Persistent bad breath or a bad taste in the mouth may be a warning sign of gum (periodontal) disease. Gum disease is caused by the buildup of plaque on teeth. Bacteria cause the formation of toxins to form, which irritate the gums. If gum disease continues untreated, it can damage the gums and jawbone.

Other dental causes of bad breath include poorly fitting dental appliances, yeast infections of the mouth, and dental caries (cavities). The medical condition dry mouth (also called xerostomia) also can cause bad breath. Saliva is necessary to moisten the mouth, neutralize acids produced by plaque, and wash away dead cells that accumulate on the tongue, gums, and cheeks.

If not removed, these cells decompose and can cause bad breath. Dry mouth may be a side effect of various medications, salivary gland problems, or continuous breathing through the mouth.

Many other diseases and illnesses may cause bad breath. Here are some to be aware of: respiratory tract infections such as pneumonia or bronchitis, chronic sinus infections, postnasal drip, diabetes, chronic acid reflux, and liver or kidney problems.