There are many conditions that can cause bad breath (halitosis):
1. Eating certain types of food (cabbage, garlic, raw onions, or coffee), vitamin supplements or drugs;
2. Disorders in the oral cavity: poor dental hygiene, dentures, cavities, gum disease (gingivitis, gingivostomatitis), abscessed tooth, impacted tooth, periodontal disease, Acute necrotizing ulcerative mucositis, acute necrotizing ulcerative gingivitis,
3. Tobacco smoking and alcoholism;
4. Respiratory disorders: ozena (atrophic rhinitis), sinusitis, throat infection, lung infection, bronchiectasis, lung abscess, foreign body in the nose etc;
5. Disorders of the digestive system: bowel obstruction (can cause breath to smell like feces), Esophageal cancer, Gastric carcinoma, Gastrojejunocolic fistula (fruity-smelling breath), Hepatic encephalopathy, Zenker's diverticulum etyc;
6. Chronic renal failure (can cause breath to smell like ammonia);
7. Diabetes mellitus (fruity or sweet chemical smell with ketoacidosis).
To diagnose the possible cause of bad breath, first you can visit your dentist. If your teeth are OK, then you can visit your GP. Your GP may send you to an ENT or internist, if necessary.