Bad breath is caused by a variety of factors. In most cases, it is caused by food remaining in the mouth – on the teeth, tongue, gums, other structures, collecting and bacteria. 

Dead and dying bacterial cells release a sulphur compound that gives your breath an unpleasant odour. Certain foods, such as garlic and onions, contribute to breath odour. Once the food is absorbed into the bloodstream, it is transferred to the lungs, where it is exhaled. Brushing, flossing and mouthwash only mask the odour. Dieters sometimes develop unpleasant breath from fasting.

Periodontal (gum) disease often causes persistent bad breath or a bad taste in the mouth, and persistent bad breath may mean a sign that you have gum disease.

Gum disease is caused by plaque – the sticky, often colourless, film of bacteria that constantly forms on teeth. Dry mouth or xerostomia may also cause bad breath due to decreased salivary flow. Saliva cleans your mouth and removes particles that may cause odour. Tobacco products cause bad breath, stain teeth, reduces your ability to taste foods and irritate your gum tissues. Bad breath may also be a sign that you have a serious health problem, such as a respiratory tract infection, chronic sinusitis, postnasal drip, chronic bronchitis, diabetes, gastrointestinal disturbance, liver or kidney ailment.

Here are characteristic bad breath odours associated with some of these illnesses:

  • Diabetes – acetone, fruity

  • Liver failure – sweetish, musty

  • Acute rheumatic fever – acid, sweet

  • Lung abscess – foul, putrefactive

  • Blood dyscrasias – resembling decomposed blood

  • Liver cirrhosis – resembling decayed blood

  • Uremia – ammonia or urine

  • Hand-Schuller-Christian disease – fetid breath and unpleasant taste

  • Scurvy – foul breath from stomach inflammation

  • Wegner`s granulomatosis – Necrotic, putrefactive

  • Kidney failure – ammonia or urine

  • Diphtheria, dysentery, measles, pneumonia, scarlet fever, tuberculosis – extremely foul, fetid odor

  • Syphilis – fetid

Bad breath may also be caused by medications you are taking, including central nervous system agents, anti-Parkinson drugs, antihistamines/decongestants, anti-psychotics, anti-cholinergics, narcotics, anti-hypertensives, and anti-depressants.