, wobbly adult teeth or denture problems.
Dry mouth, or xerostomia, is another important contributor because saliva normally helps wash away food particles, dilute bacterial metabolites, lubricate oral tissues and maintain the oral environment. Reduced salivary flow can occur during dehydration, overnight, with mouth breathing, from certain medicines or as a result of some medical conditions. When the mouth becomes dry, odour-producing compounds can accumulate more readily. Fasting and crash dieting can also contribute to bad breath; reduced food intake and changes in metabolism can alter oral conditions and breath chemistry. NHS guidance specifically lists dry mouth and fasting or crash dieting among causes of bad breath.
Diet and lifestyle can produce temporary odours without indicating disease. Strong-smelling foods such as garlic, onions and spicy foods contain volatile compounds that can enter the bloodstream after digestion and later be exhaled through the lungs as well as remaining in the mouth. Alcohol can contribute through its own odour and through dehydration, while smoking produces distinctive volatile compounds and increases the risk of gum disease, another major cause of persistent halitosis. Sugary diets also promote dental decay and plaque accumulation, indirectly increasing oral causes of bad breath. These temporary dietary causes should be distinguished from chronic halitosis that persists despite routine oral hygiene.
Non-dental medical conditions can occasionally contribute to bad breath. NHS guidance includes tonsillitis and acid reflux among possible causes, while infections and other upper-airway conditions can also affect breath odour. Tonsils can retain material in crypts, and inflammation or infection can produce an unpleasant smell.
Reflux can allow acidic or other gastric contents to reach the upper digestive tract and mouth, although the common belief that most chronic bad breath originates in the stomach is incorrect; the mouth is the predominant source in most cases. Rare systemic disorders can alter breath chemistry, but these are much less common than oral causes. Persistent unexplained halitosis should therefore be evaluated systematically rather than attributed to a rare systemic condition without evidence.
Preventing or treating bad breath requires identifying its source. Brushing the teeth and gums twice daily, cleaning between the teeth daily, gently cleaning the tongue, maintaining dental check-ups and keeping dentures clean can substantially reduce common oral causes. Sugar-free chewing gum can stimulate saliva and help temporarily after strongly flavoured foods. Antibacterial mouthwash or toothpaste may provide additional benefit in selected situations, but mouthwash should not be regarded as a replacement for brushing and interdental cleaning. Smoking cessation and moderation of alcohol can also help. If bad breath persists for several weeks despite appropriate self-care, dental assessment is appropriate because underlying gum disease, decay, infection or another condition may require specific treatment.
