Many causes of bad breath can be easily addressed. Sometimes, however, bad breath reflects a more serious underlying condition. It could also be a side effect of certain medical treatments or therapies. Whatever the cause, chronic bad breath is not something to be lived with… or ignored. The following 10 health conditions have been found to cause bad breath. A few of them relate directly to disorders of the mouth, while others may cause bad breath as a side effect.
Causes of Bad Breath
1. Gum Disease
Gum disease causes bad breath. One 2012 study found a direct relationship between periodontitis and gingivitis to bad breath.  This halitosis resulted from the bacteria inhabiting the mouths of the suffering patients.
Don't freak out and think because your mouth is unpleasant that you have cancer. But, cancers of the mouth do create many complications for quality of one’s breath. In others, bad breath may actually help reveal an early stage cancer. A director at the Cleveland Clinic’s respiratory center has tested a device designed to recognize lung cancer in 80% of patients – based on a test of the patient’s breath.  As bad as it is, a symptom such as chronic bad breath could be a life-saver.
Chemotherapy and radiation therapy can lead to dry mouth by affecting saliva production. Without adequate saliva flow, unwelcome bacteria can build up releasing the unpleasant sulfuric gas that makes breath so awful.
If you suffer from allergies, bad breath may afflict you in addition to the scratchy throat, stuffy nose and watery eyes. Mucus and nasal drip provide a fertile breeding ground for the nasty germs that cause bad breath. Often when eliminating allergy symptoms, you experience dry mouth. This too causes bad breath. While it may appear a no-win situation, eliminating the nasal drip and keeping your mouth clean and fresh can offer a degree of freedom from both allergies and bad breath.
Diabetics suffer from inadequate insulin production. This leads the body to burn fat, a condition known as ketoacidosis.  Which in turn leads to an increase in ketones, which the body attempts to eliminate through urine and the lungs. This leads to a breath that has a fruit like odor or odor like acetone. (Acetone is a by-product of ketone production. Ketones are created when the body burns fatty acids).
5. Liver Disease
Patients suffering from liver disease experience a bad breath known by science as fetor hepaticus.  This musty smell of breath has been found to indicate liver disorders, often before they manifest through other more obvious means.
6. Kidney Failure
Bad breath has also been found to indicate possible kidney failure. Researchers speculate this may be due to metabolic changes that lead to dry mouth, lack of saliva flow, and an altered sense of taste.  All of these contribute to halitosis as the saliva flow fails to cleanse the mouth.
7. Candida albicans
Yet another cause of bad breath may be Candida albicans. This yeast has been shown to be a common concern for those who choose tongue piercings. It has also been identified in denture wearers, or anyone who uses dental appliances such as retainers. 
8. Chronic Acid Reflux
Suffering from chronic acid reflux is bad enough. For those that do, keeping the mouth cleaned is imperative. A review of several studies on patients suffering from GERD (Gastroesophageal Reflux Disease) found halitosis frequently plagues these patients.  The influx of acids and other partially digested material into the esophagus and mouth cavity can create concerns for keeping the oral cavity clean.
9. Helicobacter pylori
H Pylori infection, typically associated with ulcers and other gastrointestinal concerns, has been found to occur more highly in patients with halitosis. One study found patients suffering from gastrointestinal concerns suffered from halitosis and H Pylori infection.  Fortunately, for these patients once the infection was addressed, the halitosis cleared up.
10. Sjögren's Syndrome
Sometimes dry mouth is caused by autoimmune disorders – the body attacking itself. One such medical condition known as Sjögren's syndrome occurs when the body actually attacks and inhibits exocrine glands like the salivary glands from working properly.  A concern like this leads to dry mouth and the many concerns associated with it.
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†Results may vary. Information and statements made are for education purposes and are not intended to replace the advice of your doctor. If you have a severe medical condition or health concern, see your physician.