The most common cause of bad breath is sulphur-producing bacteria that live on the tongue and throat. Sometimes these bacteria break down proteins at a high rate, releasing volatile sulphur compounds that produce a foul odour.

Other causes of halitosis include:

• Poor oral hygiene habits. If you don’t brush and floss regularly food can stay in your mouth, which leads to more bacterial growth in between the teeth, on the tongue and around the gums (for more information, see “Food traps”).
• Strong-flavoured foods like onions and garlic. The odour will go away when the foods have passed through your body.
• Gum disease (i.e. periodontitis; see “Bleeding or sore gums?”).
• Poorly fitting dental appliances (see “Ill-fitting dentures?”).
• Yeast infections in the mouth.
• Dental cavities (caries).
• Dry mouth (xerostomia). This can be a side effect of medications, salivary gland problems or continuous breathing through the mouth. It can also be caused by alcohol, stress or a medical condition.
• Many other illnesses can cause halitosis, such as respiratory tract infections (pneumonia, bronchitis), chronic sinus infections, diabetes, chronic acid reflux, and even liver or kidney problems.