Where bad breath comes from
Around nine out of ten cases originate in the mouth rather than the stomach, contrary to popular belief. The odour is produced by bacteria breaking down food debris and releasing volatile sulphur compounds.
The tongue is the usual source
The rough surface at the back of the tongue harbours far more bacteria than the teeth do. Cleaning it gently with a tongue scraper or the back of your brush, once a day, often produces a more noticeable improvement than anything else.
Other oral causes
Untreated decay, gum disease, poorly cleaned dentures and food trapping around a broken filling all contribute. Dry mouth is another major factor: saliva is the mouth's natural cleanser, which is why breath is worst on waking and why some medications make the problem worse.
What actually helps
Brush twice daily, clean between the teeth, clean the tongue, and drink water regularly. Mouthwash masks the smell but does not remove its cause, so it is a supplement rather than a solution. If the problem persists despite good hygiene, see your dentist: a hidden infection is the most likely explanation, and non-oral causes such as sinus or digestive conditions can then be investigated.