Most bad breath is produced by bacteria on the back of the tongue rather than in the stomach, and the mouthwash you rely on is often why it keeps returning.
Anaerobic bacteria break down proteins from food debris and shed cells, and the by-products are volatile sulphur compounds — the same chemistry as rotten eggs and old cabbage. Most of that population lives in the coating on the back third of the tongue, in grooves a toothbrush never reaches. Brushing the visible front of the tongue therefore achieves very little, while a scraper drawn from as far back as you can tolerate does most of the work.
Periodontal pockets deeper than four millimetres are sheltered, oxygen-poor spaces where exactly the wrong bacteria thrive, and an overhanging filling or an open crown margin creates the same trap in miniature. Saliva is the body's own rinse. Mouth breathing, dozens of common medications and alcohol-based mouthwash all reduce it, which is why a rinse that smells clean for twenty minutes can leave hour six noticeably worse.
Tonsil stones, chronic sinus drainage, reflux, poorly controlled diabetes with its sweet acetone note, and rarely liver or kidney disease all produce odour that no dental treatment touches. Roughly one case in ten sits outside the mouth. If the gums are healthy, the tongue is being cleaned properly and the smell persists, the next appointment belongs with an ear nose and throat specialist or a physician.
The examination is a full periodontal chart with pocket depths, a check of every restoration margin, and a look at saliva flow and medication. Treatment is scaling, replacing leaking margins, a tongue scraper and a short zinc or chlorhexidine protocol. What we do not do is treat halitosis with veneers. New ceramic over untreated pockets smells identical six weeks later, and the patient has paid for nothing.
Cupping your hands does not work, because you are habituated to your own odour. Lick the back of your wrist, wait ten seconds and smell it, or scrape the back of the tongue with a plastic spoon. Both are cruder than a laboratory reading and both are informative.
Only if failing old restorations were the actual source, which is a minority of cases. Diagnose first. Anyone who proposes a smile makeover as the solution to halitosis without charting your gums is selling, not treating.